Q1. [4] § 2.1 UNDERSTANDING THE CHEMICAL PROPERTIES OF ACIDS AND BASES § 2.2 WHAT DO ALL ACIDS AND ALL BASES HAVE IN COMMON? § 2.4 MORE ABOUT SALTS
In the given diagram, when electricity is passed through an aqueous solution of a common salt, a substance 'Z' is produced, along with the evolution of gases 'X' and 'Y'. When a burning matchstick is brought near the gas 'Y', it burns with a pop sound, whereas 'X' is used for disinfecting drinking water. When 'X' is passed through slaked lime, an insoluble substance 'A' is produced.
Read the following passage and answer the questions that follow :
- (a) Write the names of the gases 'X' and 'Y'. [1]
- (b) Write the balanced chemical equation for the formation of substance 'Z'. [1]
- (c) Answer (i) or (ii): [2]
Previously asked in CBSE board exam
2026 31/2/1 Q28
Generated by claude-sonnet-4-6 · 2026-06-15 06:43 · grounding stimulus
Model Answer
(a) Gas X = Chlorine (Cl₂); Gas Y = Hydrogen (H₂)
(b) Electrolysis of brine (aqueous NaCl) produces sodium hydroxide (Z):
$$2NaCl + 2H_2O \xrightarrow{\text{electricity}} 2NaOH + Cl_2 + H_2$$
(c) (i) or (ii) — answered as (i): Name substance 'A' and write its chemical equation.
When chlorine gas (X) is passed through slaked lime [Ca(OH)₂], bleaching powder is formed as substance 'A'.
$$2Ca(OH)_2 + 2Cl_2 \rightarrow Ca(OCl)_2 + CaCl_2 + 2H_2O$$
(ii) Uses of substance 'Z' (NaOH): It is used in making soap, paper, and as a drain cleaner.
Source: Chapter 2 — Acids, Bases and Salts; Electrolysis of Brine / Chemicals from Common Salt
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Explanation
- X = Chlorine because it disinfects water and reacts with slaked lime. Y = Hydrogen because it burns with a pop sound — classic test for H₂.
- The balanced equation for (b) is the standard electrolysis of brine equation — examiners award the mark only if it is balanced with correct formulae.
- For (c), bleaching powder is Ca(OCl)Cl (calcium oxychloride); examiners accept either formula. If the sub-question asked for uses of NaOH instead, soap/paper/drain cleaner are the three accepted uses from the textbook.
- Always write chemical names alongside formulae to avoid losing marks for ambiguity.
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cbse.gov.in.