Ch 2 Science Class 10 Acids Bases And Salts Questions and Answers

NCERT Solutions for Class 10 Science Chapter-2 Acids, Bases and Salts Questions and Answers

INTEXT QUESTION – Page No.- 18

Q1. You have been provided with three test-tubes  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. (i). Put the red litmus paper in all the test-tubes turn by turn. The solution which turns red litmus to blue will be a basic solution. The blue litmus paper formed here can now be used to test the acidic solution.

(ii). Put the blue litmus paper (obtained above) in the remaining test tubes one by one. The solution which turns the blue litmus paper to red will be the acidic solution.

(iii). The solution which has no effect on any litmus paper will be neutral and hence it will be distilled water.

INTEXT QUESTION – Page No.-22

Q1. Why should curd and sour substances not be kept in brass and copper vessels?

Ans. Curd and other substances contain acids which can react with the metal of brass and copper vessels to form toxic (poisonous) metal compound which can cause food poisoning and damage our health.

Q2. Which Gas is usually liberated when an acid react with a metal? illustrate with the help of an example. How will you test the presence of this gas?

Ans. When an acid reacts with a metal, then Hydrogen gas is liberated. For example: When dilute Sulphuric acid reacts with Zinc granules, then Hydrogen gas is liberated and zinc sulphate solution is formed.

Zn(s) + H2SO₄(aq) → ZnSO₄(aq) + H₂(g)

The presence of Hydrogen gas is tested by burning it. When a lighted match stick is thought near a gas jar containing Hydrogen gas, it burns with a pop sound making a little explosion.

Q3. 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 of the compounds formed is calcium chloride.

Ans. The gas that extinguishes a candle is carbon dioxide which is formed by the action of dilute hydrochloric acid On a metal carbonate (or metal hydrogen carbonate ) and produces effervescence. Now, since one of the compounds formed is Calcium Chloride, it shows that the metal compound is Calcium Carbonate (it cannot be Calcium Hydrogen carbonate because calcium hydrogen carbonate found only in solution, it is too unstable to as a solid).

Thus, the metal compound A is calcium carbonate (CaCO3) Calcium carbonate reacts with dilute hydrochloric acid to form calcium chloride, carbon dioxide and water.

This can be written as:

CaCO₃(s) + HCl(aq) →  CaCl₂(aq) + CO₂(g) + H₂0(l)

INTEXT QUESTION – Page No.25

Q1. Why do HCl, HNO₃, etc., show acidic character in aqueous solutions while solutions of compounds like alcohol and glucose do not show acidic character?

Ans. The acidic character of a substance is due to the presence of hydrogen ions [H⁺ (aq) ions]  in its aqueous solution (water solution). HCl and HNO₃ show acidic properties because they produce hydrogen ions when dissolved in water. The solutions of compounds like alcohol and glucose do not show acidic character because they do not ionise (or dissociate) in water to produce hydrogen ions.

Q2. Why does an aqueous solution of an acid conduct electricity ?

Ans. The aqueous solution of an acid conducts electricity due to the presence of charged particles called ions in it.

For example: When hydrochloric acid (HCl) is dissolved in water, then its aqueous solution contains hydrogen ions [H ⁺ (aq)] and chloride ions [Cl^– (aq)]. These ions carry electric currents. So, due to the presence of H⁺ (aq) ions and Cl^– (aq) ions, a water solution of hydrochloric acid conduct electricity.

Q3. Why does dry HCl gas not change the colour of dry litmus paper?

Ans. Dry HCl does not change the colour of dry litmus paper because it has no hydrogen ions, H⁺ (aq) ions, in it which can impart acidic properties to it.

Q4. While diluting an acid, why it recommended that the acid should be added to water and not water to the acid?

Ans. Concentrated acid is diluted by adding water to it. The process of mixing water to a concentrated acid is highly exothermic in which a large amount of heat is evolved:

(i) When the concentrated acid is added to water, then the heat is evolved gradually and easily absorbed by the large amount of water.

(ii) If water is added to a concentrated acid, then a large amount of heat is evolved at once. This heat changes some of the water to steam explosively which can splash the acid on our face or clothes and cause acid burns.

Q5. How is the concentration of hydronium ions (H3O⁺) effect when the solution of an acid is diluted?

Ans. On diluting an acid, the concentration of hydronium ions (H3O⁺) in it decreases.

