What becomes the colour of iron nails after they have been kept in the blue solution of copper sulphate for about $20$ minutes?

When ferrous sulphate is heated the following reaction takes place:

$2{\mathrm{FeSO}}_4\left(\mathrm{s}\right)\to {\mathrm{Fe}}_2{\mathrm{O}}_3\left(\mathrm{s}\right)+{\mathrm{SO}}_2\left(\mathrm{g}\right)+{\mathrm{SO}}_3\left(\mathrm{g}\right)$

The above reaction is called _____of ferrous sulphate.