Acid, bases and salt class 10 chemistry NCERT

Acid, bases and salt class 10 chemistry NCERT solution:

 

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?

If the color of red litmus does not change then it is acidic. If the color of red litmus changes to blue then it is the base. If there is slight change in the color of red litmus (such as purple) then it is distilled water.

 

 

1. Why should curd and sour substances not be kept in brass and copper vessels?

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?

Hydrogen gas is usually liberated when an acid reacts with metal.

 

 

 

Take a 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 extinguishes a burning candle. Write a balanced chemical equation for the reaction if one of the compounds formed is calcium chloride.

CaCO3 (s) + 2HCl (aq) → CaCl2 (aq) + CO2 (g) + H2O (l)

Calcium Carbonate + Hydrochloric acid → Calcium Chloride + Carbon dioxide + Water

 

 

 

 

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?

 

When HCl or HNO3 are mixed with water then they dissolve in water to form H+ or H3O+ ions which show their acidic character. For example just see the following reactions

HCl (aq) → H+ + Cl-

H+ + H2O → H3O+

When alcohol 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?

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 color of the dry litmus paper?

Dry HCl gas does not change the color 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?

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?

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 an excess base is dissolved in a solution of sodium hydroxide?

The concentration of hydroxide ions (OH−) would increase when the 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?

 

Answer

 

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?

 

Answer

 

If the concentration of H+ (aq) ions is increased (>10-7) then the solution becomes acidic and if the concentration of H+ (aq) ions is decreased (<10-7) then the solution becomes basic in nature.

 

3. Do basic solutions also have H+ (aq) ions? If yes, then why are these basic?

 

Answer

 

Yes, the 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)?

 

Answer

 

If the soil is acidic and improper for cultivation, then to increase the basicity of soil, the farmer would treat the soil with quicklime or slaked lime or chalk.

 

Page No: 33

 

1. What is the common name of the compound CaOCl2?

► Bleaching Powder.

 

2. Name the substance which on treatment with chlorine yields bleaching powder?

► Calcium hydroxide [Ca(OH)2]

 

3. Name the sodium compound which is used for softening hard water.

► Washing soda (Na2CO3.10H2O)

 

4. What will happen if a solution of sodium hydrocarbonate is heated? Give the equation of the reaction involved.

 

Answer

 

When sodium hydrogen carbonate is heated, then sodium carbonate and water are formed along with the evolution of carbon dioxide gas.

 

5. Write an equation to show the reaction between Plaster of Paris and water.

 

Answer

 

 

Page No: 34

 

Exercise

 

1. A solution turns red litmus blue, its pH is likely to be

(a) 1

(b) 4

(c) 5

(d) 10

► (d) 10

 

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) KCl

► (b) HCl

 

3. 10 mL of a solution of NaOH is found to be completely neutralized by 8 mL of a given solution of HCl. If we take 20 mL of the same solution of NaOH, the amount of HCl solution (the same solution as before) required to neutralize it will be

(a) 4 mL

(b) 8mL

(c) 12 mL 

(d) 16 mL

► (d) 16 mL

 

4. Which one of the following types of medicines is used for treating indigestion?

(a) Antibiotic

(b) Analgesic

(c) Antacid

(d) Antiseptic

► (c) Antacid

 

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 aluminum powder.

(d) dilute hydrochloric acid reacts with iron filings.

 

Answer

 

(a) H2SO4 (aq) + Zn (s) → ZnSO4 (aq) + H2 (g)

 

(b) 2HCl (aq) + Mg (s) → MgCl2 (aq) + H2 (g)

 

(c) 3H2SO4 (aq) + 2Al (s) → Al2(SO4)3 (aq) + 3H2 (g)

 

(d) 6HCl (aq) + 2Fe (s) → 2FeCl3 (aq) + 3H2 (g)

 

6. Compounds such as alcohols and glucose also contain hydrogen but are not categorized as acids. Describe an activity to prove it.

 

Answer

 

Two nails are fitted on a cork and are kept in a 100 mL beaker. The nails are then connected to the two terminals of a 6-volt battery through a bulb and a switch. Some dilute HCl is poured in the beaker and the current is switched on. The same experiment is then performed with glucose solution and alcohol solution.

 

Observations:

It will be observed that the bulb glows in the HCl solution and does not glow in the glucose solution.

 

Result:

HCl dissociates into H+and Cl−ions. These ions conduct electricity in the solution resulting in the glowing of the bulb. On the other hand, the glucose solution does not dissociate into ions. Therefore, it does not conduct electricity.

 

 

Conclusion:

From this activity, it can be concluded that all acids contain hydrogen but not all compounds containing hydrogen are acids.

That is why, though alcohols and glucose contain hydrogen, they are not categorized as acids.

 

7. Why does distilled water not conduct electricity, whereas rain water does?

Distilled water cannot conduct electricity because it does not contain ions while rain water conducts electricity as it contains ions due the presence of dissolved salts in it.

 

8. Why do acids not show acidic behavior in the absence of water?

Acids do not show acidic behavior in the absence of water because the dissociation of hydrogen ions from an acid occurs in the presence of water only.

 

9. Five solutions A, B, C, D and E when tested with a 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.

(a) Neutral →Solution D with pH 7

(b) Strongly alkaline →Solution C with pH 11

(c) Strongly acidic →Solution B with pH 1

(d) Weakly acidic →Solution A with pH 4

(e) Weakly alkaline →Solution E with pH 9

The pH can be arranged in the increasing order of the concentration of hydrogen ions as: 11 < 9 < 7 < 4 < 1.

 

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. In which test tube will the fizzing occur more vigorously and why?

