Which gas has a pungent smell that causes choking and coughing?

Which among the following statement$(s)$ is(are) true? Exposure of silver chloride to sunlight for a long duration turns grey due to
$(i)$ the formation of silver by decomposition of silver chloride
$(ii)$ sublimation of silver chloride
$(iii)$ decomposition of chlorine gas from silver chloride
$(iv)$ oxidation of silver chloride

Which one of the following is a chemical change? Give reason also.
$A$. Burning of wax
$B$. Melting of wax

Which method is used to collect $H_2$ gas in a gas jar in the laboratory?

$A$ reddish-brown coloured metal, used in electrical wires, when powdered and heated strongly in an open china dish, its colour turns black. When hydrogen gas is passed over this black substance, it regains its original colour. Based on the above information, answer the following questions:
$(i)$ Name the metal and the black coloured substance formed.
$(ii)$ Write balanced chemical equations for both the reactions.

Which of the following acts as a catalyst in the production of ammonia by Haber's process?