1. Start preparing for JEE/NEET right after your Class 10 exams. Do not waste your time. 2. JEE/NEET is NOT tough. You Just need to develop a natural interest in Physics, Chemistry and Maths – think in terms of actual scenarios and not just solving problems. 3.Self Study is absolutely Must – You must spent time on self-preparation apart from School and Coaching classes
4.The number of hours you study is more important than how much you actually get done in those hours. Concentration in whichever subject you study is the key!Focus more on the important chapters for JEE/NEET, practice more questions from these topic
5.Build a good timetable and follow it religiously. Systematic study is more important than studying concepts randomly.
6.Avoid Social Media distractions like Facebook and Mobile distractions like Whatsapp for these two years, but you could Join useful Facebook groups and Google+ communities to keep a tab on what other students are upto – how are they preparing differently.
7.Take breaks between studies. Play your favorite sports to keep yourself fit and rejuvenated.Fitness is absolutely important during these 2 years. Not being fit is one of the major reasons behind a bad JEE/NEET rank.
8.Find a mentor in your close circle. It can be your brother, sister, mother, father, cousin, class teacher – anyone. You need to be motivated throughout these 2 years.
9.If you have not been able to revise Class 11 syllabus between November20xx and January 20xx, then you must devote one or two days to do a quick revision of Class 11 syllabus as almost 40-45 per cent questions will be asked from that portion. Revisit your revision notes for a quick recapitulation of concepts, formulae, fact based information and definition etc.
10.If there are any particular type of numerical question(s) on topics which you had faced difficulty before, then try solving few representative questions just to renew your confidence. If you are still having difficulty now, simply move on. Do not get upset about it
Acids: The word ‘Acid’ came from Latin word ‘Acidus or Acere’ which means sour.
Sour taste is the most common characteristic of acid. Acid turns blue litmus paper red. There are many substances which contain acid and hence taste sour, such as curd, tamarind, lemon, etc.
Types of Acids: Acids are divided into two types on the basis of their occurrence – Natural acids and Mineral acids.
Natural Acid: Acids which are obtained from natural sources are called natural acid or organic acid.
Mineral Acids: Acids that prepare from mineral are known as mineral acids, inorganic acids, man-made acids or synthetic acid, such as hydrochloric acid, sulphuric acid, nitric acid, etc.
Acids give hydrogen gas along with respective salt when they react with a metal.
Metal + Acid ⇨ Salt + Hydrogen
Example: Hydrogen gas and zinc chloride are formed when hydrochloric acid reacts with zinc metal.
Zn + 2HCl ⇨ ZnCl2 + H2
Hydrogen gas and sodium chloride are formed when hydrochloric acid reacts with sodium metal.
2Na + 2HCl ⇨ 2NaCl + H2
Hydrogen gas and iron chloride are formed when hydrochloric acid reacts with iron.
Fe + 2HCl ⇨ FeCl2 + H2
Hydrogen gas and zinc sulphate are formed when zinc metal reacts with sulphuric acid
Zn + H2SO4 ⇨ ZnSO4 + H2
Test for hydrogen gas: The gas evolved after reaction of acid with metal can be tested by bringing a lighted candle near it. If the gas burns with pop sound, then it confirms the evolution of hydrogen gas. Burning with pop sound is the characteristic test for hydrogen gas.
Reaction of acids with metal carbonate:
Acids give carbon dioxide gas and respective salts along with water when they react with metal carbonates.
Metal carbonate + Acid ⇨ Salt + Carbon dioxide + Water
Examples: Hydrochloric acid gives carbon dioxide gas, sodium chloride along with water when reacts with sodium carbonate.
Na2CO3 + 2HCl ⇨ 2NaCl + CO2 + H2O
Hydrochloric acid gives carbon dioxide, magnesium chloride and water when it reacts with magnesium carbonate.
MgCO3 + 2HCl ⇨ MgCl2 + CO2 + H2O
Hydrochloric acid gives carbon dioxide gas, calcium chloride and water when it reacts with calcium carbonate.
CaCO3 + 2HCl ⇨ CaCl2 + CO2 + H2O
Sulphuric acid gives calcium sulphate, carbon dioxide gas, calcium sulphate and water when it reacts with calcium carbonate.
CaCO3 + H2SO4 ⇨ CaSO4 + CO2 + H2O
Sulphuric acid gives sodium sulphate, carbon dioxide gas and water when it reacts with sodium carbonate.
Na2CO3 + H2SO4 ⇨ Na2SO4 + CO2 + H2O
Nitric acid gives sodium nitrate, water and carbon dioxide gas when it reacts with sodium carbonate.
2HNO3 + Na2CO3 ⇨ NaNO3 + 2H2O + CO2
Reaction of acid with hydrogen carbonates (bicarbonates):
Acids give carbon dioxide gas, respective salt and water when they react with metal hydrogen carbonate.
Acid + Metal hydrogen carbonate ⇨ Salt + Carbon dioxide + Water
Examples: Hydrochloric acid gives carbon dioxide, sodium chloride and water when it reacts with sodium bicarbonate.
NaHCO3 + HCl ⇨ NaCl + CO2 + H2O
Sulphuric acid gives sodium sulphate, carbon dioxide gas and water when it reacts with sodium bicarbonate.
2NaHCO3 + H2SO4 ⇨ Na2SO4 + 2CO2 + 2H2O
Sodium bicarbonate is also known as sodium hydrogen carbonate, baking soda, baking powder, bread soda and bicarbonate of soda.
Test for evolution of carbon dioxide gas: Carbon dioxide turns lime water milky when passed through it. This is the characteristic test for carbon dioxide gas.
The gas evolved because of reaction of acid with metal carbonate or metal hydrogen carbonate turns lime water milky. This shows that the gas is carbon dioxide gas. This happens because of formation of white precipitate of calcium carbonate.
Ca(OH)2 + CO2 ⇨ CaCO3 + H2O
CaCO3 + CO2 + H2O ⇨ Ca(HCO3 )2
But when excess of carbon dioxide is passed through lime water, it makes milky colour of lime water disappear. This happens because of formation of calcium hydrogen carbonate. As calcium hydrogen carbonate is soluble in water, thus the milky colour of solution mixture disappears.
Reaction of acid with marble and egg shell: Since, marble and egg shell are made of calcium carbonate, hence when acid is poured over marble or egg shell, bubbles of carbon dioxide are formed.
Historical Monuments and Acid Rain:
Burning of fossil fuels releases oxides of sulphur and nitrogen. Nitrogen oxide and sulphur dioxide form nitric acid and sulphuric acid on reaction with water. When rain droplets mix with these gases; present in atmosphere because of pollution; they form acid rain.
Acid rain causes damage to the historical monuments and other buildings. For example Taj Mahal, which is made of marble, is getting damaged because of reaction with acid rain. Marble is calcium carbonate which reacts with the acid and thus gets corroded.
Acid & Base and Fire Extinguisher
Metal carbonate or metal hydrogen carbonate and acid are used in fire extinguisher to produce carbon dioxide gas. Acid and metal carbonate or bicarbonate are kept in separate chambers in a fire extinguisher. On emergency they are allowed to react with one another. The carbon dioxide gas so produce is poured over fire. As carbon dioxide does not support burning, it puts off the fire.
Common in Acid and Base
Acids give hydrogen gas when they react with metal. This shows that all acids contain hydrogen. For example: Hydrochloric acid (HCl), sulphuric acid (H2SO4), nitric acid (HNO3), etc.
When an acid is dissolved in water, it dissociates hydrogen. The dissociation of hydrogen ion in aqueous solution is the common property in all acids. Because of dissociation of hydrogen ion in aqueous solution, an acid shows acidic behavior.