The fizzing will occur strongly in test tube A, in which hydrochloric acid (HCl) is added. This is because HCl is a stronger acid than CH3COOH and therefore produces hydrogen gas at a faster speed due to which fizzing occurs.

 

11. Fresh milk has a pH of 6. How do you think the pH will change as it turns into curd? Explain your answer.

The pH of milk is 6. As it changes to curd, the pH will reduce because curd is acidic in nature. The acids present in it decrease the pH.

 

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?

(a) The milkman shifts the pH of the fresh milk from 6 to slightly alkaline because in alkaline condition, milk does not set as curd easily.

 

(b) Since this milk is slightly more basic than usual milk, acids produced to set the curd are neutralized by the base. Therefore, it takes a longer time for the curd to set.

 

13. Plaster of Paris should be stored in a moisture-proof container. Explain why?

 

Answer

 

The Plaster of Paris should be stored in a moisture-proof container as it absorbs water from moisture and turns into a hard substance (Gypsum) as shown in the following chemical equation.

 

14. What is a neutralization reaction? Give two examples.

 

Answer

 

A reaction in which an acid and base react with each other to give a salt and water is termed a neutralization reaction.For Example:

(i) NaOH + HCl → NaCl + H2O

(ii) HNO3 + KOH → KNO3 + H2O

 

15. Give two important uses of washing soda and baking soda.

 

Answer

 

Two important uses of washing soda are:

→ It is used in glass, soap, and paper industries.

→ It is used to remove permanent hardness of water.

 

Two important uses of baking soda are:

→ It is used as baking powder. Baking powder is a mixture of baking soda and a mild acid known as tartaric acid. When it is heated or mixed in water, it releases CO2 that makes bread or cake fluffy.

→ It is used in soda-acid fire extinguishers.

Acid, bases and salt class 10 chemistry NCERT MCQ

Question :  When a bee stings, immediately a paste of lime is put on the sting. Why?

• a) Bee sting contains an acid called formic acid It gets neutralized with CaO .

• b) Bee sting is made of a base

• c) Bee sting is acidic due to hydrochloric acid and this is neutralized

• d) All of the above.

Answer :  Bee sting contains an acid called formic acid It gets neutralized with CaO .

 

Question :  Why does dry HCl gas not change the color of the dry litmus paper?

• a) No H3O+ ions will be present, litmus changes color only in the presence of H3O+ ions

• b) Blue litmus becomes dry in presence of dry HCl gas

• c) HCl gas acts as a dehydrating agent

• d) None of the options

Answer :  No H3O+ ions will be present, litmus changes color only in the presence of H3O+ ions

 

Question :  How is the concentration of hydronium ions (H3O+) affected when a solution of an acid is diluted?

• a) Increases

• b) Decreases

• c) Remains the same

• d) Becomes zero

Answer :  Increases

 

Question :  A calcium compound reacts with dilute hydrochloric acid to produce effervescence. The gas extinguishes a burning candle. Identify the compound and the gas evolve

• a) Calcium Carbonate, Carbon dioxide

• b) Calcium chloride, carbon dioxide

• c) Calcium oxide, hydrogen

• d) Calcium carbonate, hydrogen

Answer :  Calcium Carbonate, Carbon dioxide

 

Question :  Acids react with bases to form salt and water. This reaction is known as:

• a) Neutralization

• b) Combination

• c) Decomposition

• d) reduction

Answer :  Neutralization

  

Question :  The milkiness (on passing excess carbon dioxide gas through lime water) disappears due to the formation of:

• a) Calcium hydrogen carbonate CaHCO3

• b) Calcium carbonate CaCO3

• c) Calcium oxide CaO

• d) Calcium Nitrate Ca(NO3)2

Answer :  Calcium hydrogen carbonate CaHCO3

 

Question :  Lime water turns milky when carbon dioxide is passed due to the formation of ______.

• a) CaCO3

• b) CaO

• c) CO2

• d) CaSO4

Answer :  CaCO3

 

Question :  Acids react with metal carbonates to liberate ______ gas

• a) Carbon dioxide

• b) Carbon monoxide

• c) Hydrogen

• d) Water

Answer :  Carbon dioxide

 

Question :  Acids react with metals to liberate _________gas

• a) Hydrogen

• b) Carbon dioxide

• c) Carbon monoxide

• d) Water

Answer :  Hydrogen

 

Question :  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. Which of them will turn red litmus to blue?

• a) Base

• b) Acid

• c) Water

• d) All of the above

Answer :  Base

 

Question : 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. Which of them will turn red litmus to blue?

a) Acid

b) Base

c) Water

d) All of the above

Answer : B

 

Question : Acids react with metals to liberate _________gas

a) Carbon dioxide

b) Carbon monoxide

c) Hydrogen

d) Water

Answer : C

 

Question : Acids react with metal carbonates to liberate ______ gas

a) Carbon dioxide

b) Carbon monoxide

c) Hydrogen

d) Water

Answer : A

 

Question : Lime water turns milky when carbon dioxide is passed due to the formation of ______.

a) CaCO3

b) CaO

c) CO2

d) CaSO4

Answer : A

 

Question : The milkiness (on passing excess carbon dioxide gas through lime water) disappears due to the formation of:

a) Calcium carbonate CaCO3

b) Calcium hydrogen carbonate CaHCO3

c) Calcium oxide CaO

d) Calcium Nitrate Ca(NO3)2

Answer : B

 

Question : Acids react with bases to form salt and water. This reaction is known as:

a) Combination

b) Decomposition

c) Neutralization

d) reduction

Answer :  C

 

