You could have noted that when copper powder is heated in a china dish, the surface of copper powder becomes coated with black coloured substance.
$(i)$ Why has this black coloured substance formed?
$(ii)$ What is this black substance?
$(iii)$ Write the chemical equation of the reaction that takes place.

Zinc liberates hydrogen gas when reacted with dilute hydrochloric acid,whereas copper does not. Explain why?

When hydrogen gas burns in the presence of oxygen,water is formed,and when water is electrolysed,hydrogen and oxygen gases are produced. State the kind of reaction that takes place:
$(a)$ In the first case
$(b)$ In the second case

Give the characteristic tests for the following gases:
$(a)$ $O_2$
$(b)$ $H_2$

$A$ small amount of quicklime is added to water in a beaker.
$(a)$ Name and define the type of reaction that has taken place.
$(b)$ Write the balanced chemical equation for the above reaction. Write the chemical name of the product obtained.
$(c)$ State two observations that you will make in the reaction.

State what happens when:
$(a)$ Gypsum is heated at $373 \, K$.
Also,write the chemical equation for the reaction.