Explain the term 'chemical reaction' with special reference to – 'reactants' and 'products'.

A chemical reaction is generally accompanied by certain external indications or characteristics. These include change of –

(a) color

(b) state

(c) smell

(d) evolution of gas

(e) formation of precipitate

(f) evolution or absorption of heat.

With reference to change of color – state the change in color seen when the following are heated –

(i) copper carbonate

(ii) zinc carbonate

(iii) mercury [II] oxide

(iv) lead [IV] oxide

Give a balanced equation for addition of iron to copper [II] sulphate solution. State the change in color seen.

In certain reactions a change of state is observed i.e. solid to liquid, liquid to gas etc. – State the change of state of the reactants – to give the respective products in each case.

(a) C + 2S ⟶ CS₂

(b) NH₃ + HCl ⇌ NH₄Cl

(c) 2H₂ + O₂ ⟶ 2H₂O

(d) $2\text{H}$2\text{O} \xrightarrow[\text{current}]{\text{electric}} 2\text{H}2 + \text{O}_2