(a) 1 gram of solid sodium chloride was taken in a clean and dry test tube and concentrated sulphuric acid was added to it. (i) Name the gas evolved in the reaction. (ii) What will be observed when this gas is tested with (I) dry, and (II) wet blue litmus paper? Write your conclusion about the nature (acidic/basic) of this gas. OR (b) Some metals react with acids to produce salt and hydrogen gas. Illustrate it with an example. How will you test the presence of this gas?

Q: (a) 1 gram of solid sodium chloride was taken in a clean and dry test tube and concentrated sulphuric acid was added to it. (i) Name the gas evolved in the reaction. (ii) What will be observed when this gas is tested with (I) dry, and (II) wet blue litmus paper? Write your conclusion about the nature (acidic/basic) of this gas. OR (b) Some metals react with acids to produce salt and hydrogen gas. Illustrate it with an example. How will you test the presence of this gas?

**(a)** (i) The gas evolved in the reaction is **hydrogen chloride gas** ($\mathrm{HCl}$). (ii) Observations: - **(I) Dry blue litmus paper:** No change in colour is observed. - **(II) Wet blue litmus paper:** The blue litmus paper turns red. **Conclusion:** Hydrogen chloride gas is acidic in nature, but it only exhibits its acidic behavior in the presence of water/moisture. This is because the separation of $\mathrm{H^+}$ ions from $\mathrm{HCl}$ molecules cannot occur in the absence of water to form hydronium ions ($\mathrm{H_3O^+}$). **OR** **(b)** **Example:** When zinc granules react with dilute sulphuric acid, zinc sulphate (salt) and hydrogen gas are produced. $$\mathrm{Zn(s) + H_2SO_4(aq) \rightarrow ZnSO_4(aq) + H_2(g)}$$ **Test for Hydrogen Gas:** Pass the evolved gas through a soap solution. Bubbles filled with the gas will form. Bring a burning candle near a gas-filled soap bubble. The gas burns with a characteristic **'pop' sound**, confirming the presence of hydrogen gas.

Acids, Bases and Salts

What Happens to an Acid or a Base in a Water Solution? / Reaction of Metals with Acids

Official Answer

(a)

(i) The gas evolved in the reaction is hydrogen chloride gas ( $\mathrm{HCl}$ ).

(ii) Observations:

(I) Dry blue litmus paper: No change in colour is observed.
(II) Wet blue litmus paper: The blue litmus paper turns red.

Conclusion: Hydrogen chloride gas is acidic in nature, but it only exhibits its acidic behavior in the presence of water/moisture. This is because the separation of $\mathrm{H^+}$ ions from $\mathrm{HCl}$ molecules cannot occur in the absence of water to form hydronium ions ( $\mathrm{H_3O^+}$ ).

(b)

Example: When zinc granules react with dilute sulphuric acid, zinc sulphate (salt) and hydrogen gas are produced.

$\mathrm{Zn(s) + H_2SO_4(aq) \rightarrow ZnSO_4(aq) + H_2(g)}$

Test for Hydrogen Gas: Pass the evolved gas through a soap solution. Bubbles filled with the gas will form. Bring a burning candle near a gas-filled soap bubble. The gas burns with a characteristic 'pop' sound, confirming the presence of hydrogen gas.

hydrogen chloride gasdry blue litmuswet blue litmusturns redhydronium ionszincsulphuric acidhydrogen gaspop soundburning candle

Marking Scheme

1For Part (a): - Naming the gas as hydrogen chloride ( $\mathrm{HCl}$ ): [0.5 mark] - Correct observations for dry (no change) and wet (turns red) litmus paper: [0.5 mark] - Correct conclusion explaining that water is necessary for $\mathrm{H^+}$ ion dissociation: [1 mark]
2For Part (b): - Writing a balanced chemical equation for a metal reacting with an acid (e.g., $\mathrm{Zn}$ and $\mathrm{H_2SO_4}$ ): [1 mark] - Describing the soap solution and burning candle 'pop' test for hydrogen gas: [1 mark]

Hint

For (a), remember that dry gases cannot dissociate without moisture. For (b), think of the reaction of zinc with dilute sulphuric acid and the classic sound made by burning hydrogen.

Quick Oral Answer

Dry $\mathrm{HCl}$ gas does not change the colour of dry litmus paper because it cannot dissociate into $\mathrm{H^+}$ ions without water. When water is present, it forms hydronium ions ( $\mathrm{H_3O^+}$ ), which turn blue litmus red.

Analysis & Explanation

This question tests two fundamental concepts from the NCERT textbook:

The role of water in acid dissociation (Activity 2.9): Acids do not show acidic behavior in their anhydrous (dry) form because the covalent bond in $\mathrm{HCl}$ requires a polar solvent like water to undergo ionization. Water molecules stabilize the released proton by forming hydronium ions ( $\mathrm{H_3O^+}$ ).
Reaction of metals with acids (Activity 2.3): Active metals displace hydrogen from dilute acids to form metal salts and hydrogen gas. The low density and highly flammable nature of hydrogen gas allow it to be easily tested using the 'pop' sound test with a flame.

Common Mistakes

1In part (a), writing 'hydrogen gas' instead of 'hydrogen chloride gas'.
2Assuming dry $\mathrm{HCl}$ gas will turn dry blue litmus paper red.
3In part (b), forgetting to write a balanced chemical equation or omitting physical states.

Interesting Facts

If the weather is highly humid during the $\mathrm{HCl}$ gas experiment, teachers use a guard tube containing calcium chloride (a drying agent) to absorb moisture from the gas so that the dry litmus test remains accurate!

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Frequently Asked Questions

How many marks does this question carry in CBSE Class 10 Science 2024?

This question carries 2 marks in the CBSE Class 10 Science 2024 examination.

Which chapter does this question come from in Science?

This question is from the chapter "Acids, Bases and Salts" in the CBSE Class 10 Science syllabus.

What topic does this question cover in Science?

This question covers the topic "What Happens to an Acid or a Base in a Water Solution? / Reaction of Metals with Acids" from CBSE Class 10 Science.

What type of question is this in the CBSE Class 10 Science 2024 paper?

This is a Very Short Answer question from Section B in the CBSE Class 10 Science 2024 paper.