Q6. How is the concentration of hydroxide ions (OH^–) affected when the excess of the base is dissolved in a solution of sodium hydroxide?

Ans. On dissolving more sodium hydroxide base in some solution, the concentration of hydroxide (OH^–) increases.

INTEXT QUESTIONS Page No. – 28

Q1. You have two solutions A and B. The pH of solution A is 6 and the pH solution B is 8. Which solution has more hydrogen ions concentration ? Which of these is acidic and which one basic?

Ans. We know that the pH of a solution is inversely proportional to its hydrogen ion concentration.This means that the solution having lower pH will have more hydrogen ion concentration. In this case, solution A (having a lower pH of 6) will have more hydrogen ion concentration.

Solution A is acidic and solution B is basic.

Q2. What effect does the concentration of H⁺ (aq) ions have on the nature of the solution?

Ans. Higher the concentration of H⁺ (aq) ions in a solution, more acidic the solution will be.

Q3. Does the basic solution also have H+(aq) ions? If yes, then why are these basic?

Ans. Even the basic solutions have hydrogen ions [H⁺ (aq)] in them which come from the ionisation of water in which the base is dissolved. They are basic because the concentration of hydroxide ions [OH^–ions] in them is much more than the concentration of hydrogen ions.

Q4. Under what soil condition do you think a farmer would treat the soil of his fields with quicklime (calcium oxide) or slaked lime (calcium hydroxide) or chalk (Calcium carbonate)?

Ans. A farmer which treat this soil of his fields with quicklime (calcium oxide), or slaked lime (calcium hydroxide) or chalk (calcium carbonate) if the soil is too acidic having low pH value. All these material analyses and hence react with the less acid present in the soil and reduce its acidity.

INTEXT QUESTION – Page No – 33

Q1. What is the common name of the compound CaCl₂?

Ans. Bleaching powder.

Q2. Name the substance which on treatment with chlorine yields bleaching powder?

Aus. Slaked lime

Q3. Name the sodium compound which is used for softening hard water?

Ans. Sodium carbonate (Washing soda).

Q4. What will happen if a solution of sodium hydrogen carbonate heated? Give the equation of the reaction involved?

Ans. When a solution of sodium hydrogen carbonate is heat it decomposes to form sodium carbonate with the evolution of carbon dioxide gas. The equation of the reaction involved is:

2NaHC₃ → Na₂CO₃ + CO₂ + H₂O

Q5. Write an equation the show the reaction between Plaster of Paris and water ?

Ans. CaSO₄.1/2H₂O +3/2 H₂O → CaCa₄.2H₂O

NCERT EXERCISE

Q1. A solution turns red litmus blue, its pH is likely to be:

1. 1
2. 4
3. 5
4. 10

Ans. (d) If a solution turns red litmus blue then the solution is basic in nature and its pH value is likely to be greater than 7.

Q2. A solution reacts with crushed egg-shell to be given that turns lime-water milky. The solution contains:

1. NaCl
2. HCl
3. LiCl
4. KCl

Ans. (b) The crushed eggshell consists of layer of calcium carbonate which reacts with dilute HCl to evolve CO₂(g). The CO₂ gas turns lime water milky.

Q3. 10mL of a solution of NaOH is found to be completely neutralised by 8 mL of a given solution of NaOH, the amount of HCl solution required to neutralise, it will be:

(a) 4mL

(b) 8 mL 

(c) 12mL

(d) 16 mL

Ans. (d) if we take double the amount same HCl solution required to neutralise it is also doubled. Hence, 16ml of HCl solution will be required to neutralise 20mL NaOH solution.

Q4. Which one of the following types of medicine is used for treating indigestion?

(a) Antibiotic

(b) Analgesic

(c) Antacid

(d) Antiseptic

Ans. (c) Antacid contain solution of mild base and hence they are used for treating indigestion.

Q5. Write a word equation and then a balanced equation for the reaction taking place when:

Ans. 1. dilute sulphuric acid reacts with zinc granules.

Sulphuric acid + Zinc → Zinc sulphate + Hydrogen

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

2. dilute hydrochloric acid reacts with magnesium ribbon.

Ans. Hydrochloric acid + Magnesium → Magnesium chloride + Hydrogen

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

3. dilute sulphuric acid reacts with aluminium power.

