Class 10 Science Chapter 2 – “Acids, Bases and Salts

In‑text Exercise (Pages ~18–33)

Question 1

You have three test tubes—one with distilled water, one acidic solution, one basic solution—and only red litmus paper. How do you identify each?

Solution:

1. Dip red litmus in each test tube. The one that turns the red litmus blue is basic.

2. Use that blue litmus to test the remaining two: the one that turns it red is acidic.

3. The remaining one shows no change on either test—it’s neutral (distilled water).

Question 2

Hydrogen gas test: How do you test for H₂ gas? What observations confirm its presence?

Solution:
Pass hydrogen gas over a burning candle or matchstick—if it burns with a ‘pop’ sound, H₂ is present. In an alternate setup, the gas trapped in soap bubbles bursts and ignites with a pop.

Question 3

What gas evolves when acid reacts with metal? Illustrate with a metal and acid. Test for the gas.

Solution:

• Acid + Metal → Salt + Hydrogen gas.
  Example:
  $\text{2HCl (dilute) + Mg(s) → MgCl₂ + H₂↑}$

• Test: pop test confirms hydrogen presence.

Question 4

Metal compound A reacts with dilute HCl, effervescence observed. Gas extinguishes a burning candle. If one product is CaCl₂, identify A and write equation.

Solution:

• Effervescence gas extinguishes flame → CO₂.

• Therefore A must be CaCO₃ (calcium carbonate).

• Reaction:
  $\text{CaCO₃ (s) + 2HCl(aq) → CaCl₂(aq) + CO₂↑ + H₂O(l)}$

Exercise Questions & Answers

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: A solution that turns red litmus blue is basic (pH > 7), and among the given options, pH 10 is the only basic value .

Q2. 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
Answer: (b) HCl
Explanation: Eggshells are made of calcium carbonate (CaCO₃). When they react with hydrochloric acid, CO₂ gas is liberated, which turns lime-water milky. Other chlorides (NaCl, LiCl, KCl) do not cause this reaction .

Q3. If 10 mL of NaOH solution is completely neutralized by 8 mL of HCl solution, how much of the HCl solution will be required to neutralize 20 mL of the same NaOH solution?

a) 4 mL b) 8 mL c) 12 mL d) 16 mL
Answer: (d) 16 mL
Explanation: Neutralization is in direct proportion. Twice the NaOH requires twice the acid: 8 mL × 2 = 16 mL would be needed .

Q4. Write the equation of reaction when dilute HCl reacts with magnesium. What gas is evolved, and how will you test for it?

Answer:
Equation: 2 HCl(aq) + Mg(s) → MgCl₂(aq) + H₂(g)
Gas evolved: Hydrogen (H₂)
Test: Bring a burning candle or matchstick near the gas; it burns with a distinct “pop” sound, confirming H₂ .

Q5. A metal compound A reacts with dilute hydrochloric acid to produce effervescence. The gas evolved extinguishes a burning candle, and one of the products is CaCl₂. Identify A and write the balanced equation.

Answer:
A is CaCO₃ (calcium carbonate).
Reaction: CaCO₃(s) + 2 HCl(aq) → CaCl₂(aq) + CO₂(g) + H₂O(l)
Observation: CO₂ gas is evolved, which extinguishes flame and turns lime-water milky .

Q6. Under what soil condition would a farmer treat the field with calcium oxide (quick lime), calcium hydroxide (slaked lime), or chalk (CaCO₃)?

Answer:
If the soil is acidic (pH less than 7), these basic substances are added to neutralize the acidity and make the soil suitable for cultivation .

Q7. Name:

a) The compound called bleaching powder.
b) The substance which yields bleaching powder upon treatment with chlorine.
c) A sodium compound used for softening hard water.
Answer:

• Bleaching powder: Calcium oxychloride (CaOCl₂)

• Precursor substance: Slaked lime, i.e. calcium hydroxide, Ca(OH)₂

• Sodium compound: Sodium carbonate (washing soda) is used to soften hard water .

📝 Tips for CBSE Exam Answers:

• Write a clear first statement of the concept (e.g., “The solution is basic…”).

• Balance all chemical equations, including physical states.

• Mention observations explicitly for practical questions.

• Use correct terminology: acidic, basic, neutral, neutralization, effervescence, etc.

• Refer to pH values when relevant, and explain indicator behavior accurately