ACID, BASES AND SALTS
Intext Questions
Page No: 18
1. 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?
Sol. If the colour of red litmus does not change then it is acid. If the colour of red litmus changes to blue then it is base. If there is slight change in the colour of red litmus (such as purple) then it is distilled water.
Page No: 22
1. Why should curd and sour substances not be kept in brass and copper vessels?
Sol. Curd and other sour substances contain acids. Therefore, when they are kept in brass and copper vessels, the metal reacts with the acid to liberate hydrogen gas and harmful products, thereby spoiling the food.
2. 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?
Sol. Hydrogen gas is usually liberated when an acid reacts with a metal.
Take few pieces of zinc granules and add 5 ml of dilute H2SO4. Shake it and pass the gas produced into a soap solution. The bubbles of the soap solution are formed. These soap bubbles contain hydrogen gas.
H2SO4 + Zn → ZnSO4 + H2 ↑
We can test the evolved hydrogen gas by its burning with a pop sound when a candle is brought near the soap bubbles.
3. 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.
Sol. CaCO3 (s) + 2HCl (aq) → CaCl2 (aq) + CO2 (g) + H2O (l)
Calcium Carbonate + Hydrochloric acid → Calcium Chloride + Carbon dioxide + Water
Page No: 25
1. Why do HCl, HNO3, etc., show acidic characters in aqueous solutions while solutions of compounds like alcohol and glucose do not show acidic character?
Sol. When HCl or HNO3 are mixed with water then they dissolve in water to form H+ or H3O+ ions which shows their acidic character. For example just see the following reactions
2. Why does an aqueous solution of an acid conduct electricity?
Sol. The presence of hydrogen (H+) or hydronium (H3O+) ions in the aqueous solution of an acid are responsible for conducting electricity.
3. Why does dry HCl gas not change the colour of the dry litmus paper?
Sol. Dry HCl gas not change the colour of the dry litmus paper because it has no Hydrogen ions (H+) in it.
4. While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid?
Sol. Since the process of dissolving an acid in water is exothermic, it is always recommended that acid should be added to water. If it is done the other way, then it is possible that because of the large amount of heat generated, the mixture splashes out and causes burns.
5. How is the concentration of hydronium ions (H3O+) affected when a solution of an acid is diluted?
Sol. When an acid is diluted, the concentration of hydronium ions (H3O+) per unit volume decreases. This means that the strength of the acid decreases.
6. How is the concentration of hydroxide ions (OH−) affected when excess base is dissolved in a
solution of sodium hydroxide?
Sol. The concentration of hydroxide ions (OH−) would increase when excess base is dissolved in a solution of sodium hydroxide.
Page No: 28
1. 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?
Sol. A pH value of less than 7 indicates an acidic solution, while greater than 7 indicates a basic solution. Therefore, the solution with pH = 6 is acidic and has more hydrogen ion concentration than the solution of pH = 8 which is basic.
2. What effect does the concentration of H+ (aq) ions have on the nature of the solution?
Sol. If the concentration of H+ (aq) ions is increased (>10-7) then the solution become acidic and if the concentration of H+ (aq) ions is decreased (
3. Do basic solutions also have H+ (aq) ions? If yes, then why are these basic?
Sol. Yes, basic solution also has H+ ions. However, their concentration is less as compared to the concentration of OH- ions that makes the solution basic.
4. 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)?
Sol. If the soil is acidic and improper for cultivation, then to increase the basicity of soil, the farmer would treat the soil with quick lime or slaked lime or chalk.
Excercise
Q.1 A solution turns red litmus blue, its pH is likely to be
(a) 1 (b) 4 (c) 5 (d) 10
Sol. (d) 10
(b) Hydrochloric acid + Magnesium → Magnesium chloride + Hydrogen
(c) Suphuric acid + Aluminium → Aluminium sulphate + Hydrogen
(d) Sulphuric acid + Iron → Ferric chloride + Hydrogen
Intext Questions
Page No: 18
1. 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?
Sol. If the colour of red litmus does not change then it is acid. If the colour of red litmus changes to blue then it is base. If there is slight change in the colour of red litmus (such as purple) then it is distilled water.
Page No: 22
1. Why should curd and sour substances not be kept in brass and copper vessels?
Sol. Curd and other sour substances contain acids. Therefore, when they are kept in brass and copper vessels, the metal reacts with the acid to liberate hydrogen gas and harmful products, thereby spoiling the food.
