(a) How does the resistance of a pure metal change if its temperature decreases?
(b) How does the presence of impurities in a metal affect its resistance?
In pure metals, the resistance of the substance increases with the increase in its temperature. The increase is large and fairly regular for normal ranges of temperature.
(b) The presence of impurities in a metal has an important effect on its resistance.
Impurities make the metal highly resistant.
So, the more impurities will be added to a metal, the more strong it will be.