What do you understand by term reduction?

Reduction:

• When a substance gains hydrogen or electropositive radical or loses electronegative radical or loses oxygen, then the reduction of substance takes place.
• Reduction reactions involve:

1. Addition of hydrogen:

$\underset{\mathrm{nitrogen}}{{\mathrm{N}}_2\left(\mathrm{g}\right)}+\underset{\mathrm{hydrogen}}{3{\mathrm{H}}_2\left(\mathrm{g}\right)}\stackrel{}{\to }\underset{}{\underset{\mathrm{ammonia}}{2{\mathrm{NH}}_3\left(\mathrm{g}\right)}(\mathrm{reduction}\mathrm{of}\mathrm{nitrogen})}$

2. Addition of electropositive element:

$\underset{\mathrm{stannous}\mathrm{chloride}}{{\mathrm{SnCl}}_2\left(\mathrm{s}\right)}+\underset{\mathrm{mercuric}\mathrm{chloride}}{{\mathrm{HgCl}}_2\left(\mathrm{s}\right)}\to \underset{\mathrm{tin}\mathrm{tetrachloride}}{{\mathrm{SnCl}}_4\left(\mathrm{l}\right)}+\underset{\mathrm{mercurrous}\mathrm{chloride}}{{\mathrm{Hg}}_2{\mathrm{Cl}}_2\left(\mathrm{s}\right)}$(reduction of mercuric chloride)

3. Removal of oxygen

$\underset{\mathrm{zinc}\mathrm{oxide}}{\mathrm{ZnO}\left(\mathrm{s}\right)}+\underset{\mathrm{carbon}}{\mathrm{C}\left(\mathrm{s}\right)}\stackrel{}{\to \underset{\mathrm{zinc}}{\mathrm{Zn}\left(\mathrm{s}\right)}+\underset{\mathrm{carbon}\mathrm{monoxide}}{\mathrm{CO}\left(\mathrm{g}\right)}}(\mathrm{reduction}\mathrm{of}\mathrm{zinc}\mathrm{oxide})$

4. Removal of electronegative element

$\underset{\mathrm{ferric}\mathrm{chloride}}{{\mathrm{FeCl}}_3\left(\mathrm{s}\right)}+{\underset{\mathrm{hydrogen}}{{\mathrm{H}}_2\left(\mathrm{g}\right)}}_{}\stackrel{}{\to }\underset{\mathrm{ferrous}\mathrm{chloride}}{{\mathrm{FeCl}}_2\left(\mathrm{s}\right)}+\underset{\mathrm{hydrochloric}\mathrm{acid}}{\mathrm{HCl}\left(\mathrm{aq}\right)}$(reduction of ferric chloride)