State the main characteristics of chemical reactions. Give at least one example in each case.

Explanation

The main characteristics of chemical reactions are:
(1) Evolution of gas :
E.g. When zinc reacts with dilute sulphuric acid, hydrogen gas is evolved, with effervescence.
`underset"[zinc]"(Zn)+underset"[dil. sulphuric acid]"(H_2SO_4)to underset"[zinc sulphate]"(ZnSO_4)+ underset"[hydrogen]"(H_2)`
(ii) Change of colour :

E.g. When a few pieces of iron are dropped into a blue coloured copper sulphate solution, the blue colour of the solution fades and eventually turns into light green due to the formation of ferrous sulphate.
`underset"[iron]"(Fe)+underset"[blue solutions]"(CuSO_4(aq))to underset"[green solution]"(FeSO_4)+underset"(red deposit)"underset"[copper]"(Cu)`
(iii) Formation of precipitates :
E.g. When a solutions of silver nitrate is added to a solutions of sodium chloride, a white insoluble substances (precipitate) silver chloride, is formed
`underset"[silver nitrate solutions]"(AgNO_3(aq))+ underset"[sodium chloride solution]"(NaCl(aq)) to underset"(white ppt)"underset"[silver chloride]"(AgCl(ppt))+ underset"[sodium nitrate solution]"(NaNO_3(aq))`
(iv)Change of state :
E.g. Ammonia gas reacts with hydrogen chloride gas to produce solid ammonium chloride.
`NH_3(g)+HCl(g) hArr NH_4Cl(s)`