Write the essential condition for the following reaction to take place: $2 AgCl \rightarrow 2 Ag + Cl_{2}$ and write one use of this reaction.

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

State whether the following statement is true or false:
Keeping food in airtight containers helps to slow down oxidation.

The smell and taste of food containing fats and oils change after some time. Why does this happen? Name the process. List two ways which help to slow down the above-mentioned process.

$A$ magnesium ribbon is burnt in oxygen to give a white compound $X$ accompanied by emission of light. If the burning ribbon is now placed in an atmosphere of nitrogen,it continues to burn and forms a compound $Y.$
$(a)$ Write the chemical formulae of $X$ and $Y.$
$(b)$ Write a balanced chemical equation,when $X$ is dissolved in water.

Which one of the following processes involves chemical reactions?