What happens when hydrogen gas is passed over the heated copper oxide (CuO)?

The substances which get oxidised easily can be kept for longer duration by:

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

The reaction which is used in black and white photograph is:

The following reaction is an example of a:

4NH₃(g) + 5O₂(g) → 4NO(g) + 6H₂O(g)

(i) displacement reaction

(ii) combination reaction

(iii) redox reaction

(iv) neutralization reaction