Question : A calcium compound reacts with dilute hydrochloric acid to produce effervescence. The gas extinguishes a burning candle. Identify the compound and the gas evolved)

a) Calcium Carbonate, Carbon dioxide

b) Calcium chloride, carbon dioxide

c) Calcium oxide, hydrogen

d) Calcium carbonate, hydrogen

Answer :  A

 

Question : How is the concentration of hydronium ions (H3O+) affected when a solution of an acid is diluted?

a) Increases

b) Decreases

c) Remains the same

d) Becomes zero

Answer : A

 

Question : Why does dry HCl gas not change the color of the dry litmus paper?

a) Blue litmus becomes dry in presence of dry HCl gas

b) No H3O+ ions will be present, litmus changes color only in the presence of H3O+ ions

c) HCl gas acts as a dehydrating agent

d) None of the above

Answer : B

 

Question : When a bee stings, immediately a paste of lime is put on the sting. Why?

a) Bee sting is made of a base

b) Bee sting contains an acid called formic acid) It gets neutralized with CaO .

c) Bee sting is acidic due to hydrochloric acid and this is neutralized

d) All of the above.

Answer : B

 

Question : An aqueous solution turns the red litmus solution blue. Excess addition of which of the following solutions would reverse the change ?

a) Baking powder 

b) Lime

c) Ammonium hydroxide solution 

d) Hydrochloric acid

 

Question : Which one of the following can be used as an acid-base indicator by a visually impaired (blind) student?

a) Litmus 

b) Turmeric 

c) Vanilla essence 

d) Petunia leaves

 

Question : Which of the following is acidic in nature ?

a) Lime juice 

b) Human blood 

c) Lime water 

d) Antacid

 

Question : In an attempt to demonstrate electrical conductivity through an electrolyte, the following apparatus was set up Which among the following statement(s) is (are) correct?

(i) Bulb will not glow because electrolyte is not acidic

(ii) Bulb will glow because NaOH is a strong base and furnishes ions for conduction.

(iii) Bulb will not glow because the circuit is incomplete.

(iv) Bulb will not glow because it depends upon the type of electrolytic solution.

a) (i) and (iii)

b) (ii) and (iv) 

c) (ii) only 

d) (iv) only

 

Question : Which of the following is not a mineral acid ?

a) Hydrochloric acid 

b) Citric acid 

c) Sulphuric acid 

d) Nitric acid

 

Question : Which among the following is not a base ?

a) NaOH 

b) KOH 

c) NH4OH 

d) C2H5OH

 

Question : Which of the following is(are) true when HCl(g) is passed through water ?

(i) It does not ionize in the solution as it is a covalent compound)

(ii) It ionizes in the solution.

(iii) It gives both hydrogen and hydroxyl ion in the solution.

(iv) It forms hydronium ions in the solution due to the combination of hydrogen ions with water molecules.

a) (i) only 

b) (iii) only 

c) (ii) and (iv) 

d) (iii) and (iv)

 

Question : Which of the following statements is true for acids ?

a) Bitter and change red litmus to blue. 

b) Sour and change red litmus to blue.

c) Sour and change blue litmus to red) 

d) Bitter and change blue litmus to red)

 

Question : Which of the following are present in a dilute aqueous solution of hydrochloric acid ?

a) H3O+ + Cl– 

b) H3O+ + OH– 

c) Cl– + OH– 

d) unionized HCl

 

 

 

 

 

Question : Which of the following statements is not correct?

a) Acids turn blue litmus solution red

b) Raw onion can be used as an olfactory indicator to check acid or base.

c) Bases are sour in taste.

d) Vanilla essence does not give odor in strongly basic solutions.

 

Question : What happens when a solution of an acid is mixed with a solution of a base in a test tube ?

(i) The temperature of the solution increases.

(ii) The temperature of the solution decreases.

(iii) The temperature of the solution remains the same.

(iv) Salt formation takes place.

a) i only 

b) i and iii 

c) ii and iii 

d) i and iv

 

Question : A sample of soil is mixed with water and allowed to settle. The clear supernatant solution turns the pH paper yellowish orange. Which of the following would change the color of this pH paper to greenish-blue ?

a) lemon juice 

b) vinegar 

c) common salt 

d) an antacid

 

Question : Which of the following gives the correct increasing order of acidic strength ?

a) Water < Acetic acid < Hydrochloric acid

b) Water < Hydrochloric acid < Acetic acid

c) Acetic acid < Water < Hydrochloric acid

d) Hydrochloric acid < Water < Acetic acid

 

Question : If a few drops of a concentrated acid accidentally spills over the hand of a student, what should be done?

a) Wash the hand with saline solution.

b) Wash the hand immediately with plenty of water and apply a paste of sodium hydrogen carbonate.

c) After washing with plenty of water, apply a solution of sodium hydroxide on the hand)

d) Neutralize the acid with a strong alkali.

 

Question : Sodium hydrogen carbonate when added to acetic acid evolves into a gas. Which of the following statements are true about how gas evolved?

(i) It turns lime water milky

(ii) It extinguishes a burning splinter

(iii) It dissolves in a solution of sodium hydroxide

(iv) It has a pungent odor

a) i and ii 

b) i, ii and iii 

c) ii, iii and iv 

d) i and iv

 

Question : To prevent tooth decay we are advised to brush our teeth regularly. The nature of the toothpaste commonly used is

a) acidic 

b) neutral 

c) basic 

d) corrosive

 

Question : Which of the following statements is correct about an aqueous solution of an acid and of a base?

