Heat is added to ice at \(0^{\circ} \mathrm{C}\). Explain why the temperature of the ice does not change. What does change?

Understanding phase changes is crucial in grasping how substances transition between solid, liquid, and gas states. These transformations occur at specific temperatures and pressures and involve the transfer of energy. During a phase change, a substance either absorbs or releases energy, which is not reflected in a temperature change but in a state change.

For instance, when heat is added to ice at its melting point (which is \(0^\circ \mathrm{C}\) for water), the added energy is utilized not for raising the temperature but for breaking the molecular bonds that hold the water molecules in a rigid structure. This is why, despite absorbing heat, the temperature of the ice doesn't rise during the melting process. The energy that goes into altering the state of a substance rather than its temperature is known as latent heat. The latent heat of fusion, specifically, is the amount of heat energy required to convert a solid into a liquid without changing its temperature.

A heating curve graphically represents the change in temperature of a substance as energy is added steadily. It's fascinating because it visually demonstrates phase changes and the concept of latent heat. On this curve, you'll notice that during phase transitions, such as melting or boiling, the temperature remains constant despite continual heat addition.

This plateau occurs because the input energy is channeled into breaking intermolecular forces, like during the melting of ice, rather than increasing the kinetic energy of molecules, which would raise the temperature. As a result, the heating curve includes flat sections where phase changes happen (at the melting and boiling points), and sloped sections that reflect temperature increases within a single phase. Therefore, it becomes a powerful tool in illustrating the specific points where substances alter their physical states while absorbing or releasing energy.

The melting point of a substance is a distinctive temperature at which it transitions from a solid to a liquid. This property is inherent to the material and is a critical identifier in chemistry and physics. For example, pure water has a melting point of \(0^\circ \mathrm{C}\), a well-known benchmark.

At the melting point, the energy being added to the solid is utilized primarily to weaken and eventually break the organized intermolecular attractions found in the solid state. Upon reaching this temperature, additional heat doesn't result in a warmer solid; instead, it initiates the transformation to a liquid, where molecules are less rigidly held and can move more freely. This is a specific instance of a phase change, and understanding it helps explain why different substances exhibit unique behaviors such as differing melting points and why some materials may be more suitable for certain applications based on their thermal properties.