Final answer:
Impurities in a sample are likely to decrease the melting temperature due to the disruption of intermolecular forces, leading to a lower energy requirement to change from solid to liquid.
Step-by-step explanation:
If a sample is impure, the likely effect on its melting temperature is that it decreases, which means option C) The melting temperature decreases is correct. Impurities in a sample disrupt the orderly structure of a solid and interfere with the strong intermolecular forces that typically determine the melting point of a pure substance. As a result, the energy required to break these interactions is reduced, causing the melting temperature to be lower than that of a pure substance.
The melting point is the temperature at which a solid changes into a liquid, and this transition occurs at a specific temperature for pure substances. However, with impurities present, the melting range becomes broader and starts at a lower temperature because different impure particles may require different amounts of energy to overcome their interactions.