(i) Higher the pH, stronger the acid

(ii) Higher the pH, weaker the acid

(iii) Lower the pH, stronger the base

(iv) Lower the pH, weaker the base

a) i and iii

b) ii and iii

c) i and iv

d) ii and iv

 

Question : The pH of the gastric juices released during digestion is

a) less than 7 

b) more than 7 

c) equal to 7 

d) equal to 0

 

Question : Equal volumes of hydrochloric acid and sodium hydroxide solutions of same concentration are mixed and the pH of the resulting solution is checked with a pH paper. What would be the color obtained ?

(You may use the color guide given in the figure) : 

a) Red 

b) Yellow 

c) Yellowish green 

d) Blue

 

Question : pH of a solution having hydrogen ion concentration of 1M is

a) 0 

b) 1 

c) 10 

d) 14

 

Question : During the preparation of hydrogen chloride gas on a humid day, the gas is usually passed through the guard tube containing calcium chloride. The role of calcium chloride taken in the guard tube is to 

a) absorb the evolved gas 

b) moisten the gas

c) absorb moisture from the gas 

d) absorb Cl– ions from the evolved gas

 

Question : Which of the following salts does not contain water of crystallization ?

a) Blue vitriol 

b) Baking soda 

c) Washing soda 

d) Gypsum

 

Question : Sodium carbonate is a basic salt because it is a salt of

a) strong acid and strong base 

b) weak acid and weak base

c) strong acid and weak base 

d) weak acid and strong base

 

Question : Calcium phosphate is present in tooth enamel. Its nature is

a) basic 

b) acidic 

c) neutral 

d) amphoteric

 

Question : Common salt besides being used in kitchen can also be used as the raw material for making

(i) washing soda (ii) bleaching powder (iii) baking soda (iv) slaked lime

a) i and ii 

b) i, ii and iv 

c) i and iii 

d) i, iii and iv

 

Question : One of the constituents of baking powder is sodium hydrogen carbonate, the other constituent is

a) hydrochloric acid 

b) tartaric acid 

c) acetic acid 

d) sulphuric acid

 

Question : Identify the correct representation of reaction occurring during the chlor-alkali process.

a) 2NaCl(l) + 2H2O(l) →2NaOH(l) + Cl2(g) + H2(g)

b) 2NaCl(aq) + 2H2O(aq) → 2NaOH(aq) + Cl2(g) + H2(g)

c) 2NaCl(aq) + 2H2O(l) →2NaOH(aq) + Cl2(aq) + H2(aq)

d) 2NaCl(aq) + 2H2O(l) →2NaOH(aq) + Cl2(g) + H2(g)

 

Question : Metal hydrogen carbonates react with acids to give

a) salt, water, chlorine 

b) salt, water and carbon dioxide

c) salt and carbon dioxide 

d) salt, hydrogen and carbon dioxide

 

Question : Bleaching powder is:

a) CaO2Cl2 

b) CaOCl2 

c) CaClO2 

d) CaCl2 + O2

 

Question : Plaster of Paris is:

a) CaSO4 

b) CaSO4.H2O 

c) CaSO4.2H2O 

d) CaSO4. 1/2 H2O

Acid, bases and salt class 10 chemistry NCERT SAQ

.(i) Give the constituents of baking powder

(ii) Why does cake or bread swell on adding baking powder? Write chemical equations.

Answer.

(i) Baking powder contains sodium hydrogen carbonate and tartaric acid.

(ii)It is due to carbon dioxide

Equal lengths of magnesium ribbon are taken in two test tubes ‘A’ and ‘B\ H2SO4 is added to test tube ‘A’ and H2CO3 in the test tube ‘B’ in equal amounts:

(a) Identify the test tube showing vigorous reaction.

(b) Give reason to support your answer.

(c) Name the gas liberated in both the tubes. How will you prove its liberation?

(d) Write chemical equations for both reactions.

(e) Out of the two acids taken above

(i) which one will have lower pH value.

(ii) lowerH+ concentration respectively.

Answer.

(a) A will show vigorous reaction.

(b) It is because H2SO4 is a strong acid.

(c) Hydrogen gas will be formed. Bring a burning splinter near the gas. It will burn with a ‘pop’ sound. It shows that the gas liberated is hydrogen.

Q.1. Write the equation for the dissociation of hydrochloric acid (HCl) in water. 

Ans. HCl + H2O → H3O+ + Cl-

or it can also be written as

HCl H+(aq) + Cl-(aq)

 

Q.2. Which one of these has a higher concentration of H+ ions? 

1M HCl or 1M CH3COOH

Ans. 1M HCl will have a higher concentration of H+ ions because HCl being a strong acid undergoes complete dissociation.

 

Q.3. While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid?

Or 

Why should water be never added dropwise to concentrated sulphuric acid?

Ans.

Fig. Adding acid to water

While diluting an acid, water should not be added to a concentrated acid because adding water to acid is a highly exothermic process and the heat generated may cause the mixture to splash out.

 

Q.4. How is the concentration of hydroxide ions (OH-) affected when excess base is dissolved in a solution of sodium hydroxide? 

Ans. The concentration of hydroxide ions (OH-) is increased when the excess base is dissolved in a solution of sodium hydroxide.

 

Q.5. Do basic solutions also have H+ (aq) ions? If yes, then why are these basic? 

Ans. Basic solutions also have H+ (aq) ions. A solution of an acid or a base always contains both H+ (aq) ions as well as OH- (aq) ions. It shows the basic character if it has more OH- (aq) ions and acidic character if it has more H+ (aq) ions.

 

Q.6. Choose strong acid and a strong base from the following: CH3COOH, NH4OH, KOH, HCl 

Ans. A strong acid is HCl and a strong base is KOH.

 

Q.7. Which is more acidic: a solution with pH = 6.0 or a solution with pH = 2.0? 

Ans. A solution with pH = 2.0 is more acidic.

 

Q.8. Which is more basic, a solution with pH = 9.0 or a solution with pH = 13.0? 