Example: Hydrochloric acid (HCl) gives hydrogen ion (H+) and chloride ion (Cl−) when it is dissolved in water.
HCl (aq) ⇨ H+ + Cl−
Sulphuric acid (H2SO4) gives hydrogen ion (H+) and sulphate ion (SO4− −) in water.
H2SO4 (aq) ⇨ H+ + SO4− −
Nitric acid (HNO3) gives hydrogen ion (H+) and nitrate ion (NO3−) in water.
HNO3 (aq) ⇨ H+ + NO3−
Acetic acid (CH3COOH) gives acetate ion (CH3COO−) and hydrogen ion (H+).
CH3COOH (aq) ⇨ CH3COO− H+
Hydrogen ion which is produced by acid (when acid is combined with water molecule), exists in the form of hydronium ion (H3O−) in aqueous solution. That’s why hydrogen ion is always written with suffix (aq), such as H+ (aq).
HCl + H2O ⇨ H3O− + Cl−
H2SO4 + H2O ⇨ H3O− + SO4− −
Thus, because of dissociation of hydrogen ions; acid shows its acidic behavior.
Acids conduct electricity in their aqueous solution because of dissociation of hydrogen ion. Hydrogen ion in aqueous solution conducts electricity.
A dry acid, such as dry hydrochloric acid does not change the colour of blue litmus paper to red because a dry acid does not dissociate hydrogen ion. This is the cause that a moist litmus paper is used to check the acidic or basic character of a gas.
Acidic behavior of carbon dioxide gas: Carbon dioxide gas produces carbonic acid when dissolved in water. This carbonic acid dissociates hydrogen ion and carbonate ion in the aqueous solution.
CO2 + H2O ⇨ H2CO3 ⇨ H+ + CO3− −
Are all compounds which contain hydrogen, necessarily acids?
No, all compounds which contain hydrogen are not acid. For example; glucose (C6H12O6), methyl alcohol (CH3OH), etc. are not acid in spite of the fact that they contain hydrogen. This is because these compounds do not dissociate hydrogen ion in their aqueous solution.
Common in all Base:
A base dissociates hydroxide ion in water, which is responsible for the basic behavior of a compound. Example:
When sodium hydroxide is dissolved in water, it dissociates hydroxide ion and sodium ion.
NaOH (aq) ⇨ Na+ + OH−
Similarly, when potassium hydroxide is dissolved in water, it dissociates hydroxide ion and potassium ion.
KOH (aq) ⇨ K+ + OH−
Thus, base shows its basic character because of dissociation of hydroxide ion.
Neutralisation Reaction:
When an acid reacts with a base, the hydrogen ion of acid combines with the hydroxide ion of base and forms water. As these ions combine together and form water; instead of remaining free, thus both neutralize each other.
OH− + H+ ⇨ H2O
Example: When sodium hydroxide (a base) reacts with hydrochloric acid, sodium hydroxide breaks into sodium ion and hydroxide ion and hydrochloric acid breaks into hydrogen ion and chloride ion. Hydrogen ion and hydroxide ion combine together and form water, while sodium ion and chloride ion combine together and form sodium chloride.
NaOH + HCl ⇨ OH− + Na+ + H+ + Cl− ⇨ NaCl + H2O Dilution of Acid and Base:
The concentration of hydrogen ion in an acid and hydroxide ion in a base; per unit volume; shows the concentration of acid or base.
By mixing of acid to water, the concentration of hydrogen ion per unit volume decreases. Similarly, by addition of base to water the concentration of hydroxide ion per unit volume decreases. This process of addition of acid or base to water is called dilution and the acid or base is called diluted.
The dilution of acid or base is exothermic. Thus, acid or base is always added to water and water is never added to acid or base. If water is added to a concentrated acid or base a lot of heat is generated, which may cause splashing out of acid or base and may cause severe damage as concentrated acid and base are highly corrosive.
Strength of Acid and Base
Acids in which complete dissociation of hydrogen ion takes place are called strong acid. Similarly, bases in which complete dissociation of hydroxide ion takes place are called strong base.
In mineral acids, such as hydrochloric acid, sulphuric acid, nitric acid, etc. hydrogen ion dissociates completely and hence they are considered as strong acid. Since, in organic acids hydrogen ions do not dissociate completely, so they are weak acid.
Alkalis are water soluble base, thus in alkali; complete dissociation of hydroxide ions takes place and they are considered as strong base.
The complete dissociation of hydrogen ions or hydroxide ions is shown by a single arrow. The incomplete dissociation of hydrogen ions or hydroxide ions is denoted by double arrow.
Example of complete dissociation:
NaOH (aq) ⇨ Na+ + OH−
HCl ⇨ H+ + Cl−
Example of incomplete dissociation:
Mg(OH)2 ⇨ Mg+ + + OH− −
CH3COOH ⇨ CH3COO− + H+
Although acetic acid being an organic acid is a weak acid, but concentrated acetic acid is corrosive and can damage the skin if poured over it.
pH – Measurement of strength of Acid and Base
The strength of acid or base depends upon the hydrogen ion concentration. If the concentration of hydrogen ion is greater than hydroxide ion, the solution is called acidic. If the concentration of hydrogen ion is smaller than the hydroxide ion, the solution is called basic. If the concentration of hydrogen ion is equal to the concentration of hydroxide ion, the solution is called neutral solution.
pH is a scale which quantifies the concentration of hydrogen ion in a solution. The range of pH scale is between 0 to 14.
The pH value decreases with increase in hydrogen ion concentration. If the value of pH is 0, this shows maximum hydrogen ion concentration. pH value equal to 14 shows lowest hydrogen ion concentration. pH value equal to 7 shows the hydrogen ion concentration is equal to hydroxide ion concentration.
A neutral solution, such as distilled water has value of hydrogen ion concentration equal to 7 on pH scale. The acidic solution has value of hydrogen ion concentration less than 7 on pH scale. The basic solution has value of hydrogen ion concentration greater than 7 on pH scale.
In pH scale ‘p’ stands for ‘potenz’. Potenz is a German word which means ‘power’ or ‘potential’. Here; ‘H’ stands for hydrogen ion. Thus, pH means the potential of hydrogen or power of hydrogen.
pH is defined as the decimal logarithm of the reciprocal of the hydrogen ion activity (a(H+ )), in a solution.
Indicator:
Substances which show the acidic or basic behavior of other substance by change in colour are known as indicator.
Type of Indicator: There are many types of indicators. Some common types of indicators are
Natural
Olfactory Indicator
Synthetic Indicator
Universal Indicator
Natural Indicator: Indicators obtained from natural sources are called natural indicators. Litmus, turmeric, red cabbage, China rose, etc. are some common natural indicators used widely to show the acidic or basic character of substances.
Litmus: Litmus is obtained from lichens. The solution of litmus is purple in colour. Litmus paper comes in two colour – blue and red.
An acid turns blue litmus paper red.
A base turns red litmus paper blue.
Turmeric: Turmeric is another natural indicator. Turmeric is yellow in colour. Turmeric solution or paper turns reddish brown with base. Turmeric does not change colour with acid.
Red cabbage: The juice of red cabbage is originally purple in colour. Juice of red cabbage turns reddish with acid and turns greenish with base.
Olfactory Indicators: Substances which change their smell when mixed with acid or base are known as olfactory indicators. For example onion, vanilla, clove, etc.
Onion: Paste or juice of onion loses its smell when added with base. It does not change its smell with acid.
Vanilla: The smell of vanilla vanishes with base, but it’s smell does not vanishes with an acid.
Olfactory indicators are used to ensure the participation of visually impaired students in laboratory.
