CBSE Class X Science Chapter 2: Acids, Bases and Salts Questions and Answers

Section 2.1: Understanding the Chemical Properties of Acids and Bases

In-Text Questions (Page 18)

Q1. 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?

To identify the contents of each test tube using only red litmus paper, follow these steps:

1. Dip the red litmus paper into each of the three test tubes one by one. The test tube that turns the red litmus paper blue contains the basic solution.
2. Take the litmus paper that has now turned blue and dip it into the remaining two test tubes. The solution that turns the blue litmus paper back to red is the acidic solution.
3. The remaining test tube, which causes no colour change on either red or blue litmus paper, contains distilled water.

Figure 2.1: Reaction of zinc granules with dilute sulphuric acid and testing hydrogen gas by burning.

Figure 2.2: Passing carbon dioxide gas through calcium hydroxide solution.

In-Text Questions (Page 22)

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

Curd and sour substances contain acidic materials (such as lactic acid). Acids react with metals like copper and brass to form poisonous metal salts and release hydrogen gas. These compounds render the food toxic and harmful for consumption.

Q2. 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?

Gas Liberated: Hydrogen gas ($H_2$) is usually liberated when an acid reacts with a metal.

Example: When zinc granules react with dilute sulphuric acid, zinc sulphate salt and hydrogen gas are formed:

Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g)

Test for Hydrogen Gas: Pass the gas being evolved through a soap solution so that soap bubbles filled with the gas are formed. Bring a burning candle near a gas-filled bubble; the gas burns with a characteristic pop sound.

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 if one of the compounds formed is calcium chloride.

The gas that extinguishes a burning candle and causes effervescence is carbon dioxide ($CO_2$). Since carbon dioxide is produced and one of the products is calcium chloride ($CaCl_2$), metal compound A must be calcium carbonate ($CaCO_3$).

Balanced Chemical Equation:

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

Section 2.2: What Do All Acids and All Bases Have in Common?

Figure 2.3: Acid solution in water conducts electricity.

Figure 2.4: Preparation of HCl gas.

Figure 2.5: Warning sign displayed on containers containing concentrated acids and bases.

In-Text Questions (Page 25)

Q1. Why do HCl, $HNO_3$, etc., show acidic characters in aqueous solutions while solutions of compounds like alcohol and glucose do not show acidic character?

Compounds like HCl and $HNO_3$ dissociate in aqueous solution to produce hydrogen ions ($H^+$ or $H_3O^+$), which are responsible for their acidic properties. In contrast, compounds such as alcohol and glucose do not ionise or dissociate in water to release $H^+$ ions, and therefore do not exhibit acidic character.

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

An aqueous solution of an acid conducts electricity because the acid dissociates in water to form free-moving cations ($H^+$ ions) and anions. These mobile ions carry electric current through the liquid solution.

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

Dry HCl gas does not dissociate to produce hydrogen ions ($H^+$) in the absence of water. Since acidic behaviour and colour changes in litmus paper require the presence of $H^+$ or $H_3O^+$ ions, dry HCl gas shows no effect on dry litmus paper.

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

The dissolution of a concentrated acid in water is a highly exothermic process. If water is added to a concentrated acid, the sudden generation of a large amount of heat may cause the mixture to splash out and cause severe acid burns, or break the glass container. Adding acid slowly to water with continuous stirring allows the large body of water to safely absorb the heat produced.

Q5. How is the concentration of hydronium ions ($H_3O^+$) affected when a solution of an acid is diluted?

When an acidic solution is diluted by adding water, the volume of the solution increases, resulting in a decrease in the concentration of hydronium ions ($H_3O^+$) per unit volume.

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

When excess base is dissolved in a solution of sodium hydroxide, more hydroxide ions ($OH^-$) are released into the solution, causing the overall concentration of $OH^-$ ions per unit volume to increase.

Section 2.3: How Strong Are Acid or Base Solutions?

Figure 2.6: Variation of pH with the change in concentration of $H^+(aq)$ and $OH^-(aq)$ ions.

Figure 2.7: pH of some common substances shown on a pH paper.

In-Text Questions (Page 28)

Q1. 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?

Hydrogen Ion Concentration: Solution A (pH = 6) has a higher hydrogen ion ($H^+$) concentration than Solution B (pH = 8), because pH is inversely proportional to $H^+$ ion concentration.

Nature of Solutions:

• Solution A (pH = 6) is acidic (pH < 7).
• Solution B (pH = 8) is basic (pH > 7).

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

The concentration of $H^+(aq)$ ions determines the acidity of a solution. A higher $H^+$ ion concentration makes the solution more acidic (lower pH), whereas a lower $H^+$ ion concentration makes the solution less acidic or basic (higher pH).