Ans. A solution with pH = 13.0 is more basic.

 

Q.9. How would you show that lemon and tomato contain acids? 

Ans. Both lemon juice and tomato juice turn blue litmus red. It shows that both of them contain acids.

 

Q.10. What is the action of the solution of sodium carbonate towards litmus? 

Ans. Solution of sodium carbonate will turn the color of red litmus into blue indicating that it is alkaline in nature.

 

Q.11. Dry ammonia gas has no action on litmus paper but a solution of ammonia in water turns red litmus paper blue. Why is it so? 

Ans. Ammonia in water forms ammonium hydroxide. These hydroxide ions turn red litmus blue.

 

Q.12. What is the action on litmus of: 

(a) Dry ammonia gas? 

(b) The solution of ammonia gas in the water? 

Ans. 

(a) Dry ammonia gas has no action on litmus. 

(b) Solution of ammonia gas in water turns red litmus blue.

 

Q.13. Why should curd and sour substances not be kept in brass and copper vessels? 

Ans. Curd and sour substances contain acids which react with brass and copper and form toxic substances whose consumption is bad for health.

 

Q.14. Why do HCl, HNO3, etc. show acidic character in aqueous solutions while solutions of compounds like C2H5OH and glucose do not show acidic character? 

Ans. A substance will show an acidic character if it gives H+ ions when dissolved in water. Among these substances, HCl and HNO3 provide H+ ions whereas C2H5OH and glucose do not give H+ ions so they do not show acidic character.

 

Q.15.  Given two unlabelled bottles, one containing dilute acid and the other water. How would you decide to label them? 

Ans. Acid and water can be identified by testing with litmus. Water will not change the color of red or blue litmus whereas acid will change blue litmus into red.

 

Q.16. Why does distilled water not conduct electricity, whereas rainwater does? 

Ans. The electric current is carried by ions in solutions. Distilled water has no ions whereas rainwater is slightly acidic and contains ions so rainwater conducts electricity.

 

Q.17. What happens when carbon dioxide gas is passed through a sodium hydroxide solution? 

Ans. When carbon dioxide gas is passed through a sodium hydroxide solution, sodium carbonate is formed.

 2NaOH + CO2 → Na2CO3 + H2O

 

 

 

A gas ‘X’ reacts with lime water and forms a compound ‘Y’ which is used as a bleaching agent in the chemical industry. Identify ‘X’ and ‘Y\ Give the chemical equation of the reactions involved.

Answer. ‘X’ is chlorine; ‘Y’ is bleaching powder.

(i) Name the compound which is obtained from baking soda and is used to remove permanent hardness of water.

(ii) Write its chemical formula.

(iii) What happens when it is recrystallised from its aqueous solution?

Answer.

(i) Sodium carbonate is obtained from baking soda and is used to remove the hardness of water.

(ii) Na2CO3   .

(iii) It changes to washing soda,Na2CO3. 10H2O  .

 

 

 

 

Compounds like alcohol and glucose also contain hydrogen but are not categorized as acids. Discuss an activity to prove it.

Answer.Take a beaker of 250 ml and place two nails fixed with the help of cork.

• Connect the nails to the two terminals of a 6 volt battery as shown in the figure.

• Now add some water containing ethanol and put the switch ON.

• Repeat the experiment with glucose solution.

Observation : K The bulb will not glow and the needle of the ammeter will not show deflection because glucose and ethanol do not conduct electricity.

Conclusion: The experiment shows glucose and ethanol do not ionize in aqueous solution, that is, they do not give H+ ions, therefore cannot conduct electricity. Thus, glucose and ethanol are not categorized as acids.

 

 

 

 

What happens when chlorine is passed over slaked lime at 313K? Write the chemical equation of the reaction involved and state two uses of the product obtained.

Answer. Bleaching powder is formed.

(i) It is used as a bleaching agent in the paper and textile industries.

(ii)It is used as a disinfectant in purification of drinking water.

 

 

.What is meant by ‘water of crystallization’ of a substance ?

Describe an activity to show that blue copper sulfate crystals contain water of crystallization.

Answer.The water molecules associated with a crystalline substance is called ‘water of crystallization’.

To show crystalline salts contain water of crystallization.

Materials Required: CuS04.5H20 (Blue vitriol), boiling tube, burner, cork, delivery tube, test tube, clamp stand.

Procedure:

1. Take 2g of CuS04.5H20 in a boiling tube fitted in a clamp stand.

2. Observe its color. Fit it with a cork and delivery tube bent at two right angles which dip into a test tube.

3. Heat crystals in a boiling tube. ‘

4. Observe vapors being condensed in the test tube.

5. Cool the crystals and add a few drops of water into it.

Observation: Water vapors get condensed in a test tube and the color of blue crystals changes into white. On adding water to anhydrous copper sulfate, it changes into blue again.

Chemical Reaction:

Blue vitriol White

Conclusion: Crystalline substances have water of crystallization which are lost on heating.

 

 

 

 

Q.18. Name the sodium compound which is used for softening hard water. 

Ans: The sodium compound used for softening hard water is sodium carbonate (Na2CO3.10H2O) also called sodium carbonate decahydrate

 

Q.19. What is the chemical name and formula of baking soda? 

Ans. The chemical name of baking soda is sodium hydrogen carbonate and its formula is NaHCO3.

 

Q.20. A compound 'X' is an important ingredient of an antacid. It is also used in fire extinguishers. Identify 'X'. 

Ans. Compound 'X' is sodium hydrogen carbonate (NaHCO3).

 

Q.21. Fresh milk has a pH of 6. How do you think the pH will change as it turns into curd? Explain your answer. 