Synthetic Indicator: Indicators that are synthesized in laboratory are known as synthetic indicators. For example; phenolphthalein, methyl orange, etc.
Phenolphthalein is a colourless liquid. It remains colourless with acid but turns into pink with a base.
Methyl orange is originally orange in colour. It turns into red with acid and turns into yellow with base.
Indicator
Original colour
Acid
Base
Red litmus
Red
No change
Blue
Blue litmus
Blue
Red
No change
Turmeric
Yellow
No change
Reddish brown
Red cabbage juice
Purple
Reddish
Greenish yellow
Phenolphthalein
Colourless
Colourless
Pink
Methyl orange
Orange
Red
Yellow
Onion
n/a
No change
Smell vanishes
Vanilla
n/a
No change
Smell vanishes
Universal Indicator:
Using a litmus paper, phelophthalein, methyl orange, etc. only the acidic or basic character of a solution can be determined, but use of these indicators does not give the idea about the strength of acid or base. So, to get the strength as well as acidic and basic nature of a given solution universal indicator is used.
Universal indicator shows different colour over the range of pH value from 1 to 14 for a given solution. Universal indicator is available both in the form of strips and solution. Universal indicator is the combination of many indicators, such as water, propanol, phelophthalein, sodium salt, sodium hydroxide, methyl red, bromothymol blue monosodium salt, and thymol blue monosodium salt. The colour matching chart is supplied with universal indicator which shows the different colours for different values of pH.
Characteristics of salt: Most of the salts are crystalline solidSalts may be transparent or opaqueMost of the salts are soluble in waterSolution of salts conducts electricity. Salts conduct electricity in their molten state alsoThe salt may be salty, sour, sweet, bitter and umami (savoury)Neutral salts are odourlessSalts can be colourless or colouredSaltSalts are the ionic compounds which are produced after the neutralization reaction between acid and base. Salts are electrically neutral. There are number of salts but sodium chloride is the most common among them. Sodium chloride is also known as table salt or common salt. Sodium chloride is used to enhance the taste of food.Family of Salt:Salts having common acidic or basic radicals are said to belong to same family.Example: Sodium chloride (NaCl) and Calcium chloride (CaCl2) belong to chloride family.Calcium chloride (CaCl2) and calcium sulphate (CaSO4) belong to calcium family.Zinc chloride (ZnCl2) and Zinc sulphate (ZnSO4) belong to zinc family.Acidic, Basic and Neutral SaltsNeutral Salt: Salts produced because of reaction between strong acid and strong base are neutral in nature. The pH of value of such salts is equal to 7, i.e. neutral. Example; Sodium chloride, sodium sulphate, potassium chloride, etc. Sodium chloride (NaCl) is formed after the reaction between hydrochloric acid (a strong acid) and sodium hydroxide (a strong base).NaOH + HCl ⇨ NaCl + H2OSodium sulphate (Na2SO4): It is formed after the reaction between sodium hydroxide (a strong base) and sulphuric acid (a strong acid).2NaOH + H2SO4 ⇨ Na2SO4 + 2H2Potassium chloride (KCl): It is formed after the reaction between potassium hydroxide (a strong base) and hydrochloric acid (a strong acid).KOH + HCl ⇨ KCl + H2OAcidic salt:Salts which are formed after the reaction between a strong acid and weak base are called acidic salt. The pH value of acidic salt is lower than 7. Example: ammonium sulphate, ammonium chloride, etc.Ammonium chloride is formed after reaction between hydrochloric acid (a strong acid) and ammonium hydroxide (a weak base).NH4OH + HCl ⇨ NH4Cl + H2OAmmonium sulphate is formed after reaction between ammonium hydroxide (weak base) and sulphuric acid (a strong acid).2NH4OH + H2SO4 ⇨ (NH4 )2SO4 + 2H2OBasic Salt: Salts which are formed after the reaction between weak acid and strong base are called basic salt. For example; sodium carbonate, sodium acetate, etc.Sodium carbonate is formed after the reaction between sodium hydroxide (a strong base) and carbonic acid (a weak base).H2CO3 + 2NaOH ⇨ Na2CO3 + 2H2OSodium acetate is formed after the reaction between a strong base, sodium hydroxide and a weak acid, acetic acid.CH3COOH + NaOH ⇨ CH3COONa + H2O Cause of formation of acidic, basic and neutral salt:When a strong acid reacts with a weak base, the base is unable to fully neutralize the acid. Due to this an acidic salt is formed in this case.When a strong base reacts with a weak acid, the acid is unable to fully neutralize the base. Due to this a basic salt is formed in this case.When equally strong acid and base react they fully neutralize each other. Due to this a neutral salt is formed in this case.pH value of salt:Neutral salt: The pH value of a neutral salt is almost equal to 7.Acidic salt: The pH value of an acidic salt is less than 7.Basic salt: The pH value of a basic salt is more than 7.Common Salt (Sodium Chloride)Sodium chloride (NaCl) is also known as common or table salt. It is formed after the reaction between sodium hydroxide and hydrochloric acid. It is a neutral salt. The pH value of sodium chloride is about 7. Sodium chloride is used to enhance the taste of food. Sodium chloride is used in manufacturing of many chemicals.Important chemicals from sodium chloride:Sodium Hydroxide (NaOH): Sodium hydroxide is a strong base. It is also known as caustic soda or Iye. It is obtained by the electrolytic decomposition of solution of sodium chloride (brine). In the process of electrolytic decomposition of brine (aqueous solution of sodium chloride), brine decomposes to form sodium hydroxide. In this process, chlorine is obtained at anode and hydrogen gas is obtained at cathode as byproducts. This whole process is known as Chlor-Alkali process.2NaCl + 2H2O ⇨ 2NaOH + Cl2 + H2Use of products after the electrolysis of brine:Hydrogen gas is used as fuel, margarine, in making of ammonia for fertilizer, etc.Chlorine gas is used in water treatment, manufacturing of PVC, disinfectants, CFC, pesticides. It is also used in manufacturing of bleaching powder and hydrochloric acid.Sodium hydroxide is used for de-greasing of metals, manufacturing of paper, soap, detergents, artificial fibres, bleach, etc.Bleaching Powder (CaOCl2 ):Bleaching powder is also known as chloride of lime. It is a solid and yellowish white in colour. Bleaching powder can be easily identified by the strong smell of chlorine.When calcium hydroxide (slaked lime) reacts with chlorine, it gives calcium oxychloride (bleaching powder) and water is formed.Ca(OH)2 + Cl2 ⇨ CaOCl2 + H2OAqueous solution of bleaching powder is basic in nature. The term bleach means removal of colour. Bleaching powder is often used as bleaching agent. It works because of oxidation. Chlorine in the bleaching powder is responsible for bleaching effect.Use of bleaching powder:Bleaching powder is used as disinfectant to clean water, moss remover, weed killers, etc.Bleaching powder is used for bleaching of cotton in textile industry, bleaching of wood pulp in paper industry.Bleaching powder is used as oxidizing agent in many industries, such as textiles industry, paper industry, etc.Baking Soda (NaHCO3 )Baking soda is another important product which can be obtained using byproducts of chlor-alkali process. The chemical name of baking soda is sodium hydrogen carbonate (NaHCO3 ) or sodium bicarbonate. Bread soda, cooking soda, bicarbonate of soda, sodium bicarb, bicarb of soda or simply bicarb, etc. are some other names of baking soda.Baking soda is obtained by the reaction of brine with carbon dioxide and ammonia. This is known as Solvay process.NaCl + CO2 + NH3 + H2O ⇨ NH4Cl + NaHCO3In this process, calcium carbonate is used as the source of CO2 and the resultant calcium oxide is used to recover ammonia from ammonium chloride.Properties