Impure substance melts sharply the melting point. The melting point of the impure substance would be lower Since this sample is an impure solid as explained in question 1 (melting point is over a wide range), it's most likely to melt at 139°C. A pure substance melts quite sharply at the melting point. The melting point is a physical property of a solid and can be used to help identify a substance. When a pure substance like table sugar (with a melting point of 185 °C) or ice (with a melting point of 0 °C) is heated, it changes state from solid to liquid at a specific temperature due to the consistent strength of intermolecular attractions. Reasons and Explanations. Similar to the melting point, the boiling point of pure substances occurs at a specific temperature, and the transition from liquid to gas phase happens sharply. Instead, an impure substance has a melting point range, which means it can melt over a variety of temperatures. This explains the melting point depression (lowering) observed from impure solids. Which of the following is an effect that impurities do not typically have on the boiling or melting point of a substance? [A] Deviating from the reference value for the melting or boiling point [B] Making the melting point occur over a larger temperature range [C] Increasing the boiling point [D] Making the boiling point occur sharply at a definite temperature [E] Decreasing the melting point For example, while a pure substance may melt sharply within a range of about 1-2 °C, an impure sample might have a melting range that exceeds 5 °C.