(a) Write the formula of the ions which (i) acids, and (ii) bases generate in water solutions.

(b) Dry HCl gas does not change the colour of dry litmus paper. Why?

Q: (a) Write the formula of the ions which (i) acids, and (ii) bases generate in water solutions. (b) Dry HCl gas does not change the colour of dry litmus paper. Why?

**(a) Ions generated in water:** (i) **Acids** generate the **hydrogen ion, $H^+(aq)$** (which combines with water to exist as the **hydronium ion, $H_3O^+$**) in aqueous solution. Example: $HCl + H_2O \rightarrow H_3O^+ + Cl^-$ (ii) **Bases** generate the **hydroxide ion, $OH^-(aq)$** in aqueous solution. Example: $NaOH(s) \xrightarrow{H_2O} Na^+(aq) + OH^-(aq)$ **(b) Why dry HCl gas does not turn dry litmus paper red-to-... (i.e., shows no acidic behaviour):** As described in Activity 2.9 of the chapter, hydrogen ions from HCl are produced **only in the presence of water** — *"The separation of $H^+$ ion from HCl molecules cannot occur in the absence of water."* Dry HCl gas therefore contains no free $H^+/H_3O^+$ ions to impart acidic character, so it does not change the colour of dry litmus paper. It is only when HCl is dissolved in water that it ionises ($HCl + H_2O \rightarrow H_3O^+ + Cl^-$) and shows acidic behaviour, changing the colour of moist/wet litmus paper.

Acids, Bases and Salts

Ionisation of Acids and Bases in Water

Official Answer

(a) Ions generated in water:

(i) Acids generate the hydrogen ion, $H^+(aq)$ (which combines with water to exist as the hydronium ion, $H_3O^+$ ) in aqueous solution.

Example: $HCl + H_2O \rightarrow H_3O^+ + Cl^-$

(ii) Bases generate the hydroxide ion, $OH^-(aq)$ in aqueous solution.

Example: $NaOH(s) \xrightarrow{H_2O} Na^+(aq) + OH^-(aq)$

(b) Why dry HCl gas does not turn dry litmus paper red-to-... (i.e., shows no acidic behaviour):

As described in Activity 2.9 of the chapter, hydrogen ions from HCl are produced only in the presence of water — "The separation of $H^+$ ion from HCl molecules cannot occur in the absence of water." Dry HCl gas therefore contains no free $H^+/H_3O^+$ ions to impart acidic character, so it does not change the colour of dry litmus paper. It is only when HCl is dissolved in water that it ionises ( $HCl + H_2O \rightarrow H_3O^+ + Cl^-$ ) and shows acidic behaviour, changing the colour of moist/wet litmus paper.

Marking Scheme

11 mark for (a): correct ions $H^+(aq)/H_3O^+$ for acids and $OH^-(aq)$ for bases ( $\frac{1}{2}$ mark each).
21 mark for (b): correct reasoning that ionisation of HCl requires water, hence no $H^+$ ions form in the dry gas.

Hint

Think about which ion is common to all acids and which is common to all bases when dissolved in water (Section 2.2), and recall Activity 2.9 about dry HCl gas.

Quick Oral Answer

Acids give $H^+(aq)$ (as $H_3O^+$ ) and bases give $OH^-(aq)$ in water; dry HCl gas has no water to ionise it into $H^+$ and $Cl^-$ , so it shows no acidic property until dissolved in water.

Analysis & Explanation

This question directly tests Section 2.2 and Activity 2.9. Part (a) requires stating the specific ions ( $H^+(aq)$ / $H_3O^+$ for acids, $OH^-(aq)$ for bases) rather than a general description. Part (b) requires linking the absence of water to the absence of ionisation — the NCERT text explicitly states hydrogen ions 'cannot exist alone' and must always be shown as $H^+(aq)$ or $H_3O^+$ , proving that water is essential for an acid to display its characteristic acidic (H⁺-generating) behaviour. Since dry HCl has no water molecules to ionise it, no $H^+$ ions are generated, and hence no colour change is observed with dry litmus paper.

Common Mistakes

1Writing only 'H+' for acids without noting it exists as $H_3O^+$ in water.
2Answering part (b) with 'litmus paper needs to be wet' without explaining WHY — i.e., without connecting it to the ionisation of HCl requiring water.

Interesting Facts

The apparatus in Activity 2.9 uses concentrated sulphuric acid and solid NaCl to generate dry HCl gas, which is then tested with both dry and wet blue litmus paper to show the contrast.

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Frequently Asked Questions

How many marks does this question carry in CBSE Class 10 Science 2025?

This question carries 2 marks in the CBSE Class 10 Science 2025 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 "Ionisation of Acids and Bases in Water" from CBSE Class 10 Science.

What type of question is this in the CBSE Class 10 Science 2025 paper?

This is a Very Short Answer question from Section B in the CBSE Class 10 Science 2025 paper.