Ans. sulphuric acid + Aluminum → Aluminum sulphate + Hydrogen

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

4. dilute hydrochloric acid reacts with iron fillings.

Ans. Hydrochloric acid + Iron → Iron (II) chloride + Hydrogen

2 HCL (aq) + Fe (s) → FeCl₂ (aq) + H₂ (g)

Q6. Compounds such as alcohols and glucose also contain hydrogen but are not categorised as acid. Describe an activity to prove it?

Ans. Alcohol and glucose contain hydrogen but are not categorised as acids because they do not ionise in the solution to produce H+(aq) ions and hence can’t conduct electricity.

Activity: set the apparatus as shown below and test for the solution of HCl, alcohol and glucose.

Diagram

Observation:

(i) The bulb glows when dilute HCl was added to beaker.

(ii) The bulb did not glow when alcohol and glucose solution was added to the beaker.

Conclusion:

(i) Alcohol and glucose can’t form ions in solution and hence, can’t conduct electricity.

(ii) Dilute HCl solution form (aq) H⁺ ions and hence, can conduct electricity suggesting that electric current is carried through the solution by ions formed by them.

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

Ans. Distilled water does not contain any ions and have no electrolytes. Hence, it does not conduct electricity. But in rainwater, a small amount of electrolyte is present in the form of acid. These acids are produced due to the dissolution of acidic oxides (eg. SO2, NO₂) in rainwater and make it a better conductor of electricity.

Q8. Why do not acids show acidic behaviour in the absence of water?

Ans. Ions are produced only in aqueous solution and presence of H+ ions are responsible for the existence of acidic properties. Hence, acid show acidic behaviour only in the presence of water and not in its absence.

Q9. Five solution 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 H+ ion concentration.

Ans.

(a) Neutral = D (pH = 7)

(b) strongly alkaline = C (pH = 11)

(c) strongly acidic =B (pH = 1)

(d) weakly acidic = A (ph = 4)

(e) weakly alkaline = E (pH=9)

pH in increasing order of H⁺ ion concentration is C<E<D<A<B.

Q10. Equal lengths of magnesium ribbon 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.(i). Reaction occurs in test tube A:

2HCl(aq) + Mg(s) –> MgCl2(aq) +H2(g)

(ii). Reaction doors in test tube B.

2 CH3 COOH (aq) + Mg(s) –> (CH3COO)2Mg(aq) + H2(g)

Fizzing occurs in both the test tubes due to the evolution of H2 gas but it is faster in test tube A because of HCl (strong acid) as compared to B as CH3COOH is not completely dissociated (weak acid).

Q11. Fresh milk has a ph 6. How do you think the pH will change as it turns into curd? explain.

Ans. pH will decrease from t because during und formation lactic acid is produced which makes it acidic then milks changes into curd its pH changes in and becomes less than 6.

Q12. A milkman adds a very small amount of baking soda to fresh milk.

(i). Why does he shift the pH of the fresh milk from 6 to slightly alkaline?

Ans. Fresh milk is slightly acidic naturally. Fresh milk is made slightly alkaline by
adding a little of baking soda so that it may not get sour easily due to the
formation of lactic acid in it.

(ii). Why does this milk take a longer time to set as curd?

Ans. The slightly alkaline milk takes a longer time to set into curd because the lactic acid being formed during curdling has to first neutralise the alkali present in it.

Q13. Plaster of Paris should be stored in moisture proof containers. Explain why?

Ans. Plaster of Paris should be stored in a moisture-proof container because the presence of moisture (or water) can cause its slow setting by bringing about its hydration. That will make the plaster of Paris useless at same time.

Q14. What is neutralisation reaction ? Give two examples?

Ans. The reaction between an acid and base to form salt and water is called neutralisation reaction.

Acid + Base –> Salt +Water

For example:

HNO3(aq) + KOH (aq) –> KNO3 (aq) + H2O(l)
HCl (aq) + NaOH (aq) –> NaCl (aq) + H₂O (l)

Q15. Give two important uses of washing soda and baking soda?

Ans. Uses of washing soda are:

(i). it is used to remove the permanent hardness of the water.

(ii). It is used as a cleansing agent.

Uses of baking soda are:

(i). It is used in bakery.

(ii). It is used for extinguishing fire (in soda and fire extinguishers).