Balance the following chemical equations.

$(a)$ $HNO _{3}+ Ca ( OH )_{2} \longrightarrow Ca \left( NO _{3}\right)_{2}+ H _{2} O$

$(b)$ $NaOH + H _{2} SO _{4} \longrightarrow Na _{2} SO _{4}+ H _{2} O$

$(c)$ $NaCl + AgNO _{3} \longrightarrow AgCl + NaNO _{3}$

$(d)$ $BaCl _{2}+ H _{2} SO _{4} \longrightarrow BaSO _{4}+ HCl$

Explain the "Oxidation'' in terms of gain or loss of oxygen with two examples.

Identify the substances that are oxidised and the substances that are reduced in the following reactions.

$(i)$ $4 Na ( s )+ O _{2}( g ) \rightarrow 2 Na _{2} O ( s )$

$(ii)$ $CuO ( s )+ H _{2}( g ) \rightarrow Cu ( s )+ H _{2} O ( l )$

Translate the following statements into chemical equations and then balance them.

$(a)$ Hydrogen gas combines with nitrogen to form ammonia.

$(b)$ Hydrogen sulphide gas burns in air to give water and sulpur dioxide.

$(c)$ Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate.

$(d)$ Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.

Oil and fat containing food items are flushed with nitrogen. Why ? 

Explain the ''Rancidity'' terms with one example.