The reaction, $3 MnO_{2}(s) + 4 Al(s) \longrightarrow 3 Mn(l) + 2 Al_{2}O_{3}(s) + \text{Heat}$, is an example of:

Why do we store silver chloride in dark-coloured bottles?

$(i)$ 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.

Write the chemical equation of the reaction with an example in which a change in temperature takes place.

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

On adding dilute hydrochloric acid to copper oxide powder,the solution formed is blue-green. Predict the new compound formed which imparts a blue-green colour to the solution.