Which of the following is not a physical change?

$A$ metal $P$,when exposed to moist air for a longer period of time,loses its shiny brown surface and gains a green coat. Why has this happened? Identify the metal. Write the name and chemical formula of this green-coloured compound. List two ways to prevent this process.

$(i)$ When a metal $X$ is added to the salt solution of a metal $Y$,the following chemical reaction takes place: Metal $X +$ Salt solution of $Y \rightarrow$ Salt solution of $X +$ Metal $Y$.
$(ii)$ Mention the inference you draw regarding the reactivity of metal $X$ and $Y$ and also about the type of reaction. State the reason for your conclusions.

Why is a combustion reaction considered an oxidation reaction?

What happens when a piece of $(i)$ Silver metal is added to copper sulphate solution? Also,write the balanced chemical equation if the reaction occurs.

Describe an activity to show a decomposition reaction in which light is used to decompose a reactant. Write the chemical equation of the reaction and state its one use.