Question

What happens to the entropy of a sample of matter when it changes state from a solid to a liquid? From a liquid to a gas?

Short answer

The entropy increases when a sample of matter changes from solid to liquid and from liquid to gas because the particles are less ordered and have more freedom to move in higher energy states.

Step-by-step solution

Understanding the concept of entropy

Entropy is a measure of the disorder or randomness in a system. When a substance changes from a solid to a liquid, its particles gain more freedom to move, and its entropy increases. Similarly, when a liquid changes to a gas, the particles gain even more freedom and the entropy increases further.

Entropy change from solid to liquid

During the phase change from solid to liquid, known as melting, the substance absorbs heat, and its particles vibrate more vigorously until they can move past each other. This increase in movement corresponds to an increase in entropy since the liquid state is more disordered than the solid state.

Entropy change from liquid to gas

In the transition from liquid to gas, called vaporization, the substance must absorb heat to overcome the intermolecular forces that keep it in the liquid state. As the particles spread out and occupy a greater volume with less order, the disorder of the system increases, leading to an increase in entropy.

Key concepts

Entropy in Thermodynamics

Entropy is a fundamental concept within thermodynamics, which is often described as a measure of the disorder or randomness of particles within a system. In more technical terms, entropy quantifies the number of microscopic configurations that correspond to the macroscopic state of a system. For instance, if we consider a room filled with air, the entropy is high because there are many ways in which the particles can be arranged while still maintaining the overall temperature and pressure of the room.

From a macroscopic viewpoint, the second law of thermodynamics tells us that the total entropy of an isolated system can never decrease over time; it either remains constant for a reversible process or increases for an irreversible process. This implies that natural processes tend to move towards a state of maximum entropy. Therefore, when a substance undergoes a phase change, the entropy change is a crucial aspect of the process, reflecting the transition towards greater randomness and disorder in the system.

Solid to Liquid Phase Change

The phase transition from a solid to a liquid is called melting or fusion. During melting, a substance must absorb a specific amount of energy, known as the latent heat of fusion, to overcome the rigid intermolecular forces holding the particles in place within the solid lattice structure. As the substance absorbs heat, the particles start to vibrate more vigorously and eventually have enough energy to move past each other, forming a liquid.

The important aspect of melting in the context of entropy is the increase in freedom of particle movement: the liquid state is inherently more disordered than the solid state because the particles are no longer fixed in place. The increase in entropy can be qualitatively understood by considering the increase in the number of microstates available to the substance. In a liquid, the particles can switch places and spread out, leading to a multitude of possible arrangements, which corresponds to an increase in entropy.

Liquid to Gas Phase Change

Vaporization, the phase change from a liquid to a gas, involves an even greater increase in entropy than the transition from solid to liquid. When a liquid becomes a gas, it must absorb energy in the form of heat to overcome the intermolecular forces that keep it a liquid. This energy, known as the latent heat of vaporization, provides the particles with enough kinetic energy to break free from the surface of the liquid and disperse into the gas phase.

In a gaseous state, the particles are much further apart and have a vast amount of space to move relative to one another, which means there are significantly more microstates available than in the liquid state. Consequently, the entropy of a gas is typically much higher than that of the corresponding liquid. The greater freedom of movement and the higher kinetic energy of the particles in the gas phase translate directly to an increase in randomness and disorder, markedly elevating the entropy of the system. This reflects the dispersal of energy and matter, which is a central theme in the study of entropy within thermodynamics.