NCERT Solutions for Class 10 Science CBSE Chapter 2: Acids, Bases and Salts

Access Answers of NCERT class 10 Science Chapter 2 – Acids, Bases and Salts Exercise

Q.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?

Ans: The content of the test tube would be identified by the change in the colour of red litmus paper. For example, when we wet the red litmus paper with the basic solution, it changes into blue colour. Put the changed litmus paper in the solution which turns blue to red, will be the acidic solution. The solution which has no effect on the litmus paper will be neutral and hence it will be distilled water.

Q.2 What is the common name of the compound CaOCl2?

Ans: Common name of CaOCl2 is bleaching powder.

Q.3 Name the substance which on treatment with chlorine yields bleaching powder

Ans: Calcium hydroxide [Ca(OH)2] yields bleaching powder when treated with chlorine.

Q.4 Name the sodium compound which is used for softening hard water.

Ans: Sodium carbonate (Washing soda) is used for softening the hard water

Q.5 What will happen if a solution of sodium hydrocarbonate is heated? Give the equation of the reaction involved.

Ans: When solution of sodium hydrocarbonate is heated, sodium carbonate, water and carbon dioxide gas is formed.

2NaHCO₂ ⇨ Na₂CO₃ + CO₂ + H₂O

Q.6 Write an equation to show the reaction between Plaster of Paris and water.

Ans: CaSO₄. ½ H₂O + ½ H₂O ⇨ CaSO₄.2H₂O

Q.7 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?

Ans: Step 1: Let us mark the three test tubes as A, B, and C.

Step 2: A drop of the solution in test tube A is put on the red litmus paper. Same is repeated with solutions B and C. If either of them changes colour to blue, then it is basic. Therefore, out of three, one is eliminated.

Step 3: Out of the remaining two, any one can be acidic or neutral. Now a drop of basic solution is mixed with a drop of each of the remaining two solutions separately and then a drop of each solution is put on the red litmus paper.

If the colour of red litmus turns blue, then that solution is neutral and if there is no change in colour, then that solution is acidic. This is because acidic and basic solutions neutralise each other. Hence, we can distinguish between the three types of solutions.

Q.8 Why should curd and sour substances not be kept in brass and copper vessels?

Ans:Curd and other sour substances contain acids. Therefore, when they are kept in brass and copper vessels, metals react with the acids to form harmful toxic products, thereby spoiling the food and damaging our health.

Q.9 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?

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

Example:

Procedure:

Step 1: Take few pieces of zinc granules and add 5 ml of dilute H2SO4.

Step 2: Shake it and pass the gas produced into a soap solution. Bubbles are formed in the soap solution. These soap bubbles contain hydrogen gas.

Zn + dil. H₂SO₄ ⇨ZnSO₄ + H₂

Test for hydrogen gas: The evolved hydrogen gas can be tested by bringing a burning candle near the soap bubbles. Hydrogen gas burns with a pop sound.

Q.10 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.

Ans: Metal compound A should be a compound of Ca as a calcium salt is formed in the product. We know that CO2 extinguishes a burning candle. Also, since carbon dioxide is liberated, therefore, it must be a carbonate. Hence, it is calcium carbonate.

CaCO₃(S) + 2HCl (aq)⇨CaCl₂(S) + CO₂(g) + H₂O (l)

Q.11 Why do HCl, HNO₃, etc., show acidic characters in aqueous solutions while solutions of compounds like alcohol and glucose do not show acidic character?

Ans:HCl and HNO₃ show acidic character because they dissociate in the presence of water to form hydrogen or hydronium ions.Although aqueous solutions of glucose and alcohol contain hydrogen, these do not dissociate in water to form hydrogen or hydronium ions. Hence, they do not show acidic character.

Q.12 Why does an aqueous solution of acid conduct electricity?

Ans:Acids dissociate in aqueous solutions to form charged particles called 'ions'. These ions are responsible for conduction of electricity.