Or 

Fresh milk has a pH of 6. When it changes into curd (yogurt) will its pH value increase or decrease? Why?  

Ans. The pH will decrease from 6 because it becomes more acidic when milk is converted into curd and more acidic solutions have lower pH value. 

Fig: pH scale

 

Q.22. What happens when crystals of washing soda are left open in dry air? What is this change called? Name two industries based on the use of washing soda. 

Ans. When crystals of washing soda are left open in dry air, they lose nine molecules of water of crystallization and become white powder.

This change is called efflorescence. 

Two industries based on the use of washing soda are: 

(i) manufacture of glass 

(ii) paper and textile industries.

 

Q.23. What will happen if the solution of sodium hydrogen carbonate is heated? Give the equation of the reaction involved. 

Or

(i) Name the products formed when sodium hydrogen carbonate is heated.

(ii) Write the chemical equation for the reaction involved in the above. 

Ans. When the solution of sodium hydrogen carbonate is heated, it decomposes to form sodium carbonate with the evolution of carbon dioxide gas. 

 

Q.24. How is Plaster of Paris chemically different from gypsum? How may they be interconverted? Write one use of Plaster of Paris. 

Or 

How was the Plaster of Paris obtained? What reaction is involved in the setting of a paste of Plaster of Paris?

Or

State the chemical difference between Plaster of Paris and gypsum. Describe their either-way interconversions. 

 

Fig: Plaster of Paris

Ans. Plaster of Paris is chemically different from gypsum in terms of water or crystallisation. Gypsum has 2 moles of water per mole of CaSO4, (CaSO4.1/2H2O). It can also be written as if one mole of water of crystallization is present for two moles of CaSO4, (2CaSO4. H2O). Gypsum on heating at 373 K gets converted into the Plaster of Paris.

When the Plaster of Paris is mixed with water, it gets converted into gypsum.

The plaster of Paris is used for making statues and for setting fractured bones.

 

Q.25. Name three compounds of calcium that are used in day-to-day life and write one important use of each of them. 

Ans. The three compounds of calcium and their uses are :

(i) Slaked lime [Calcium hydroxide, Ca(OH)2] used for the manufacture of bleaching powder.

(ii) Bleaching powder [Calcium oxychloride, CaOCl2] used as a bleaching agent in the laundry. 

(iii) Plaster of Paris [Calcium sulfate hemihydrate, CaSO4. 1/2H2O] used to plaster the fractured bones.

 

PREVIOUS YEARS' BOARD QUESTIONS

Q.26. A chemical compound having the smell of chlorine is used to remove the yellowness of white clothes in the laundry. Name the compound and write the chemical equation involved in its preparation.

Ans. The compound is bleaching powder (CaOCl2). It removes yellowness from clothes due to its bleaching action. For details, consult the text part.

 

Q.27. Explain giving reasons: 

(i) Tartaric acid is a component of baking powder used in making cakes.

(ii) Gypsum, CaSO4.2H2O is used in the manufacture of cement.

Ans. 

(i) Role of tartaric acid in baking powder (mixture of tartaric acid and sodium hydrogen carbonate) is to neutralize sodium carbonate formed upon heating sodium hydrogen carbonate.

Sodium hydrogen carbonate Sodium carbonate 

(ii) The role of gypsum (CaSO4.2H2O) in the manufacture of cement is to slow down the process of setting of cement.

 

Q.28. How is chloride of lime chemically different from calcium chloride? Why does the chloride of lime gradually lose its chlorine when kept exposed to air? 

Ans. Chloride of lime is calcium oxychloride [(Ca(OCl)Cl] also known as bleaching powder. Calcium chloride is CaCl2. Bleaching powder gives identific smell of chlorine when exposed to air because it reacts with CO2 and chlorine gas is evolved. 

 

Q.29. What is the chemical name of washing soda? Name three raw materials used in making washing soda by Solvay process.

Ans. Chemical name: Sodium carbonate decahydrate (Na2CO3.10H2O).

Raw materials: Brine, limestone, ammonia.

 

Q.30. State the chemical property in each case on which the following uses of baking soda are based 

(i) as an antacid. 

(ii) as a constituent of baking powder. 

Ans.

(i) It is weakly alkaline in nature and neutralizes acid (HCl) formed in the stomach.

NaHCO3 + HCl  → NaCl + H2O + CO2

(ii) It evolves CO2 in the form of bubbles when cake is made by baking. As a result, the cake becomes porous as well as fluffy.

 

 

State the chemical name of Plaster of Paris. Write a chemical equation to show the reaction between Plaster of Paris and water.

Answer. Calcium sulfate hemihydrate.

 

.State in brief the preparation of washing soda from baking soda. Write balanced

chemical equation of the reaction involved.

Answer. Sodium hydrogen carbonate (baking soda) on heating gives sodium carbonate which on recrystallisation gives washing soda

 

 

 

 What is the color of FeSO4.7H2O crystals? How does this color change upon heating? Give a balanced chemical equation for the changes.

Answer. Pale green is the color of FeSO4.7H2O crystals. It becomes dirty white on heating.

 

 

Classify the following salts into acidic, basic and neutral: Potassium sulfate, ammonium chloride, sodium carbonate, sodium chloride.

Answer. Neutral: Potassium sulfate, Sodium chloride Acidic: Ammonium chloride Basic: Sodium carbonate

 

 

A student dropped a few pieces of marble in dilute HC1 contained in a test tube. The evolved gas was passed through lime water.

(i) What change would be observed in lime water?

(ii) Write a balanced chemical equation for the above change.

Answer.

(i) Lime water will turn milky due to the formation of calcium carbonate.

