chapter 2 science class 10 solution

In-text Questions Set 1 – Page 18

Question: You are given three test tubes. The three test tubes contain distilled water, an acidic solution and the basic solution, respectively. There is only red litmus paper available in order to identify what is there in each test tube. How will you find out what is in each of the test tubes?

Solution: We can identify the content in each of the test tubes using red litmus paper. This can be done by noticing the colour change of the red litmus paper.
- On litmus paper, the three solutions in the test tubes are poured separately.
- The solution which turns red litmus to blue contains a basic solution.
- Divide the formed blue litmus paper into two parts.
- The solution from the test tube, which turns blue litmus paper to red, will be the acidic solution.
- The solution of the test tube, which does not change either red or blue litmus paper, contains water.
Note: After immediate distillation, distilled water has a pH of 7. However, within a few hours after distillation, it absorbs carbon dioxide from the atmosphere and turns slightly acidic with a pH of 5.8.

In-text Questions Set 2 – Page 22

Question: Why should curd and sour substances not be kept in brass and copper vessels?

Solution: Curd and sour food substances contain acids; these acidic substances combine with metal. This reaction turns food into poison, which damages people’s health.
Question: 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?

Solution: When an acid reacts with any metal, salt and hydrogen gas are formed.
Metal + Acid → Salt + Hydrogen gas
Question: 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.

Solution: As the metal compound released is Calcium Chloride, the gas evolved here is CO₂. Hence, metal A should be Calcium Carbonate. The reaction between Calcium Carbonate and HCl is:
CaCO₃ (s) + 2HCl (Aq) → CaCl₂ (Aq) + CO₂ (g) + H₂O (l)

In-text Questions Set 3 – Page 25

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

Solution: Release of H⁺ ion in water will make a compound acidic or non-acidic. Acids upon dissociating with water result in the production of Hydrogen ions. Some compounds show acidic character as they dissociate in the aqueous solution, resulting in the production of hydrogen ions (acids like HCl, HNO₃). Compounds like glucose or alcohol contain hydrogen, but their hydrogen does not separate to form hydrogen ions on dissolving in water.
Question: Why does an aqueous solution of acid conduct electricity?

Solution: Charged particles (ions) are responsible for the conductance of electricity in an acid.
Question: Why does dry HCl gas not change the colour of dry litmus paper?

Solution: HCl does not give out Hydrogen ions in dry state; therefore, HCl does not show acidic behaviour, and the colour of the litmus paper remains unchanged.
Question: While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid?

Solution: If water is added to concentrated acid, it releases a huge amount of heat which may result in splashing and burns. Hence, it is safe to add acid to water but not water to acid.
Question: How is the concentration of hydronium ions (H₃O⁺) affected when a solution of an acid is diluted?

Solution: On dilution, hydronium ions per volume of solution decrease, lowering hydronium concentration.
Question: How is the concentration of hydroxide ions (OH⁻) affected when excess base is dissolved in a solution of sodium hydroxide?

Solution: Hydroxide ion concentration increases until saturation; after that, adding more base does not increase it further.

In-text Questions Set 4 – Page 33

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

Solution: Lower pH means higher hydrogen ion concentration. Solution A has more hydrogen ion concentration and is acidic; B is basic.
Question: What effect does the concentration of H⁺ (aq) ions have on the nature of the solution?

Solution: Higher hydrogen ion concentration makes the solution acidic; lower concentration makes it basic.
Question: Do basic solutions also have H⁺ (aq) ions? If yes, then why are these basic?

Solution: Basic solutions have H⁺ ions, but the concentration of hydroxide ions is higher, making the solution basic.
Question: 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)?

Solution: If the soil is acidic (pH below 7), it should be treated with quick lime, slaked lime, or chalk.

In-text Questions Set 5 – Page 34–35

Question: What is the common name of the compound CaOCl₂?

Solution: Bleaching powder.
Question: Name the substance, which on treatment with chlorine, yields bleaching powder.

Solution: Calcium hydroxide.
Question: Name the sodium compound which is used for softening hard water.

Solution: Sodium carbonate.
Question: What will happen if a solution of sodium hydrocarbonate is heated? Give the equation of the reaction involved.

Solution: Heating sodium hydrocarbonate yields sodium carbonate, and carbon dioxide gas is liberated.
2NaHCO₃ → Na₂CO₃ + H₂O + CO₂
Question: Write an equation to show the reaction between Plaster of Paris and water.