Q.13 Why does dry HCl gas not change the colour of the dry litmus paper?

Ans:Dry HCl gas does not change the colour of the dry litmus paper because it does not contain hydrogen or hydronium ions.

Q.14 While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid?

Ans:It is recommended that the acid should be added to water and not water to the acid because the process of dissolving an acid in water is exothermic. If water is added to acid, since large amount of acid is present, a large amount of heat is generated at once. Hence, the mixture can splash out and cause burns. But, if acid is added to water, then heat is evolved gradually and easily absorbed by the large amount of water.

Q.15 How is the concentration of hydronium ions (H₃O+) affected when a solution of an acid is diluted?

Ans:When an acid is diluted, the concentration of hydronium ions (H₃O+) per unit volume decreases. This means that the strength of the acid decreases.

Q.16 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-) would increase when excess base is dissolved in a solution of sodium hydroxide.

Q.17 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?

Ans:A pH value of less than 7 indicates an acidic solution, while greater than 7 indicates a basic solution. The pH of a solution is inversely proportional to its hydrogen ion concentration. Hence, the solution having lower pH will have more hydrogen ion concentration. Therefore, the solution with pH = 6 is acidic and has more hydrogen ion concentration than the solution of pH = 8 which is basic.

Q.18 What effect does the concentration of H+(aq) ions have on the nature of the solution?

Ans:If the concentration of H+ ions is increased, the solution becomes more acidic. If the concentration of H+ ions is decreased, the solution becomes less acidic or more basic.

Q.19 Do basic solutions also have H+(aq) ions? If yes, then why are these basic?

Ans:Yes, a basic solution also has H+(aq) ions. These come from the ionization of water in which the base is dissolved. However, their concentration is less as compared to the concentration of OH-(aq) ions that makes the solution basic.

Q.20 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)?

Ans:If the soil is too acidic and improper for cultivation, then to neutralise the acidity of the soil, the farmer would treat the soil with bases like quick lime or slaked lime or chalk.

Q.21 What is the common name of the compound CaOCl₂?

Ans:The common name of the compound CaOCl₂ is bleaching powder.

Q.22 Name the substance which on treatment with chlorine yields bleaching powder?

Ans:Calcium hydroxide or slaked lime [Ca(OH)₂], on treatment with chlorine, yields bleaching powder.

Q.23 Name the sodium compound which is used for softening hard water.?

Washing soda (Na₂CO₃.10 H₂O) is used for softening hard water.

Q.24 What will happen if a solution of sodium hydrocarbonate is heated? Give the equation of the reaction involved.

When a solution of sodium hydrogencarbonate is heated, sodium carbonate and water are formed with the evolution of carbon dioxide gas.

Q.25 Write an equation to show the reaction between Plaster of Paris and water.

Ans:The chemical equation for the reaction of Plaster of Paris and water can be represented as:

Q.26 Write an equation to show the reaction between Plaster of Paris and water.

Ans: The chemical equation for the reaction of Plaster of Paris and water is

CaSO₄.1/2H₂O + 3/2H₂O → CaSO₄.2H₂O

Q.27 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

Ans:HCl

Q.28 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 of HCl solution (the same solution as before) required to neutralise it will be

(a) 4 mL

(b) 8mL

(c) 12 mL

(d) 16 mL

Ans:(d) 16 mL

Q.29 Which one of the following types of medicines is used for treating indigestion?

       (a) Antibiotic

      (b) Analgesic

      (c) Antacid

       (d) Antiseptic

Ans:(c) Antacid

Q.30 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

Ans:

(a) Sulphuric acid + Zinc ⇨ Zinc sulphate + Hydrogen

H₂S0₄(aq) + Zn(s) ⇨ ZnSO₄(aq) + H₂(g)

(b) Hydrochloric acid + Magnesium ⇨ Magnesium chloride + Hydrogen

2HCl (aq) + Mg (s) ⇨ MgCl₂(aq) + H₂(g)

(c) Sulphuric acid + Aluminium ⇨ Aluminium sulphate + Hydrogen

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

(d) Hydrochloric acid + Iron ⇨ Ferric chloride + Hydrogen

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

Q.31 Compounds such as alcohols and glucose also contain hydrogen but are not categorized as acids. Describe an activity to prove it.

