Q21
2 marksVery Short AnswerSection B

(A) A student took a small amount of copper oxide in a conical flask and added dilute hydrochloric acid to it with constant stirring. He observed a change in colour of the solution.

(i) Write the name of the compound formed and its colour.

(ii) Write a balanced chemical equation for the reaction involved.

OR

(B) The industrial process used for the manufacture of caustic soda involves electrolysis of an aqueous solution of compound 'X'. In this process, two gases 'Y' and 'Z' are liberated. 'Y' is liberated at cathode and 'Z', which is liberated at anode, on treatment with dry slaked lime forms a compound 'B'. Name X, Y, Z and B.

Acids, Bases and Salts
Chemical Properties of Acids and Bases
Official Answer

(A)

(i) The compound formed is copper(II) chloride (or cupric chloride). Its colour is blue-green.

(ii) The balanced chemical equation for the reaction is:

CuO(s)+2HCl(aq)→CuCl2(aq)+H2O(l)CuO(s) + 2HCl(aq) \rightarrow CuCl_2(aq) + H_2O(l)


OR


(B)

  • X: Sodium chloride (NaClNaCl)
  • Y: Hydrogen gas (H2H_2)
  • Z: Chlorine gas (Cl2Cl_2)
  • B: Bleaching powder or Calcium oxychloride (CaOCl2CaOCl_2)
copper(II) chlorideblue-greenCuO+2HCl→CuCl2+H2OCuO + 2HCl \rightarrow CuCl_2 + H_2OSodium chlorideHydrogenChlorineBleaching powderCaOCl2CaOCl_2

Marking Scheme

  • 1For (A)(i): Correct name (copper(II) chloride) and colour (blue-green) (1 mark)
  • 2For (A)(ii): Correctly balanced chemical equation with state symbols (1 mark)
  • 3For (B): Correct identification of X (NaClNaCl) and Y (H2H_2) (1 mark)
  • 4For (B): Correct identification of Z (Cl2Cl_2) and B (CaOCl2CaOCl_2) (1 mark)

Hint

For (A), recall the reaction of a metal oxide with an acid. For (B), think of the chlor-alkali process and the reaction of slaked lime with a gas liberated at the anode.

Quick Oral Answer

Copper oxide reacts with hydrochloric acid to form copper(II) chloride, which is blue-green. In the chlor-alkali process, electrolysis of brine (NaCl) yields hydrogen at the cathode, chlorine at the anode, and caustic soda.

Analysis & Explanation

(A) When basic metal oxides like copper(II) oxide (CuOCuO) react with acids like hydrochloric acid (HClHCl), they undergo a neutralization-like reaction to form a salt and water. The copper(II) chloride (CuCl2CuCl_2) salt formed dissolves in water, giving the solution a characteristic blue-green colour.


(B) The industrial manufacture of caustic soda (NaOHNaOH) is called the chlor-alkali process.

  • Electrolysis of an aqueous solution of sodium chloride (brine, X) produces sodium hydroxide, hydrogen gas, and chlorine gas.
  • Hydrogen gas (Y) is liberated at the cathode.
  • Chlorine gas (Z) is liberated at the anode.
  • When chlorine gas (Z) is treated with dry slaked lime (Ca(OH)2Ca(OH)_2), it undergoes a reaction to form bleaching powder (B), chemically known as calcium oxychloride (CaOCl2CaOCl_2).

Common Mistakes

  1. 1Writing the formula of copper chloride as CuClCuCl instead of CuCl2CuCl_2.
  2. 2Confusing the gases liberated at the anode and cathode (remember: Chlorine is negative and goes to the positive anode; Hydrogen is positive and goes to the negative cathode).

Interesting Facts

Copper(II) oxide is black, but when it dissolves in hydrochloric acid, it forms a beautiful blue-green solution of copper(II) chloride.

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Frequently Asked Questions

How many marks does this question carry in CBSE Class 10 Science 2023?

This question carries 2 marks in the CBSE Class 10 Science 2023 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 "Chemical Properties of Acids and Bases" from CBSE Class 10 Science.

What type of question is this in the CBSE Class 10 Science 2023 paper?

This is a Very Short Answer question from Section B in the CBSE Class 10 Science 2023 paper.