(ii) Ca(OH) 2  (aq) + CO2   (g)—> Ca CO3  (s) + HaO(l)

 

 

 

.(a) What is a universal indicator?

(b) Write the chemical equation involved in the preparation of sodium hydroxide. Name the process.

Answer. (a) Universal indicator is the mixture of synthetic indicators which is used to find the pH of solutions.

Q.31. 

 

(a) Name the raw materials used in the manufacture of sodium carbonate by Solvay process.

(b) How is sodium hydrogen carbonate formed during the Solvay process separated from a mixture of NH4Cl and NaHCO3 ?

(c) How is sodium carbonate obtained from sodium hydrogen carbonate? 

Ans. 

(a) The raw materials used are: NaCl, limestone or CaCO3 and NH3.

(b) Sodium hydrogen carbonate (NaHCO3) is sparingly soluble or less soluble in water and gets separated as a precipitate while NH4Cl remains in solution. The precipitate is removed by filtration.

(c) Sodium hydrogen carbonate is converted to sodium carbonate upon heating.

For further details, consult the text part.

 

Q.32. 

(a) What is the action of red litmus on (i) dry ammonia gas (ii) solution of ammonia gas in water

(b) State the observations you would make on adding ammonium hydroxide to an aqueous solution of

(i) ferrous sulfate (ii) aluminum chloride.

Ans. 

(a) 

(i) Red litmus has no action on dry ammonia gas because it does not release any hydroxyl ions (OH-)

(ii) When passed through water, ammonia (NH3) is converted to ammonium hydroxide (NH4OH). It dissociates to give hydroxyl ions (OH-) and the solution is basic in nature. Red litmus acquires a blue color.

(b) 

(i) A green precipitate of ferrous hydroxide will be formed by a double decomposition reaction. 

(ii) A white precipitate of aluminum hydroxide will be formed by a double decomposition reaction.

 

Q.33. Name the gas that evolved when dilute HCl reacts with sodium hydrogen carbonate. How is it recognised?

Ans. The gas that evolved is carbon dioxide (CO2). When the gas is bubbled through lime water, it becomes milky due to the formation of CaCO3.

 

Q.34. 

(a) Name the two chief chemicals used for making a soda acid fire extinguisher. 

(b) How does the soda-acid fire extinguisher help to extinguish the fire? 

Ans. 

(a) The two chief chemicals are sodium hydrogen carbonate (NaHCO3) and sulphuric acid (H2SO4) 

(b) For the details of the operation, consult the text part.

 

Q.35. How will you test for the gas which is liberated when hydrochloric acid reacts with an active metal?

Ans. Hydrogen gas is evolved when hydrochloric acid reacts with an active metal such as sodium, potassium,

Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)

calcium or magnesium. In order to test the gas, bring either a burning match stick or candle near the gas. The gas will immediately catch fire and burn with a ‘pop’ sound.

 

Q.36. An aqueous solution has a pH value of 7.0. Is this solution acidic, basic, or neutral? 

Ans. The solution with a pH value of 7.0 is neutral in nature

 

Q.37. Out of calcium compounds calcium carbonate, quick lime, and slaked lime, which one can be used for removing moisture from ammonia gas and why?

Ans. Quick lime (CaO) can be used to remove moisture from ammonia gas because of its hygroscopic nature. Therefore, it can act as the best dehydrating agent for ammonia.

 State the chemical properties on which the following uses of baking soda are based:

(i) as an antacid

(ii) as a soda acid fire extinguisher

(iii) to make bread and cake soft and spongy.

Answer.

(b) How is washing soda obtained from baking soda? Write a balanced chemical equation.

(a) (i) It is weakly basic in nature and naturalizes hyperacidity.

(ii)It liberates CO2 with H2SO4, which extinguishes fire.

(iii) It liberates CO2 on heating which makes bread and cake soft and sponge.

) Write the name given to bases that are highly soluble in water. Give an example.

(b) How is tooth decay related to pH? How can it be prevented?

(c) Why does bee sting cause pain and irritation? Rubbing baking soda on the sting area gives relief. How?

Answer.(a) Alkali, e.g. NaOH (Sodium hydroxide).

(b) Lower the pH, more will be tooth decay. Acid reacts with Ca3(PO4)2 and causes tooth decay.

It can be prevented by brushing teeth after every meal.

(ic) It is due to formic acid. Sodium hydrogen carbonate (Baking soda) neutralizes formic acid giving relief.

 A white powder is added while baking bread and cakes to make them soft and fluffy. Write the name of the powder. Name its main ingredients. Explain the function of each ingredient. Write the chemical reaction taking place when the powder is heated during baking.

Answer. Baking powder.

It consists of sodium hydrogen carbonate and tartaric acid.

Sodium hydrogen carbonate gives CO2 which makes cake soft and fluffy. Tartaric acid neutralizes the bitterness due to the sodium carbonate produced.

17 “Sodium hydrogencarbonate is a basic salt”. Justify the statement. How is it converted into washing soda? Explain.

Answer.Sodium hydrogencarbonate is a salt of sodium hydroxide (strong base) and carbonic acid (weak acid).

It is basic salt. It is converted into washing soda by heating followed by crystallization.

Acid, bases and salt class 10 chemistry NCERT LONG QUESTION

.State reason for the following statements:

(i) Tap water conducts electricity whereas distilled water does not.

(ii) Dry hydrogen chloride gas does not turn blue litmus red whereas dilute hydrochloric acid does.

(iii) During the summer season, a milkman usually adds a very small amount of baking soda to fresh milk.

(iv) For a dilution of acid, acid is added into water and not water into acid.

(v) Ammonia is a base but does not contain hydroxyl groups.

Answer.