of sodium bicarbonate:Sodium bicarbonate is white crystalline solid, but it appears as fine powder.Sodium hydrogen carbonate is amphoteric in nature.Sodium hydrogen carbonate is sparingly soluble in water.Thermal decomposition of sodium hydrogen carbonate (baking soda).When baking soda is heated, it decomposes into sodium carbonate, carbon dioxide and water.2NaHCO3 + heat ⇨ Na2CO3 + CO2 + H2OSodium carbonate formed after thermal decomposition of sodium hydrogen carbonate; decomposes into sodium oxide and carbon dioxide on further heating.Na2CO3 ⇨ Na2O + CO2This reaction is known as dehydration reaction.Use of Baking Soda:Baking soda is used in making of baking powder, which is used in cooking as it produces carbon dioxide which makes the batter soft and spongy.Baking soda is used as antacid.Baking soda is used in toothpaste which makes the teeth white and plaque free.Baking soda is used in cleansing of ornaments made of sliver.Since, sodium hydrogen carbonate gives carbon dioxide and sodium oxide on strong heating, thus it is used as fire extinguisher.Baking powder:Baking powder produces carbon dioxide on heating, so it is used in cooking to make the batter spongy. Although baking soda also produces carbon dioxide on heating, but it is not used in cooking because on heating; baking soda produces sodium carbonate along with carbon dioxide. The sodium carbonate; thus produced; makes the taste bitter.2NaHCO3 + heat ⇨ Na2CO3 + CO2 + H2OBaking powder is the mixture of baking soda and a mild edible acid. Generally, tartaric acid is mixed with baking soda to make baking powder.NaHCO3 + C4H6O6 ⇨ CO2 + H2O + Na2C4H4O6When baking powder (mixture of baking soda and an edible acid) is heated, the sodium carbonate formed because of heating of baking soda neutralizes after reacting with tartaric acid and sodium tartarate salt is formed. The smell of sodium tartarate is pleasant and taste is good. This makes the cake or any other food tasty.Washing Soda (Sodium carbonate)Sodium carbonate is manufactured by the thermal decomposition of sodium hydrogen carbonate obtained by Solvay process.NaCl + CO2 + NH3 + H2O ⇨ NH4Cl + NaHCO3 NaHCO3 + C4H6O6 ⇨ CO2 + H2O + Na2C4H4O6The sodium carbonate obtained in this process is dry. It is called soda ash or anhydrous sodium carbonate. Washing soda is obtained by rehydration of anhydrous sodium carbonate.Na2CO3 + 10H2O ⇨ Na2CO3.10H2OSince there are 10 water molecules in washing soda, hence it is known as Sodium bicarbonate decahydrate.Sodium carbonate is a crystalline solid and it is soluble in water when most of the carbonates are insoluble in water. Use of sodium carbonate:It is used in cleaning of cloths; especially in rural areas.In making of detergent cake and powder.In removing permanent hardness of water.It is used in glass and paper industries.Water of crystallization: Many salts contain water molecule and are known as hydrated salts. The water molecule present in salt is known as water of crystallization. Examples:Copper sulphate pentahydrate (CuSO4.5H2O):Blue colour of copper sulphate is due to presence of 5 molecules of water. When copper sulphate is heated, it loses water molecules and turns into grey-white colour, which is known as anhydrous copper sulphate. After adding water; anhydrous copper sulphate becomes blue again.CuSO4.5H2O + heat ⇨ CuSO4Ferrous Sulphate heptahydrate (FeSO4.7H2O): The green colour of Ferrous sulphate heptahydrate; commonly known as ferrous sulphate; is due to the presence of 7 molecules of water in it.Plaster of Paris:Plaster of Paris is obtained by heating of gypsum, a hydrated salt of calcium.CaSO4.2H2O + Heat ⇨ CaSO4.(0.5)H2O + (1.5)H2OAfter addition of water Plaster of Paris is again converted into gypsum.CaSO4.(0.5)H2O + (1.5)H2O ⇨ CaSO4.2H2OPlaster of Paris is used in making of toys, designer false ceiling, etc. Doctors use Plaster of Paris to set the fractured bone.
In Text Solution(NCERT) - Part - I
You have been provided with three test tubes. One of them contains distilled water and the other two contain an acidic solution and a basic solution, respectively. If you are given only red litmus paper, how will you identify the contents of each test tube? Answer: The acidic and basic nature of the solution can be identified by using litmus paper. A basic solution turns red litmus to blue and an acidic solution turns the blue litmus to red. The solution which changes the red litmus to blue is basic solution. Now, between two solutions, which turn the litmus which turned blue because of basic solution, to red, is acidic solution. The last one solution is distilled water.
Why should curd and sour substances not be kept in brass and copper vessels? Answer: Curd and other sour substances reacts with brass and copper vessels and forms hydrogen and other harmful substances due to presence of acid in them. That’s why curd and other sour substances should not be kept in brass and copper vessels.
Which gas is usually liberated when an acid reacts with a metal? Illustrate with an example. How will you test for the presence of this gas? Answer: When acid reacts with metal, it liberates hydrogen gas usually. This can be shown by following activity:Fig: Reaction of metal and acid
Take some zinc granules in the test tube and pour sulphuric acid in it.
Allow the gas pass through the solution of soap.
When gas starts coming out from the soap solution bubbles are formed.
Bring a burning candle near the bubbles of gas it burns with a pop sound.
Burning of gas with pop sound proves that it is hydrogen gas.
Reaction involved in this process can be written as follows:
Zn + H2SO4 ⇨ ZnSO4 + H2
Metal compound A reacts with dilute hydrochloric acid to produce effervescence. The gas evolved extinguishes a burning candle. Write a balanced chemical equation for the reaction if one of the compounds formed is calcium chloride. Answer: When calcium carbonate reacts with hydrochloric acid, it produces carbon dioxide gas with effervescence. Carbon dioxide used as fire extinguisher. Hence, it extinguishes a burning candle. Therefore, the metal compound A is calcium carbonate.
CaCO3 + 2HCl ⇨ CaCl2 + CO2 + H2O
Why do HCl, HNO3, etc., show acidic characters in aqueous solutions while solutions of compounds like alcohol and glucose do not show acidic character? Answer: HCl, HNO3, etc. form hydronium ions in aqueous solutions while alcohol and glucose do not dissociates hydronium ion in the solution instead they contain hydrogen. Hence, HCl, HNO3, etc show acidic characters in aqueous solution while solutions of compounds like alcohol and glucose do not.
Why does an aqueous solution of an acid conduct electricity? Answer: Acid dissociates ions in aqueous solution, which conducts electricity.
Why does dry HCl gas not change the colour of the dry litmus paper? Answer: Because dry HCl gas does not contain hydronium ions. Hence, it does not changes the colour of dry litmus paper.
While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid? Answer: When acid is mixed with water it evolves heat. The reaction takes place in this process is exothermic. If water is mixed with acid, then because of presence of large amount of acid, more heat would be evolved, which may splashes out and can be resulted into accident due to burning. Therefore, it is recommended that the acid should be added to water and not water to the acid.
In Text Solution(NCERT Book) - Part - II
How is the concentration of hydronium ions (H3O+) affected when a solution of an acid is diluted? Answer: The concentration of hydronium ions (H3O+) decreases when a solution of an acid is diluted.
How is the concentration of hydroxide ions (OH−) affected when excess base is dissolved in a solution of sodium hydroxide? Answer: The concentration of hydroxide ions increase when excess base is dissolved in a solution of sodium hydroxide.
