Question

When hydrogen is passed over a black solid compound A, the products are a 'colorless liquid' and a 'reddish-brown metal B'.

Substance B is divided into two parts each placed in separate test tubes.

Dilute HCl is added to one part of substance B and dilute HNO₃ to the other.

1. Name the substances A and B.
2. Give two tests for the colourless liquid formed in the experiment.
3. What happens to substance A when it reacts with hydrogen? Give reasons for your answer.
4. Write an equation for the reaction between hydrogen and substance A.
5. Is there any reaction between substance B and dilute hydrochloric acid? Give reasons for your answer.

Answer 1

1. A = CuO, B = Cu
2. Blue and red litmus paper, when dipped in the colourless liquid, do not change colour. This confirms the liquid formed is neutral and is water. It changes white anhydrous copper sulphate to blue salt.
3. Black copper oxide (A) on heating with hydrogen reduces copper oxide to reddish-brown copper and itself gets oxidised to water. Hydrogen is a strong reducing agent and removes oxygen from less active metals, i.e. it removes oxygen from heated metal oxides when passed over them and itself gets oxidised to water.
4. \[\ce{CuO + H2 ->[\Delta] Cu + H2O}\]
5. Cu + HCl →No reaction
   Copper is less reactive than hydrogen and hence cannot displace it from HCl.