The statement that "impurities tend to lower the temperature of substances" is not entirely accurate without context. Impurities can affect various properties of substances, including their melting and boiling points, but they do not directly lower the temperature of a substance.
For example, in the case of melting point depression, when a non-volatile solute (an impurity) is added to a pure solvent, the melting point of the solvent is lowered. This phenomenon occurs because the presence of impurities disrupts the ability of the solvent's molecules to form a solid structure during freezing.
Similarly, boiling point elevation occurs when a solute is added to a solvent, causing the boiling point to increase.
In summary, while impurities can alter the thermal properties of substances (such as melting and boiling points), they do not indiscriminately "lower the temperature" of the substances themselves. The impact of impurities depends on the specific substance and the nature of the impurities involved.