You have two solutions, A and B. The pH of solution A is 6 and pH of solution B is 8. Which solution has more hydrogen ion concentration? Which of this is acidic and which one is basic? Answer: An acidic solution has pH value less than 7 and a basic solution has pH value more than 7. Hence, the solution A having pH value is 6 is acidic consequently has more hydrogen ion concentration. The solution B which has pH value is 8 is basic solution.
What effect does the concentration of H+(aq) ions have on the nature of the solution? Answer: The concentration of H+ (aq) ions determines the acidic nature of solution. Hence, acidity of a solution increases with increases in concentration of H+ (aq) ions and vice versa.
Do basic solutions also have H+(aq) ions? If yes, then why are these basic? Answer: Yes basic solutions also have H+(aq) ions. They are basic because the concentration of hydroxide (OH−) is more than the H+(aq) ions.
Under what soil condition do you think a farmer would treat the soil of his fields with quick lime (calcium oxide) or slaked lime (calcium hydroxide) or chalk (calcium carbonate)? Answer: If the soil is acidic then it should be treated with quick lime or slaked lime to make it neutralize so that soil can be made proper for cultivation.
What is the common name of the compound CaOCl2? Answer: Common name of CaOCl2 is bleaching powder.
Name the substance which on treatment with chlorine yields bleaching powder. Answer: Calcium hydroxide [Ca(OH)2] yields bleaching powder when treated with chlorine.
Name the sodium compound which is used for softening hard water. Answer: Sodium carbonate (Washing soda) is used for softening the hard water.
What will happen if a solution of sodium hydrocarbonate is heated? Give the equation of the reaction involved. Answer: When solution of sodium hydrocarbonate is heated, sodium carbonate, water and carbon dioxide gas is formed.
2NaHCO3 ⇨ Na2CO3 + CO2 + H2O
Write an equation to show the reaction between Plaster of Paris and water. Answer:
CaSO4.(0.5)H2O + (1.5)H2O ⇨ CaSO4.2H2O
NCERT Exercise Solution - Part I
Write word equations and then balanced equations for the reaction taking place when
Dilute hydrochloric acid reacts with iron filings. Answer: Sulphuric acid + Iron → Ferric chloride + Hydrogen
2Fe + 6HCl ⇨ 2FeCl3 + 3H2
A solution turns red litmus blue, its pH is likely to be
1
4
5
10
Answer: (d) 10
A solution reacts with crushed egg-shells to give a gas that turns lime-water milky. The solution contains
NaCl
HCl
LiCl
KCl
Answer: (b) HCl
10 mL of a solution of NaOH is found to be completely neutralised by 8 mL of a given solution of HCl. If we take 20 mL of the same solution of NaOH, the amount HCl solution (the same solution as before) required to neutralise it will be
4 mL
8 mL
12 mL
16 mL
Answer: (d) 16 mL
Which one of the following types of medicines is used for treating indigestion?
Antibiotic
Analgesis
Antacid
Antiseptic
Answer: (c) Antacid
Compounds such as alcohols and glucose also contain hydrogen but are not categorised as acids. Describe an Activity to prove it. Answer: Following activity can be done for this:
Take solutions of glucose, alcohol, hydrochloric acid, sulphuric acid, etc.
Fix two nails on a cork, and place the cork in a 100 mL beaker.
Connect the nails to the two terminals of a 6 volt battery through a bulb and a switch, as shown in figure.
Now pour some glucose solution in the beaker and switch on the current.
Repeat with alcohol solution and hydrochloric acid.
It is observed that bulb does not glow using glucose or alcohol solution. While if a solution of acid is used instead of glucose or alcohol solution, bulb glows.
Bulb glows when the solution of hydrochloric acid is used, because it dissociates hydronium ion in aqueous solution. But glucose and alcohol do not dissociate hydronium ion in the water, which is responsible for the conduction of electricity. Hence solution of glucose and alcohol do not conduct electricity.
Why does distilled water not conduct electricity, whereas rain water does? Answer: Rain water contains acids and many other substances as impurities, which produce hydronium ion, while distilled water does not contain any ionic substance that can dissociate hydronium ion. That’s why rain water conduct electricity and distilled water does not.
Why do acids not show acidic behaviour in the absence of water? Hydronium ions are responsible for behavior as acid. Acids do not dissociate hydronium ion in the absence of water, hence they do not show acidic behavior in absence of water.
Five solutions A,B,C,D and E when tested with universal indicator showed pH as 4, 1, 11, 7 and 9, respectively. Which solution is
neutral? Answer: D is neutral, as its pH value is equal to 7
strongly alkaline? Answer: C is strongly alkaline, as its pH value is 11.
strongly acidic? Answer: B is strongly acidic, as its pH value is equal to 1
weakly acidic? Answer: A is weakly acidic, as its pH value is equal to 4 and more than solution B.
weakly alkaline? Answer: E is weakly alkaline, as its pH value is equal to 9 and less than C.
pH value of the given solution in Increasing order of hydrogen-ion concentration: 11 < 9 < 7 < 4 < 1
Equal lengths of magnesium ribbons are taken in test tubes A and B. Hydrochloric acid (HCl) is added to test tube A, while acetic acid (CH3COOH) is added to test tube B. Amount and concentration taken for both the acids are same. In which test tube will the fizzing occur more vigorously and why? Answer: In test tube A fizzing occurs more vigorously. Because HCl is stronger acid than acetic acid. Hence, HCl liberates hydrogen gas more vigorously, which causes fizzing more vigorously.
Fresh milk has a pH of 6. How do you think the pH will change as it turns into curd? Explain your answer. Answer: The value of pH of milk decreases, when it turns into curd. Curd is acidic in nature. Because an acidic substance has less value of pH.
A milkman adds a very small amount of baking soda to fresh milk.
Why does he shift the pH of the fresh milk from 6 to slightly alkaline? Answer: By making the milk more alkaline, it is prevent for more time to turn into curd. That’s why milkman shifts the pH of fresh milk to slightly alkaline by adding a very small amount of baking soda to it.
Why does this milk take a long time to set as curd? Answer: As this milk is slightly more alkaline than other, therefore, acid produced to set into curd will be neutralized by baking soda added by milkman. Hence, this milk takes a longer time to set as curd.
Plaster of Paris should be stored in a moisture-proof container. Explain why? Answer: Because plaster of Paris turns into Gypsum after reacting with moisture present in air. That’s why Plaster of Paris should be stored in a moisture-proof container.
What is a neutralisation reaction? Give two examples. Answer: When acid is added to base, it produces salt and water after neutralizing each other, such reactions are known as neutralization reaction.
Example – 1: When Sodium hydroxide is added to hydrochloric acid. Sodium chloride and water is formed.
NaOH + HCl ⇨ NaCl + H2O
Example - 2: Milk of magnesia, which is a base, is given as antacid in the case of indigestion, to neutralize the more acid produced in stomach.
Mg(OH)2 + 2HCl ⇨ MgCl2 + 2H2O
Give two important uses of washing soda and baking soda. Answer: Two important uses of washing soda and baking soda are as follows:
Washing soda is used to remove the permanent hardness of water.
Washing soda is used in making soap.
Baking soda is used as baking powder, in the making of bread, and other many items.