Solution: The chemical equation for the reaction is:
CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O

Exercise Questions

Question: A solution turns red litmus blue, its pH is likely to be
(a) 1 (b) 4 (c) 5 (d) 10
Solution: The answer is 10 because blue litmus indicates a basic solution (pH more than 7).
Question: A solution reacts with crushed eggshells to give a gas that turns lime-water milky. The solution contains
(a) NaCl (b) HCl (c) LiCl (d) KCl
Solution: The answer is HCl.
Eggshells contain calcium carbonate, which on reaction with HCl liberates CO₂ gas, turning lime water milky.
CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂
Question: 10 mL of a solution of NaOH is completely neutralised by 8 mL of HCl. If we take 20 mL of NaOH, the amount of HCl required to neutralise will be
(a) 4 mL (b) 8 mL (c) 12 mL (d) 16 mL
Solution: 20 mL NaOH requires 16 mL HCl.
Question: Which one of the following types of medicines is used for treating indigestion?
(a) Antibiotic (b) Analgesic (c) Antacid (d) Antiseptic
Solution: Indigestion is treated by antacids.
Question: Write word equations and then balanced equations for the reactions:
- (a) Dilute sulphuric acid reacts with zinc granules.
  Dilute sulphuric acid + zinc → Zinc sulphate + hydrogen
  H₂SO₄(aq) + Zn → ZnSO₄(aq) + H₂(g)
- (b) Dilute hydrochloric acid reacts with magnesium ribbon.
  Dilute hydrochloric + magnesium → Magnesium chloride + hydrogen
  2HCl(aq) + Mg → MgCl₂(aq) + H₂(g)
- (c) Dilute sulphuric acid reacts with aluminium powder.
  Dilute sulphuric acid + aluminium → Aluminium sulphate + hydrogen
  3H₂SO₄(aq) + 2Al(s) → Al₂(SO₄)₃(aq) + 3H₂(g)
- (d) Dilute hydrochloric acid reacts with iron filings.
  Dilute hydrochloric acid + iron → Ferrous chloride + hydrogen
  6HCl(aq) + 3Fe(s) → 3FeCl₂(aq) + 3H₂(g)
Question: Compounds such as alcohols and glucose also contain hydrogen but are not categorised as acids. Describe an activity to prove it.
Solution: Insert two nails into a cork and place on a beaker. Connect the nails to a bulb, battery, and switch. Dip nails in alcohol/glucose. Bulb does not glow. Now add HCl, bulb glows. This proves acids conduct electricity; alcohol and glucose do not.
Question: Why does distilled water not conduct electricity, whereas rain water does?
Solution: Distilled water lacks ions; rainwater contains dissolved acidic gases like CO₂, forming carbonic acid and ions, enabling it to conduct electricity.
Question: Why do acids not show acidic behaviour in the absence of water?
Solution: Hydrogen ions are produced only in the presence of water; water is needed for acidic behaviour.
Question: Five solutions (A, B, C, D, E) have pH values of 4, 1, 11, 7, and 9. Which is
(a) Neutral?
(b) Strongly alkaline?
(c) Strongly acidic?
(d) Weakly acidic?
(e) Weakly alkaline?
Solution:
- pH 11 (C): Strongly alkaline
- pH 9 (E): Weakly alkaline
- pH 7 (D): Neutral
- pH 4 (A): Weakly acidic
- pH 1 (B): Strongly acidic
Question: Equal lengths of magnesium ribbons are taken in test tubes A and B. Hydrochloric acid (HCl) is added to A, acetic acid (CH₃COOH) to B, same amount and concentration. In which tube will fizzing occur more and why?
Solution: HCl is a strong acid, acetic is weaker. More hydrogen gas (fizzing) in test tube A.
Question: Fresh milk has a pH of 6. How will the pH change as it turns into curd?
Solution: Milk turns to curd due to lactic acid, lowering pH.
Question: A milkman adds a small amount of baking soda to fresh milk.
(a) Why does he shift the pH from 6 to slightly alkaline?
(b) Why does this milk take longer to set as curd?
Solution:
- (a) Shifting pH prevents milk from souring quickly due to lactic acid.
- (b) Neutralises acid first, delaying curd formation.
Question: Plaster of Paris should be stored in a moisture-proof container. Explain why.
Solution: Moisture causes hydration, spoiling the plaster and slowing setting.
Question: What is a neutralisation reaction? Give two examples.
Solution: Acid + base → salt + water is neutralisation.
Examples:
NaOH + HCl → NaCl + H₂O
Mg(OH)₂ + H₂CO₃ → MgCO₃ + 2H₂O

Question: Give two important uses of washing soda and baking soda.
Solution:

Washing Soda	Baking Soda
Used as an electrolyte	Tests garden soil acidity (bubble formation indicates acidity)
Water softener for laundry	Removes bug bodies from car paint safely