Which of the following is not a physical change?

What is the difference between displacement and double displacement reactions? Write equations for these reactions.

$(a)$ Give an example of a combination reaction which is exothermic.
$(b)$ Identify the oxidizing agent and reducing agent in the following reaction:
$H_{2}S + Cl_{2} \rightarrow 2HCl + S$
$(c)$ Name the phenomenon due to which the taste and smell of oily food changes when kept for a long time in the open. Suggest one method to prevent it.

Observe the given figure and answer the following questions:
$(i)$ Write the complete balanced chemical equation for the reaction.
$(ii)$ Write the type of reaction involved.
$(iii)$ Is there any precipitate formed?
$(iv)$ If any precipitate is formed,write the colour of the precipitate.

Which of the following gases is combustible?

Why do fireflies glow at night?