Chemical Reaction: The transformation of chemical substance into another chemical substance is known as Chemical Reaction. For example: rusting of iron, setting of milk into curd, digestion of food, respiration, etc. In chemical reaction new substance is formed which is completely different in properties from the original substance, so in a chemical reaction chemical change takes place. Signs of Chemical Reaction Change of state of substance Change of colour of substance Evolution of heat Absorption of heat Evolution of gas Evolution of light Evolution of sound, etc. Reactant and Product: Reactant: Substances which take part in a chemical reaction are called reactants. Product: New substance formed after a chemical reaction is called product. Chemical Equation: Representation of chemical reaction using symbols of substances is called chemical equation. Example: A + B ⇨ C + D In this equation, A and B are called reactants and C and D are called the products. Arrow shows the direction of chemical reaction. Condition, if any, is written generally above the arrow. When hydrogen reacts with oxygen, it gives water. This reaction can be represented by following chemical equation. Hydrogen + Oxygen ⇨ Water H2 + O2 ⇨ H2O In first equation words are used and in second symbols of substances are used to write the chemical equation. For convenience, symbol of substance is used to represent chemical equations. Chemical Equation is a way to represent the chemical reaction in concise and informative way. Chemical equation can be divided into two types – Balanced Chemical Equation and Unbalanced Chemical Equation. Balanced Chemical Equation: A balanced chemical equation has number atoms of each element equal on both sides. Example: Zn + H2SO4 ⇨ ZnSO4 + H2 In this equation, numbers of zinc, hydrogen and sulphate are equal on both sides, so it is a balanced chemical equation. According to Law of Conservation of Mass, mass can neither be created nor destroyed in a chemical reaction. To obey this law, the total mass of elements present in reactants must be equal to the total mass of elements present in products. Unbalanced Chemical Equation: If the number of atoms of each element in reactants is not equal to the number of atoms of each element present in product, then the chemical equation is called unbalanced chemical equation. Example: Fe + H2O ⇨ Fe3O4 + H2 In this example number atoms of elements are not equal on two sides of the reaction. For example, on the left hand side only one iron atom is present, while three iron atoms are present on the right hand side. Therefore, it is an unbalanced chemical equation.Balancing a chemical equation: To balance the given or any chemical equation, follow these steps: Fe + H2O ⇨ Fe3O4 + H2 Write the number of atoms of elements present in reactants and in products in a table; as shown here. Name of atomNo. of atoms in reactantNo. of atoms in product Iron 1 3 Hydrogen 2 2 Oxygen 1 4 Balance the atom which is the maximum in number; on either side of chemical equation. In this equation, the number of oxygen atom is the maximum on the RHS. To balance the oxygen one needs to multiply the oxygen on the LHS by 4; so that the number of oxygen atoms becomes equal on both sides. Fe + 4 x H2O ⇨ Fe3O4 + H2 Now, the number of hydrogen atoms becomes 8 on the LHS; which is more than that on the RHS. To balance it, one needs to multiply the hydrogen on the RHS by 4. Fe + 4 x H2O ⇨ Fe3O4 + 4 x H2 After that number of oxygen and hydrogen atoms becomes equal on both sides. The number of iron is one on the LHS, while it is three on the RHS. To balance it, multiply the iron on the LHS by 3. 3 x Fe + 4 x H2O ⇨ Fe3O4 + 4 x H2 Now the number of atoms of each element becomes equal on both sides. Thus, this equation becomes a balanced equation. Name of atomNo. of atoms in reactantNo. of atoms in product Iron33 Hydrogen88 Oxygen44 After balancing, the above equation can be written as follows. 3Fe + 4H2O ⇨ Fe3O4 + 4H2 Balance the given equations. H2 + O2 ⇨ H2O Answer: 2H2 + O2 ⇨ 2H2O Fe + H2O ⇨ Fe2O3 + H2 Answer: 2Fe + 3H2O ⇨ Fe2O3 + 3H2 CO2 + H2O ⇨ C6H12O6 + O2 Answer: 6CO2 + 6H2O ⇨ C6H12O6 + 6O2 Fe2O3 + C ⇨ Fe + CO2 Answer: 2Fe2O3 + 3C ⇨ 4Fe + 3CO2 Writing the symbols of Physical States of substances in Chemical equation: By writing the physical states of substances a chemical equation becomes more informative. Gaseous state is represented by symbol ‘g’ Liquid state is represented by symbol ‘l’ Solid state is written by symbol ‘s’ Aqueous solution is written by symbol ‘aq’ Writing the condition in which reaction takes place: The condition is generally written above and/or below the arrow of a chemical equation. Thus, by writing the symbols of physical state of substances and condition under which reaction takes place, a chemical equation can be made more informative. Double Displacement Reaction:Reactions in which ions are exchanged between two reactants forming new compounds are called double displacement reactions. AB + CD ⇨ AC + BD Example: When solution of barium chloride reacts with the solution of sodium sulphate, white precipitate of barium sulphate is formed along with sodium chloride. BaCl2 + Na2SO4 ⇨ BaSO4 + 2NaCl When sodium hydroxide (a base) reacts with hydrochloric acid, sodium chloride and water are formed. NaOH + HCl ⇨ NaCl + H2O Double displacement reaction, in which precipitate is formed, is also known as precipitation reaction. Neutralisation reactions are also examples of double displacement reaction. Exothermic and Endothermic Reaction Reactions which produce energy are called exothermic reaction. On the other hand, reactions which absorb energy are called endothermic reaction. Most of the combination reactions are endothermic. Most of the decomposition reactions are exothermic. Respiration is a decomposition reaction in which energy is released. When quick lime (calcium carbonate) is added to water, it decomposes and releases energy. Cooking involves chemical reactions which are endothermic as cooking is possible because of heating. Oxidation and Reduction Reaction: Oxidation: Addition of oxygen or non-metallic element or removal of hydrogen or metallic element from a compound is known as oxidation. Elements or compounds in which oxygen or non-metallic element is added or hydrogen or metallic element is removed are called to be oxidized. Oxidizing agent: Compounds which can add oxygen or a non-metallic compound or remove hydrogen or metallic element are known as oxidizing agents. Reduction: Addition of hydrogen or metallic element or removal of oxygen or non-metallic element from a compound is called reduction. The compound or element which goes under reduction is called to be reduced. Reducing agent: Compounds or elements which can cause reduction are called reducing agents. In a chemical reaction oxidation and reduction both take place simultaneously and such reactions are also known as REDOX REACTIONS. In the word REDOX, ‘Red’ stands for reduction and ‘Ox’ stands for oxidation. Example: When iron reacts with air, it forms iron oxide (rust) 4Fe + 3O2 ⇨ 2Fe2O3 In this reaction, oxygen is added to iron, thus, iron is oxidized. Here oxygen is oxidizing agent. When cupric oxide reacts with hydrogen, it gives copper and water. CuO + H2 ⇨ Cu + H2O In this reaction, oxygen is removed from copper and oxygen is added to hydrogen. So, cupric oxide is reduced to copper and hydrogen is oxidized to water. Cupric oxide is oxidizing agent and hydrogen is reducing agent. When sodium hydroxide reacts with hydrochloric acid, it gives sodium chloride and water. NaOH + HCl ⇨ NaCl + H2O In this reaction, sodium hydroxide is reduced to sodium chloride since hydrogen is removed from sodium hydroxide. Hydrochloric acid is oxidized to water, since oxygen is added to hydrogen chloride and non-metallic