Question

Viewed at the molecular level, what is happening when a dynamic equilibrium is established between the liquid and solid forms of a substance? What is the temperature called when a liquid and a solid are in equilibrium?

Short answer

At dynamic equilibrium between liquid and solid forms, molecules are continuously moving back and forth between the two phases at equal rates. The temperature at which this equilibrium occurs is known as the melting or freezing point.

Step-by-step solution

Understanding Dynamic Equilibrium

Dynamic equilibrium in the context of the liquid and solid phases of a substance occurs when the rate of the substance melting from solid to liquid equals the rate of the substance freezing from liquid to solid. This means that the number of molecules transitioning from one phase to the other is the same over time, leading to no net change in the amount of each phase despite the constant exchange.

Identifying the Temperature at Equilibrium

The temperature at which the liquid and solid phases of a substance are in equilibrium is known as the melting point if approached from the solid side or freezing point if approached from the liquid side. At this temperature, the substance can exist as both a liquid and a solid in a stable balance.

Phase Change and Equilibrium

At the molecular level, when a substance reaches the temperature of its melting/freezing point, the energy of the molecules allows for the phase change to occur without a change in temperature. The energy supplied to the system breaks intermolecular bonds allowing solids to melt, or is released to the surroundings as the liquid freezes, but the overall energy (and thus temperature) of the system remains constant as long as both phases are present.

Key concepts

Phase Change

In chemistry, a phase change is a transition of matter from one state (solid, liquid, gas) to another. During a phase change, substances absorb or release energy in the form of heat. When a solid turns into a liquid, this process is known as melting. Conversely, when a liquid becomes a solid, it's called freezing.

At the molecular level, a phase change involves a rearrangement of particles. In a solid, particles are closely packed in an orderly pattern, while in a liquid, they are further apart and can move more freely. During melting, energy absorbed by the solid causes its particles to vibrate intensely until they overcome the forces holding them in place, leading to a state where they can move around and the solid becomes a liquid.

Melting Point

The melting point of a substance is the temperature at which it changes from a solid to a liquid. At the melting point, the intermolecular forces within the solid are weakened to the point that molecules can move past one another, resulting in a phase change to liquid.

The melting point is a characteristic property of a pure substance; it's specific and can be used to identify substances. It's important to note that during the melting process, all the energy provided to the substance is used to break the intermolecular bonds, which is why the temperature remains constant during this phase transition.

Freezing Point

Conversely, the freezing point is the temperature at which a liquid becomes a solid. This is when the motion of particles slows down enough for their intermolecular forces to pull them together into fixed positions, forming a solid.

Just like the melting point, the freezing point is also a characteristic property of a substance. Interestingly, for pure substances under normal pressure, the melting point and freezing point are the same temperature; this is why we often refer to them together as the substance's melting/freezing point.

Intermolecular Forces

The interactions between molecules are governed by intermolecular forces. These forces are responsible for holding substances together and play a critical role in determining whether a substance is at its solid or liquid state at a given temperature.

There are several types of intermolecular forces, including hydrogen bonds, dipole-dipole interactions, and Van der Waals forces. Stronger intermolecular forces will result in a higher melting and freezing point, because more energy is required to overcome these forces during a phase change. Conversely, substances with weaker intermolecular forces will have lower melting and freezing points.