2. 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?
Sol. Hydrogen gas is usually liberated when an acid reacts with a metal.
Take few pieces of zinc granules and add 5 ml of dilute H2SO4. Shake it and pass the gas produced into a soap solution. The bubbles of the soap solution are formed. These soap bubbles contain hydrogen gas.
H2SO4 + Zn → ZnSO4 + H2 ↑
We can test the evolved hydrogen gas by its burning with a pop sound when a candle is brought near the soap bubbles.
3. 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.
Sol. CaCO3 (s) + 2HCl (aq) → CaCl2 (aq) + CO2 (g) + H2O (l)
Calcium Carbonate + Hydrochloric acid → Calcium Chloride + Carbon dioxide + Water
Page No: 25
1. Why do HCl, HNO3, etc., show acidic characters in aqueous solutions while solutions of compounds like alcohol and glucose do not show acidic character?
Sol. When HCl or HNO3 are mixed with water then they dissolve in water to form H+ or H3O+ ions which shows their acidic character. For example just see the following reactions
HCl (aq) → H+ + Cl-
H+ + H2O → H3O+
When alcohols and glucose are mixed with water then they do not dissolve to form ions. Hence they do not show acidic character.2. Why does an aqueous solution of an acid conduct electricity?
Sol. The presence of hydrogen (H+) or hydronium (H3O+) ions in the aqueous solution of an acid are responsible for conducting electricity.
3. Why does dry HCl gas not change the colour of the dry litmus paper?
Sol. Dry HCl gas not change the colour of the dry litmus paper because it has no Hydrogen ions (H+) in it.
4. While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid?
Sol. Since the process of dissolving an acid in water is exothermic, it is always recommended that acid should be added to water. If it is done the other way, then it is possible that because of the large amount of heat generated, the mixture splashes out and causes burns.
5. How is the concentration of hydronium ions (H3O+) affected when a solution of an acid is diluted?
Sol. When an acid is diluted, the concentration of hydronium ions (H3O+) per unit volume decreases. This means that the strength of the acid decreases.
6. How is the concentration of hydroxide ions (OH−) affected when excess base is dissolved in a
solution of sodium hydroxide?
Sol. The concentration of hydroxide ions (OH−) would increase when excess base is dissolved in a solution of sodium hydroxide.
Page No: 28
1. 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?
Sol. A pH value of less than 7 indicates an acidic solution, while greater than 7 indicates a basic solution. Therefore, the solution with pH = 6 is acidic and has more hydrogen ion concentration than the solution of pH = 8 which is basic.
2. What effect does the concentration of H+ (aq) ions have on the nature of the solution?
Sol. If the concentration of H+ (aq) ions is increased (>10-7) then the solution become acidic and if the concentration of H+ (aq) ions is decreased (
3. Do basic solutions also have H+ (aq) ions? If yes, then why are these basic?
Sol. Yes, basic solution also has H+ ions. However, their concentration is less as compared to the concentration of OH- ions that makes the solution basic.
4. 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)?
Sol. If the soil is acidic and improper for cultivation, then to increase the basicity of soil, the farmer would treat the soil with quick lime or slaked lime or chalk.
Excercise
Q.1 A solution turns red litmus blue, its pH is likely to be
(a) 1 (b) 4 (c) 5 (d) 10
Sol. (d) 10
Q.2 A solution reacts with crushed egg-shells to give a gas that turns lime-water milky. The solution contains
(a) NaCl (b) HCl (c) LiCl (d) KC
Sol. (b) HCl
(a) NaCl (b) HCl (c) LiCl (d) KC
Sol. (b) HCl
Q.3 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
(a) 4 mL (b) 8 mL (c) 12 mL (d) 16 mL
Sol. (d) 16 mL
(a) 4 mL (b) 8 mL (c) 12 mL (d) 16 mL
Sol. (d) 16 mL
Q.4 Which one of the following types of medicines is used for treating indigestion?
(a) Antibiotic (b) Analgesic (c) Antacid (d) Antiseptic
Sol. (c) Antacid
(a) Antibiotic (b) Analgesic (c) Antacid (d) Antiseptic
Sol. (c) Antacid
Q.5 Write word equations and then balanced equations for the reaction taking place when –
(a) dilute sulphuric acid reacts with zinc granules.