element chloride is removed. Sodium hydroxide is oxidising agent and hydrochloric acid is reducing agent. In this reaction oxidation and reduction both takes place simultaneously, thus it is an example of redox reaction. Significance of Oxidation Reduction in Everyday Life: Respiration is oxidation reaction in which food is oxidized to produce energy. Iron gets oxidized to form rust; which leads to corrosion of iron in the long run. Most of the metals react with atmospheric oxygen and it leads to formation of a layer on the metal article. The metal gets corroded in the long run. Rusting of iron can be prevented by painting the iron article. This can also be prevented by applying a layer of zinc over iron article. This process is known as galvanization. Fried food gets oxidized when exposed to air. This spoils the taste of the food and the food becomes unfit for consumption. The spoiling of fried food because of oxidation is called rancidity. Fried food is often packed in airtight packets to prevent rancidity. We are able to utilize various types of fuel because of oxidation. Oxidation of fuel helps in producing energy. Combination Reaction: Reactions in which two or more reactants combine to form one product are called COMBINATION REACTION. A general combination reaction can be represented by the chemical equation given here. A + B ⇨ AB Example: When magnesium is burnt in air (oxygen), magnesium oxide is formed. In this reaction, magnesium is combined with oxygen. Mg + O2 ⇨ 2MgO Magnesium + Oxygen ⇨ Magnesium oxide When carbon is burnt in oxygen (air), carbon dioxide is formed. In this reaction, carbon is combined with oxygen. C + O2 ⇨ CO2 Carbon + Oxygen ⇨ Carbon dioxide When hydrogen reacts with chlorine, hydrogen chloride is formed. H2 + Cl2 ⇨ 2HCl Hydrogen + Chlorine ⇨ Hydrogen chloride When calcium oxide reacts with water, calcium hydroxide is formed CaO + H2O ⇨ Ca(OH)2 Calcium oxide + Water ⇨ Calcium hydroxide When carbon monoxide reacts with oxygen, carbon dioxide is formed. 2CO + O2 ⇨ 2CO2 Carbon monoxide + Oxygen ⇨ Carbon dioxide Decomposition Reaction: Reactions in which one compound decomposes in two or more compounds or element are known as DECOMPOSITION REACTION. Decomposition reaction is just opposite of combination reaction. A general decomposition reaction can be represented as follows: AB ⇨ A + B Example: When calcium carbonate is heated, it decomposes into calcium oxide and carbon dioxide CaCO3 ⇨ CaO + CO2 Calcium carbonate ⇨ Calcium oxide + Carbon dioxide When ferric hydroxide is heated, it decomposes into ferric oxide and water 2Fe(OH)3 ⇨ Fe2O3 + 3H2O Ferric hydroxide ⇨ Ferric oxide + Water When lead nitrate is heated, it decomposes into lead oxide, nitrogen dioxide and oxygen. 2Pb(NO3)2 ⇨ 2PbO + 4NO2 + O2 Lead nitrate ⇨ Lead oxide + Nitrogen oxide + Oxygen In above examples, compound is decomposed because of heating, so, these reactions are called THERMAL DECOMPOSITION REACTION. Electrolytic Decomposition: Reactions in which compounds decompose into simpler compounds because of passing of electricity, are known as ELECTROLYTIC DECOMPOSITION. This is also known as ELECTROLYSIS. Example: When electricity is passed in water, it decomposes into hydrogen and oxygen. 2H2O ⇨ 2H2 + O2 Photolysis or Photo Decomposition Reaction: Reactions in which a compound decomposes because of sunlight are known as PHOTOLYSIS or PHOTO DECOMPOSITION REACTION. Example: When silver chloride is put in sunlight, it decomposes into silver metal and chlorine gas. 2AgCl ⇨ 2Ag + Cl2 Similarly, when silver bromide is put under sunlight, it decomposes into silver metal and bromine gas. 2AgBr ⇨ 2Ag + Br2 Photographic paper has coat of silver chloride, which turns into grey when exposed to sunlight. It happens because silver chloride is colourless while silver is a grey metal. Displacement Reaction Reactions in which atoms or ions move from one compound to other to form new compound are known as DISPLACEMENT REACTION. Displacement reaction is also known as Substitution Reaction or Single displacement /Replacement Reaction. A general displacement reaction can be represented using chemical equation as follows: A + BC ⇨ AC + B Displacement reaction takes place only when ‘A’ is more reactive than B. If ‘B’ is more reactive than ‘A’, then ‘A’ will not displace ‘C’ from ‘BC’ and reaction will not be taken place. Example: When zinc reacts with hydrochloric acid, it gives hydrogen gas and zinc chloride. Zn + 2HCl ⇨ ZnCl2 + H2 When zinc reacts with copper sulphate, it forms zinc sulphate and copper metal. Zn + CuSO4 ⇨ ZnSO4 + Cu When silver metal is dipped in copper nitrate, no reaction takes place because silver metal is less reactive than copper. Ag + Cu(NO3 )2 ⇨ No reaction takes place Solutions to In Text Questions NCERT Book Write the balanced equation for the following chemical reactions. Hydrogen + Chlorine ⇨ Hydrogen Chloride Answer: H2 + Cl2 ⇨ 2HCl Barium chloride + Aluminium sulphate ⇨ Barium sulphate + Aluminium chloride Answer: 3BaCl+ Al2(SO4 )3 ⇨ 3BaSO4 + 2AlCl3 Sodium + Water ⇨ Sodium hydroxide + Hydrogen Answer: 2Na + 2H2O ⇨ 2NaOH + H2 Write a balanced chemical equation with state symbols for the following reactions. Solutions of barium chloride and sodium sulphate in water react to give insoluble barium sulphate and the solution of sodium chloride. Answer: BaCl2 (aq) + Na2SO4 ⇨ BaSO4 (s) + 2NaCl (aq) Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride solution and water. Answer: NaOH (aq) + HCl ⇨ NaCl (aq) + H2O (l) A solution of a substance ‘X’ is used for white washing. Name the substance ‘X’ and write its formula. Answer: The chemical name of substance, which is used for white washing is Calcium oxide. Its common name is lime or quick lime. The chemical formula of Calcium Oxide is CaO. Write the reaction of the substance ‘X’ named in (i) above with water. Answer: CaO + H2O ⇨ Ca(OH)2 When calcium oxide reacts with water it forms calcium hydroxide. Calcium hydroxide is also known as slaked lime. Why is the amount of gas collected in one of the test tubes in Activity 1.7 double of the amount collected in the other? Name this gas. Answer: (Hydrolysis of water has been performed in the activity 1.7 in the text book.) When electric current is passed through water it decomposes into its constituent components, i.e. hydrogen and oxygen gases. During electrolysis of water hydrogen gas collected in one test tube and has double amount than oxygen collected in another test tube. This is because a water molecule is formed by the combination of hydrogen and oxygen in the ratio of 2:1. Answer the following questions: Why does the colour of copper sulphate solution change when an iron nail is dipped in it? Answer: The colour of copper sulphate solution is blue. When an iron nail is dipped in the solution of copper sulphate, it forms ferrous sulphate solution and copper metal by the displacement of copper from the solution. The colour of ferrous sulphate is green. That’s why the solution becomes green. Give an example of a double displacement reaction other than the one given in Activity 1.10. Answer: When lead nitrate reacts with potassium iodide, potassium nitrate and lead iodide is formed. In this reaction the both the reactants exchanges their irons after reaction. This type of reaction is known as double displacement reaction. Pb(NO3 )2 (aq) + KI (aq) ⇨ KNO3 + PbI2 Identify the substances that are oxidised and the substances that are reduced in the following reactions. 4Na (s) + O2 (g) ⇨ 2NaO (s) Answer: In this reaction sodium is combined with oxygen, i.e. gains oxygen and hence oxidized. Consequently oxygen is reduced. CuO (s) + H2 (g) ⇨ Cu (s) + H2O (l) Answer: In this reaction Hydrogen gains oxygen and forms water, hence it is oxidised. On the other hand copper loses oxygen and hence, reduced.