Q3. Do basic solutions also have $H^+(aq)$ ions? If yes, then why are these basic?

Yes, basic solutions also contain $H^+(aq)$ ions. However, they are basic because the concentration of hydroxide ions ($OH^-(aq)$) is significantly greater than the concentration of hydrogen ions ($H^+(aq)$).

Q4. 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)?

A farmer treats the soil with quick lime (calcium oxide), slaked lime (calcium hydroxide), or chalk (calcium carbonate) when the soil is too acidic (low pH) for optimal crop growth. These calcium compounds are basic in nature and neutralise the excess soil acidity.

Section 2.4: More About Salts

Figure 2.8: Important products from the chlor-alkali process.

Figure 2.9: Removing water of crystallisation.

In-Text Questions (Page 33)

Q1. What is the common name of the compound $Ca(ClO)_2$ / $CaOCl_2$?

The common name of $CaOCl_2$ (or $Ca(ClO)_2$) is Bleaching powder.

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

Dry slaked lime [calcium hydroxide, $Ca(OH)_2$] yields bleaching powder when treated with chlorine gas.

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

Washing soda (Sodium carbonate decahydrate, $Na_2CO_3 \cdot 10H_2O$) is used for removing permanent hardness of water.

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

When a solution of sodium hydrogencarbonate (baking soda) is heated, it decomposes to produce sodium carbonate, water, and carbon dioxide gas.

Chemical Equation:

2NaHCO₃ --(Heat)--> Na₂CO₃ + H₂O + CO₂

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

Plaster of Paris absorbs water and sets into a hard solid mass of gypsum.

Chemical Equation:

CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O

(Plaster of Paris + Water → Gypsum)

End-of-Chapter Exercises

Q1. A solution turns red litmus blue, its pH is likely to be
(a) 1    (b) 4    (c) 5    (d) 10

Answer: (d) 10

Explanation: Basic solutions turn red litmus paper blue and have a pH value greater than 7. Since 10 is the only value above 7, option (d) is correct.

Q2. 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

Answer: (b) HCl

Explanation: Crushed egg-shells contain calcium carbonate ($CaCO_3$). Hydrochloric acid (HCl) reacts with calcium carbonate to produce carbon dioxide gas ($CO_2$), which turns lime water milky.

Q3. 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

Answer: (d) 16 mL

Explanation: Since 10 mL of NaOH requires 8 mL of HCl for complete neutralisation, doubling the volume of NaOH to 20 mL will require double the volume of HCl solution: $2 \times 8\text{ mL} = 16\text{ mL}$.

Q4. Which one of the following types of medicines is used for treating indigestion?
(a) Antibiotic    (b) Analgesic    (c) Antacid    (d) Antiseptic

Answer: (c) Antacid

Explanation: Indigestion is caused by excess acid in the stomach. Antacids are mild bases that neutralise excess stomach acid to provide relief.

Q5. 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.
(a) dilute sulphuric acid reacts with aluminium powder.
(d) dilute hydrochloric acid reacts with iron filings.

(a) Dilute sulphuric acid + Zinc granules:

Word Equation: Sulphuric acid + Zinc → Zinc sulphate + Hydrogen

Balanced Equation: H₂SO₄(aq) + Zn(s) → ZnSO₄(aq) + H₂(g)

(b) Dilute hydrochloric acid + Magnesium ribbon:

Word Equation: Hydrochloric acid + Magnesium → Magnesium chloride + Hydrogen

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

(c) Dilute sulphuric acid + Aluminium powder:

Word Equation: Sulphuric acid + Aluminium → Aluminium sulphate + Hydrogen

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

(d) Dilute hydrochloric acid + Iron filings:

Word Equation: Hydrochloric acid + Iron → Iron(II) chloride + Hydrogen

Balanced Equation: 2HCl(aq) + Fe(s) → FeCl₂(aq) + H₂(g)

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

Activity Setup:

1. Prepare separate aqueous solutions of glucose, alcohol, hydrochloric acid, and sulphuric acid.
2. Fix two iron nails on a rubber cork and place it inside a 100 mL beaker.
3. Connect the nails to two terminals of a 6V battery through an electric bulb and a switch using connecting wires.
4. Pour dilute HCl solution into the beaker and switch on the electric current. Observe if the bulb glows.
5. Repeat the procedure separately with sulphuric acid solution, glucose solution, and alcohol solution.

Observation: The bulb glows brightly with acid solutions (HCl and $H_2SO_4$) but does not glow at all with glucose and alcohol solutions.