(i) Tap water contains ions which conduct electricity, distilled water does not contain ions.

(ii)Dry HCl does not form ions but HCl gives H+ and Cl–.

(iii) Baking soda does not allow milk to change to lactic acid which makes milk sour.

(iv) Adding water to acid is highly exothermic. Therefore water is added to acid very slowly with cooling.

(v) Ammonia dissolves in water and forms H– Therefore, it is basic in nature.

 Write the chemical formula of hydrated copper sulfate and anhydrous copper sulfate. Giving an activity illustrates how these are inter convertible.

(b) Write chemical names and formulas of plaster of paris and gypsum.

Answer.

(a)CuSO4.5H2O is hydrated copper sulfate. CuSO4 is anhydrous copper sulfate.

Aim: To show crystalline salts contain water of crystallization.

Material Required: CuSO4.5H2O (Blue vitriol), boiling tube, burner, cork,

delivery tube, test tube, clamp stand.

Procedure: 1.Take 2g of CuSO4.5H2O in a boiling tube fitted in a clamp stand.

2.Observe its color. Fit it with a cork and delivery tube bent at two right angles which dip into a test tube.

3.Heat crystals in a boiling tube.

4.Observe vapors being condensed in the test tube.

5.Cool the crystals and add a few drops of water into it.

Observation:Water vapors get condensed in a test tube and color

of blue crystals changes into white. On adding water to anhydrous copper sulfate it changes into blue again.

Chemical Reaction :

Conclusion : Crystalline substances have water of crystallization which are lost on heating. When we add water inCuSO4till a saturated solution is formed. On cooling, it gets converted into CuSO4.5H2Ocrystals and it shows that both are inter convertible.

.Describe an activity with a diagram to illustrate that the reaction of metal carbonates and metal bicarbonates with acids produces carbon dioxide. Write the relevant equations of all the reactions that take place. Name any two forms in which calcium carbonate is found in nature.

Answer.

Aim: To show acid reacts with metal carbonate to liberate carbon dioxide,

Material Required: CaCO3(marble chips), Woulfe-bottle, thistle funnel, dil. HCl, gas jar, matchbox, delivery tube bent at two right angles, lime water.

Procedure:

1.Take marble chips in a Woulfe-bottle.

2.Set the apparatus as shown in the diagram.

3.Add dil. HCl with the help of thistle funnel.

4.Collect the gas in a gas jar by upward displacement of air.

5.Bring a burning matchstick near the gas jar and record your observations.

6.Pass the gas evolved through lime water and note down your observations. Observations: The burning matchstick will get extinguished because carbon dioxide is neither combustible nor a supporter of combustion. Lime water will turn milky due to the formation of insoluble calcium carbonate. It can be used as a test for CO2 .The chemical reactions taking place are as follows:

Conclusion: Metal carbonates react with dilute acids to liberate carbon dioxide. Limestone, chalk, and marble are different forms of calcium carbonate. All metal carbonates and hydrogen carbonates react with acids to form corresponding salts, water and carbon dioxide.

Identify the acid and the base whose combination forms the common salt that you use in your food. Write the formula and chemical name of this salt. Name the source from where it is obtained.

(b) What is rock salt? Mention its color and the reason due to which it has this color.

(c) What happens when electricity is passed through brine? Write the chemical equation for it.

Answer.

(a) HCl is acid and NaOH is the base whose combination forms the common salt. Its formula is NaCl (Sodium chloride). It is obtained from sea water.

(b) Rock salt is the common name for the mineral “halite”. Its chemical formula is NaCl.

It may be white or light blue or yellow depending upon impurities present in it.

 Explain why hydrochloric acid is a strong acid and acetic acid, a weak acid. How can it be verified?

(ii) Explain why an aqueous solution of an acid conducts electricity.

(iii) You have four solutions A, B, C and D. The pH of solution A is 6, B is 9, C is 12 and D is 7,

(a) Identify the most acidic and most basic solutions.

(b) Arrange the above four solutions in the increasing order of H+ ion concentration.

(c) State the change in color of pH paper on dipping in solution C and D.

Answer.

(i) HCl is completely ionized in aqueous solution whereas acetic acid is partially ionized in aqueous solution. HCl gives dark red color with pH paper whereasCH3COOH gives orange color

(ii) It is because acid ions in aqueous solution and these ions conduct electricity. (Hi) (a) ‘A’ is most acidic and ‘C’ is most basic.

(b) C (10-12) < B (10-9) < D (10-7) < A (10-6)

(c) pH paper will become blue in ‘C’ and green in ‘D’.

 Dry pellets of a base ‘X’ when kept in bpen absorbs moisture and turns sticky. The compound is also formed by the chlor-alkali process. Write the chemical name and formula of X. Describe the chlor-alkali process with a balanced chemical equation. Name the type of reaction that occurs when X is treated with dilute hydrochloric acid. Write the chemical equation. (ii) While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid?

Answer.

(i) X is sodium hydroxide, NaOH.

When sodium chloride solution (brine solution) is electrolysed, sodium hydroxide solution is formed. H2 and Cl2 gasses are liberated. This is the chlor-alkali process.

The above reaction is a neutralization reaction.

(ii)It is because the process is highly exothermic. If water is added to acid, the bottle of acid will break.

A compound which is prepared from gypsum has the property of hardening when mixed with the proper quantity of water.

(i) Identify the compound.

(ii) Write the chemical equation for its preparation.

(iii) Mention one important use of this compound.

Answer.

58.Name the products formed in each case when

(a) hydrochloric acid reacts with caustic soda.

(b) granulated zinc reacts with caustic soda.

(c) carbon dioxide is passed into lime water.

Answer.