Melting Point vs. Freezing Point — What's the Difference?

Melting point and freezing point refer to temperatures at which substances change states between solid and liquid, but they represent these changes in opposite directions. While the melting point is the temperature at which a solid becomes a liquid, indicating the end of solidity, the freezing point marks the temperature at which a liquid solidifies, signifying the loss of fluidity. This distinction is fundamental in understanding the behavior of substances in response to temperature changes.

Both points are crucial for characterizing substances, especially in chemistry and physics. The melting point is particularly important for identifying substances, as pure substances have specific melting points. In contrast, the freezing point is key in meteorology and food preservation, where the transition of water to ice and vice versa has practical implications.

Interestingly, for many pure substances, the melting point and freezing point are the same temperature. This phenomenon occurs under stable conditions, where the transition from solid to liquid (melting) or liquid to solid (freezing) happens at a constant temperature. However, the presence of impurities can cause these points to differ due to melting point depression or freezing point depression.

In practice, the determination of melting and freezing points is used to assess purity and to understand the thermal properties of materials. The melting point helps in determining the purity of a substance, as impurities usually lower the melting point. Conversely, the freezing point can indicate the presence of solutes in a liquid, as solutes typically lower the freezing point of a solution compared to the pure solvent.

Environmental conditions, especially pressure, also play a significant role in these temperatures. The melting point of ice, for example, decreases under pressure, which is why ice skates can glide over ice. The freezing point, similarly, can be affected by pressure, but is more commonly influenced by the presence of solutes, which is why salt is used to melt ice on roads.