Conclusion: Acidic solutions conduct electricity because they produce free $H^+$ ions. Although glucose and alcohol contain hydrogen atoms, they do not ionise in water to form $H^+$ ions and therefore do not show acidic character.

Q7. Why does distilled water not conduct electricity, whereas rain water does?

Distilled water is pure water devoid of dissolved salts or ionic impurities, so it does not contain free ions to transport electric current. Rainwater absorbs atmospheric gases such as carbon dioxide ($CO_2$), which dissolve in it to form weak acids like carbonic acid ($H_2CO_3$). These dissociate into ions ($H^+$ and $HCO_3^-$), allowing rainwater to conduct electricity.

Q8. Why does dry HCl gas not show acidic behaviour in the absence of water?

Acidic behaviour is exhibited only when hydrogen ions ($H^+$ or hydronium ions $H_3O^+$) are present. HCl molecules can dissociate into $H^+$ ions only in the presence of water. In the absolute absence of water, dry HCl gas does not produce $H^+$ ions and hence shows no acidic properties.

Q9. 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.

Categorisation of Solutions:

• (a) Neutral: Solution D (pH = 7)
• (b) Strongly alkaline: Solution C (pH = 11)
• (c) Strongly acidic: Solution B (pH = 1)
• (d) Weakly acidic: Solution A (pH = 4)
• (e) Weakly alkaline: Solution E (pH = 9)

pH in Increasing Order of Hydrogen-Ion Concentration:

Higher hydrogen-ion concentration corresponds to lower pH value. Thus, the order from lowest $H^+$ concentration to highest $H^+$ concentration is:

11 < 9 < 7 < 4 < 1 (Solutions: C < E < D < A < B)

Q10. 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 ($CH_3COOH$) 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?

Fizzing will occur more vigorously in test tube A.

Reason: Hydrochloric acid (HCl) is a strong acid that ionises completely in water, producing a high concentration of $H^+$ ions. Acetic acid ($CH_3COOH$) is a weak acid that ionises only partially, yielding a much lower $H^+$ ion concentration. The reaction between magnesium and $H^+$ ions proceeds much faster in test tube A, resulting in a more rapid evolution of hydrogen gas.

Q11. Fresh milk has a pH of 6. How do you think the pH will change as it turns into curd? Explain your answer.

When fresh milk turns into curd, its pH value decreases below 6 (becomes more acidic). This occurs because bacteria convert lactose present in milk into lactic acid. The production of acid increases $H^+$ ion concentration, lowering the pH.

Q12. 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) Reason for shifting pH to slightly alkaline: Fresh milk is slightly acidic (pH = 6) and easily turns sour due to lactic acid formation. Adding baking soda (a mild base) shifts the pH to slightly alkaline, preventing the milk from spoiling quickly during storage or delivery.

(b) Reason for taking longer to set as curd: The lactic acid produced by bacteria must first neutralise the added alkali (baking soda) before reducing the pH to acidic conditions necessary to coagulate milk into curd.

Q13. Plaster of Paris should be stored in a moisture-proof container. Explain why?

Plaster of Paris ($CaSO_4 \cdot \frac{1}{2}H_2O$) absorbs moisture/water from the surroundings and reacts to form gypsum ($CaSO_4 \cdot 2H_2O$), which sets into a hard solid mass. Once converted into gypsum, it loses its setting properties and becomes useless. Therefore, it must be stored in moisture-proof containers.

Q14. What is a neutralisation reaction? Give two examples.

A reaction in which an acid reacts with a base to form salt and water by mutual nullification of their effects is called a neutralisation reaction.

General Equation: Acid + Base → Salt + Water

Example 1: Reaction of Hydrochloric Acid with Sodium Hydroxide

HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)

Example 2: Reaction of Sulphuric Acid with Potassium Hydroxide

H₂SO₄(aq) + 2KOH(aq) → K₂SO₄(aq) + 2H₂O(l)

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

Two Uses of Washing Soda ($Na_2CO_3 \cdot 10H_2O$):

1. Used as a cleaning agent for domestic purposes and in soap, glass, and paper industries.
2. Used for removing permanent hardness of water.

Two Uses of Baking Soda ($NaHCO_3$):

1. Used as an ingredient in antacids to neutralise excess stomach acidity.
2. Used in making baking powder (for baking soft cakes and bread) and in soda-acid fire extinguishers.

Figure 2.10: Soda-acid fire extinguisher setup showing ignition tube containing dilute sulphuric acid suspended in a wash-bottle with sodium hydrogencarbonate and resulting carbon dioxide discharge.