Solutions to NCERT Questions What is a balanced chemical equation? Why should chemical equations be balanced? Answer: When the number of atoms of reactants is equal to the number of atoms of the products, the reaction is called a balanced chemical equation. According to the Law of conservation of mass, total mass of the elements present in the reactants must be equal to the total mass of the elements present in products. That’s why a chemical equations should be balanced always. Answer the following questions: Which of the statements about the reaction below are incorrect? 2PbO (s) + C (s) ⇨ 2PbS (s) + CO2 (g) Lead is getting reduced. Carbon dioxide is getting oxidised. Carbon is getting oxidised. Lead oxide is getting reduced. a and b a and c a, b and c All Answer: (1) a and b The following reaction is an example of: Fe2O3 + 2Al ⇨ Al2O3 + 2Fe Combination reaction Double displacement reaction Decomposition reaction Displacement reaction Answer: (d) Displacement reaction What happens when dilute hydrochloric acid is added to iron fillings? Tick the correct answer. Hydrogen gas and iron chloride are produced Chlorine gas and iron hydroxide are produced No reaction takes place Iron salt and water are produced Answer: (a) Hydrogen gas and iron chloride are produced Translate the following statements into chemical equations and then balance them. Hydrogen gas combines with nitrogen to form ammonia. Hydrogen sulphide gas burns in air to give water and sulpur dioxide. Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate. Potassium metal reacts with water to give potassium hydroxide and hydrogen gas. Answer: (a) 3H2 + N2 ⇨ 2NH3 (b) 2H2S + 3O2 ⇨ 2H2O + 2SO2 (c) 3BaCl2 + Al2(SO4 )3 ⇨ 2AlCl3 + 3BaSO4 (d) 3K + 2H2O ⇨ 2KOH + H2 Write the balanced chemical equation for the following and identify the type of reaction in each case. Potassium bromide(aq) + Barium iodide(aq) → Potassium iodide(aq) + Barium bromide(s) Zinc carbonate(s) → Zinc oxide(s) + Carbon dioxide(g) Hydrogen(g) + Chlorine(g) → Hydrogen chloride(g) Magnesium(s) + Hydrochloric acid(aq) → Magnesium chloride(aq) + Hydrogen(g) Answer: (a) 2KBr (aq) + BaI2 (aq) ⇨ 2KI (aq) + BaBr2 (s) It is a double displacement reaction. (b) ZnCO3 (s) ⇨ ZnO (s) + CO2 (g) It is a decomposition reaction. (c) H2 (g) + Cl2 (g) ⇨ 2HCl It is a combination reaction. (d) Mg (s) + 2HCl (aq) ⇨ MgCl2 (aq) + H2 (g) It is a displacement reaction. What does one mean by exothermic and endothermic reactions? Give examples. Answer: Exothermic reaction: A reaction in which heat is released is called the exothermic reaction. Example: Burning of fuel is an example of exothermic reaction. When methane is burnt it gives heat along with carbon dioxide and water. CH4 (g) + 2O2 (g)⇨ CO2 (g) + 2H2O Endothermic reaction: A reaction in which heat is absorbed is called endothermic reaction. Example: When silver chloride is left in the sunlight, it absorbs heat and turns grey because of formation of silver metal. 2AgCl (s) ⇨ 2Ag (s) + Cl2 (g) Why is respiration considered an exothermic reaction? Explain. Answer: In the course of respiration glucose is broken into carbon dioxide and energy is released. That’s why it is considered as an exothermic reaction. C6H12O6 (aq) + 6O2 (g) ⇨ 6CO2 (aq) + 6H2O (l) + Energy
Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions.
Answer: In a combination reaction two or more substances are combined to form new substance while in the decomposition reaction one substance decomposes into two or more substances. Hence, the decomposition reactions are called opposite of combination reactions.
CaCO3 (s) ⇨ CaO (s) + CO2 (g) Decomposition reaction
H2 (g) + Cl2 ⇨ 2HCl (g) Combination reaction
Answer the following questions:
Write one equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity.
Answer:
CaCO3 (s) ⇨ CaO (s) + CO2 (g)
In this reaction energy is supplied in the form of heat.
2AgCl (s) + sunlight ⇨ 2Ag (s) + Cl2 (g)
In this reaction energy is supplied in the form of light,i.e.sunlight
2H2O (l) + electricity ⇨ 2H2 + O2
In this reaction energy is supplied in the form of electricity.
What is the difference between displacement and double displacement reactions? Write equations for these reactions.
Answer: When a more reactive element displaces a less reactive element from its salt solution, then it is called displacement reaction. While there is an exchange of ions between the reactants to produce new substances, it is called double displacement reaction.
In displacement reaction one displacement takes place. While in double displacement reaction two displacements takes place. Following are examples of displacement reaction and double displacement reaction.
Mg (s) + 2HCl (aq) ⇨ MgCl2 + H2 (g)
2KBr (aq) + BaCl2 ⇨ 2KCl (aq) + BaBr2 (s)
In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal. Write down the reaction involved.
Answer:
2AgNO3 (aq) + Cu (s) ⇨ Cu(NO3 )2 (l) + 2Ag (s)
What do you mean by a precipitation reaction? Explain by giving examples.
Answer: Reactions in which precipitate is formed are called precipitation reactions. Generally in a precipitation reaction insoluble salts are settled down as precipitate.
Na2SO4 (aq) + BaCl2 (aq) ⇨ 2NaCl (aq) + BaSO4 (s)
In the above reaction, when sodium sulphate reacts with barium chloride, the white precipitate of barium sulphate is formed along with the sodium chloride.
Explain the following: (a) Oxidation (b) Reduction
Answer: Oxidation: When a substance gains oxygen or loses hydrogen during a reaction, it is called that the substance is oxidized. This phenomenon is called oxidation. In the following reaction, copper gets oxygen and hence is oxidised.
2Cu + O2 + Heat ⇨ 2CuO
CuO + H2 + Hear ⇨ Cu + H2O
In the above reaction hydrogen gains oxygen, and hence oxidized.
Reduction: If a substance loses oxygen or gains hydrogen during a reaction, it is that the substance is reduced. This phenomenon is called reduction. In the following reaction zinc loses oxygen, and hence called is reduced.
ZnO + C ⇨ ZnCO
MnO2 + 4HCl ⇨ MnCl2 + 2H2O + Cl2
In this reaction Mangnese dioxide loses oxygen and hence is called readuced.
A shiny brown coloured element ‘X’ on heating in air becomes black in colour. Name the element ‘X’ and the black coloured compound formed.
Answer: Copper is a brown coloured shiny element, when heated in air, it becomes black in colour.
2Cu + O2 + Heat ⇨ 2CuO
Hence, the element ‘X’ is copper and black coloured compound formed is copper oxide.
Why do we apply paint on iron articles?
Answer: When articles made of iron come in contact with the moisture present in air, it forms iron oxide, which is known as rust. Hence, to prevent an iron article to come in contact with moisture present in air paint is applied. Paint prevents the iron to get rusted.
Oil and fat containing food items are flushed with nitrogen. Why?
Answer: When fats and oils are oxidised, they become rancid and their smell and taste change. Hence, food items containing oil and fat are flushed with nitrogen which prevents them to get oxidized and becoming rancid.
Explain the following terms with one example each: (a) Corrosion (b) Rancidity
Answer: Corrosion - Metals react with oxygen which is present in the atmospheric moisture. This leads to the formation of metal oxides. In due course of time, the metal keeps on changing into its oxide and finally the whole metal is lost due to oxidation. This process is called corrosion.
Example: Iron articles; like iron gates or bridges tend to rust because of oxidation by atmospheric moisture. We know that rust is nothing but iron oxide. Conversion of iron into rust leads to corrosion of the iron articles. Due to this, the iron articles weaken and finally wither away.
Rancidity: When fats and oils are oxidised, their smell and taste change. This process is called rancidity. Oily food often become rancid and start giving out obnoxious smell. The taste also becomes bad. Such oily food is not fit for eating.
Example: When packets of potato chips or other oily snacks are kept open for a long time; their taste and smell become bad. The oily food is no longer safe to eat.
