The presence of an impurity in a substance will cause the melting point to

Correct answer: reduce

Explanation: An impurity disrupts the orderly crystal lattice of a pure solid. This makes it easier for particles to leave the solid phase, so the melting point is lowered and the melting occurs over a broader range (less sharp) than for the pure substance.

• be zero — Incorrect. Impurities do not make the melting point 0.
• reduce — Correct. Impurities lower the melting point and widen the melting range.
• increase — Incorrect. The typical effect is depression, not elevation.
• be stable — Incorrect. With impurities, the melting point becomes less sharp (less “stable”).