What are oxidising agents?

Which process occurs at the cathode during the process of electrolysis?

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.

Which of the following statements about the given reaction are correct?
$3 Fe (s) + 4 H_2 O (g) \rightarrow Fe_3 O_4 (s) + 4 H_2 (g)$
$(i)$ Iron metal is getting oxidised
$(ii)$ Water is getting reduced
$(iii)$ Water is acting as reducing agent
$(iv)$ Water is acting as oxidising agent

The colour formed on the surface of copper powder when it is heated in a china dish is:

Identify the substances that are oxidised and the substances that are reduced in the following reactions.
$(i)$ $4Na(s) + O_2(g) \rightarrow 2Na_2O(s)$
$(ii)$ $CuO(s) + H_2(g) \rightarrow Cu(s) + H_2O(l)$