Who discovered hydrogen gas?

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

Complete the missing components/variables given as $x$ and $y$ in the following reactions:
$(a)$ $Pb(NO_3)_2(aq) + 2KI(aq) \longrightarrow PbI_2(x) + 2KNO_3(y)$
$(b)$ $Cu(s) + 2AgNO_3(aq) \longrightarrow Cu(NO_3)_2(aq) + x(s)$

Which gas is the lightest in weight?

The following diagram displays a chemical reaction. Observe it carefully and answer the following questions:
$(a)$ Identify the type of chemical reaction that will take place and define it. How will the colour of the salt change?
$(b)$ Write the chemical equation of the reaction that takes place.
$(c)$ Mention one commercial use of this salt.

Write balanced chemical equations for the following reactions:
$(i)$ Calcium carbonate on reaction with hydrochloric acid gives calcium chloride,water and carbon dioxide.
$(ii)$ Nitrogen reacts with hydrogen under pressure to form ammonia.
$(iii)$ Carbon disulphide burns in air to give carbon dioxide and sulphur dioxide.