(b) dilute hydrochloric acid reacts with magnesium ribbon.
(c) dilute sulphuric acid reacts with aluminium powder.
(d) dilute hydrochloric acid reacts with iron filings.
Sol.
(a) dilute sulphuric acid reacts with zinc granules.
(b) dilute hydrochloric acid reacts with magnesium ribbon.
(c) dilute sulphuric acid reacts with aluminium powder.
(d) dilute hydrochloric acid reacts with iron filings.
Sol.
(a) Sulphuric acid + Zinc → Zinc sulphate + Hydrogen
(b) Hydrochloric acid + Magnesium → Magnesium chloride + Hydrogen
(c) Suphuric acid + Aluminium → Aluminium sulphate + Hydrogen
(d) Sulphuric acid + Iron → Ferric chloride + Hydrogen
Q.6 Compounds such as alcohols and glucose also contain hydrogen but are not categorised as acids. Describe an Activity to prove it.
Sol.
Sol.
(i) Take solutions of glucose, alcohol, hydrochloric acid, sulphuric acid, etc.
(ii) Fix two nails on a cork, and place the cork in a 100 mL beaker.
(iii) Connect the nails to the two terminals of a 6 volt battery through a bulb and a switch, as shown in figure.
(iv) Now pour some glucose solution in the beaker and switch on the current.
(v) Repeat with alcohol solution and hydrochloric acid.
(ii) Fix two nails on a cork, and place the cork in a 100 mL beaker.
(iii) Connect the nails to the two terminals of a 6 volt battery through a bulb and a switch, as shown in figure.
(iv) Now pour some glucose solution in the beaker and switch on the current.
(v) 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.
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.
Q.7 Why does distilled water not conduct electricity, whereas rain water does?
Sol.
Sol.
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.
That’s why rain water conduct electricity and distilled water does not.
Q.8 Why do acids not show acidic behaviour in the absence of water?
Sol.
Sol.
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.
Q.9 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
(a) neutral? (b) strongly alkaline?
(c) strongly acidic? (d) weakly acidic?
(e) weakly alkaline?
Arrange the pH in increasing order of hydrogen-ion concentration.
Sol.
(a) neutral? (b) strongly alkaline?
(c) strongly acidic? (d) weakly acidic?
(e) weakly alkaline?
Arrange the pH in increasing order of hydrogen-ion concentration.
Sol.
(a) D - neutral. As its pH value is equal to 7
(b) C – Strongly alkaline. As its pH value is 11.
(c) B – Strongly acidic. As its pH value is equal to 1
(d) A – Weakly acidic. As its pH value is equal to 4 and more than solution B.
(e) E – 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
(b) C – Strongly alkaline. As its pH value is 11.
(c) B – Strongly acidic. As its pH value is equal to 1
(d) A – Weakly acidic. As its pH value is equal to 4 and more than solution B.
(e) E – 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
Q.10 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?
Sol.
Sol.
In test tube A fizzing occur more vigorously. Because HCl is stronger acid than acetic acid. Hence, HCl liberates hydrogen gas more vigorously, which causes fizzing more vigorously.
Q.11 Fresh milk has a pH of 6. How do you think the pH will change as it turns into curd? Explain your answer.
Sol.
Sol.
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.
Q.12 A milkman adds a very small amount of baking soda to fresh milk.
(a) Why does he shift the pH of the fresh milk from 6 to slightly alkaline?
(b) Why does this milk take a long time to set as curd?
Sol.
(a) Why does he shift the pH of the fresh milk from 6 to slightly alkaline?
(b) Why does this milk take a long time to set as curd?
Sol.
(a) 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.
(b) 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.
(b) 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.
Q.13 Plaster of Paris should be stored in a moisture-proof container. Explain why?
Sol.
Sol.
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.
Q.14 What is a neutralisation reaction? Give two examples.
Sol.
Sol.
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.
Example – 1
When Sodium hydroxide is added to hydrochloric acid. Sodium chloride and water is formed.
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.
Milk of magnesia, which is a base, is given as antacid in the case of indigestion, to neutralize the more acid produced in stomach.
Q.15 Give two important uses of washing soda and baking soda.
Sol.
Sol.