Ans:

Activity Procedure:

Step 1: Two nails are fitted on a cork and are kept it in a 100 mL beaker.

Step 2: The nails are then connected to the two terminals of a 6-volt battery through a bulb and a switch.

Step 3: Some dilute HCl is poured in the beaker and the current is switched on.

Step 4: 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 glucose and alcohol solution. HCl dissociates into H+ (aq) and Cl- (aq) ions. These ions conduct electricity in the solution which results in the glowing of the bulb. On the other hand, glucose and alcohol solution do not dissociate into ions. Therefore, they do 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 categorised as acids.

Q.32 Why does distilled water not conduct electricity, whereas rain water does?

Ans: Distilled water is a pure form of water and is devoid of any ionic species. Therefore, it does not conduct electricity. When rain water falls to the earth, it dissolves an acidic gas 'carbon dioxide' from the air and forms carbonic acid. This acid provides some hydrogen ions and carbonate ions to rain water. Hence, due to presence of these ionic species, rain water conducts electricity.

Q.33 Why do acids not show acidic behaviour in the absence of water

Ans: Acids do not show acidic behaviour in the absence of water because there are no hydrogen ions available for conduction of electricity. Acids dissociate in the presence of water to give free hydrogen ions. It is the hydrogen ions that are responsible for the acidic behaviour.

Q.34 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.

Ans:

(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

pH is inversely proportional to hydrogen ion concentration. Hence, the pH can be arranged in the increasing order of the concentration of hydrogen ions as: 11 < 9 < 7 < 4 < 1

Q.35 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?

Ans: 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 contains a much greater amount of hydrogen ions; therefore it produces hydrogen gas at a faster speed due to which fizzing occurs.

Q.36 Fresh milk has a pH of 6. How do you think the pH will change as it turns into curd? Explain your answer.

Ans: The pH of milk is 6. As it changes to curd, the pH will reduce because curd is acidic in nature due to formation of lactic acid.

Q.37 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?

Ans: (a) The milkman shifts the pH of the fresh milk from 6 to slightly alkaline because in alkaline condition, milk does not get sour easily.

(b) Since this milk is slightly basic than usual milk, lactic acid produced to set the curd is neutralised by the base. Therefore, it takes a longer time for the curd to set.

Q.38 Plaster of Paris should be stored in a moisture-proof container. Explain why?

Ans: Plaster of Paris should be stored in a moisture-proof container because Plaster of Paris, a powdery mass, absorbs water (moisture) to form a hard solid known as gypsum.

Q.39 What is a neutralisation reaction? Give two examples.

Ans: A reaction in which an acid and base react with each other to give a salt and water is termed as neutralisation reaction. In this reaction, energy is evolved in the form of heat.

For example:

(i) NaOH (base) + HCl (acid)⇨ NaCl (salt) + H₂O (water)

(ii) During indigestion (caused due to the production of excess of hydrochloric acid in the stomach), we administer an antacid (generally milk of magnesia, Mg(OH)₂sub> which is basic in nature). The antacid neutralises the excess of acid produced and thus gives relief from indigestion.

Mg(OH)₂ + 2 HCl⇨ MgCl₂ + 2H₂O

Q.40 Give two important uses of washing soda and baking soda.

Ans: Two important uses of washing soda and baking soda are as follows:

(1) Washing soda:

(a) It is used in glass, soap, and paper industries.

(b) It is used to remove permanent hardness of water.

(2) Baking soda:

(a) 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 gas that makes bread or cake fluffy.

(b) It is used in soda-acid fire extinguishers.