What is a real-life example of how impurities can affect melting points?

Impurities can significantly affect the melting points of substances, and the melting of ice serves as a common real-life example to illustrate this phenomenon. Pure ice has a melting point of 0 degrees Celsius. However, when impurities such as salt are present, the melting point decreases, resulting in what is known as freezing point depression. This is why roads are often salted during winter; the salt prevents ice from forming and causes existing ice to melt at lower temperatures than it would otherwise. This change in melting point can have practical implications in everyday scenarios, like maintaining safe driving conditions on icy roads.

In contrast, the other options do not directly convey the impact of impurities on melting points in the same way. For example, while metals do melt at high temperatures, the question specifically pertains to the effects of impurities, which isn't illustrated in that context. Wet sand drying quickly is more related to evaporation rather than melting points, and glass shattering upon heating involves stress and thermal shock rather than changes in melting point due to impurities. Thus, ice melting at different temperatures when impurities are present serves as the best example.