Question

Does the entropy of the system increase, decrease, or stay the same when (a) a solid melts, (b) a gas liquefies, \((\mathbf{c})\) a solid sublimes?

Short answer

(a) When a solid melts to become a liquid, the entropy of the system **increases** due to increased disorder and freedom of movement of particles. (b) When a gas liquefies, the entropy of the system **decreases** as the particles become more ordered and have less freedom of movement. (c) During sublimation, when a solid transitions directly into a gas, the entropy of the system **increases** due to the significant increase in disorder and freedom of movement.

Step-by-step solution

(a) Solid melts to Liquid

When a solid melts to become a liquid, the particles within the substance become more disordered and gain more freedom of movement. This means that the entropy of the system increases. So, when a solid melts, the **entropy increases**.

(b) Gas liquefies

When a gas condenses to become a liquid, the particles within the substance become more ordered and have less freedom of movement. This means that the entropy of the system decreases. So, when a gas liquefies, the **entropy decreases**.

(c) Solid sublimes

When a solid directly transitions into a gas (sublimation), the particles within the substance become significantly more disordered and have a huge increase in freedom of movement compared to the solid phase. This means that the entropy of the system increases. So, when a solid sublimes, the **entropy increases**.

Key concepts

Phase Transitions

Entropy plays a crucial role during phase transitions. Phase transitions occur when a substance changes from one state of matter to another, such as from solid to liquid, liquid to gas, or solid to gas. During these transitions, the organization of particles in the system changes significantly.
Entropy is a measure of this disorder or randomness in a system. When particles move from a structured state to a more random one, entropy increases. Conversely, if the particles become more organized, entropy decreases. Understanding how entropy changes during phase transitions helps us grasp what is happening at a molecular level.
In summary, when watching molecules re-arrange themselves during a phase transition, keep an eye on entropy—it holds key information about the process.

Melting

Melting is the phase transition from a solid to a liquid. As a solid melts, its particles gain energy, break out of their fixed positions, and begin to move more freely.
During melting:

• The regular, ordered structure of the solid is disrupted.
• Particles move more randomly.
• As a result, the system's entropy increases.

This increase in entropy reflects the greater freedom and higher level of randomness in a liquid compared to a solid. In everyday life, we can observe melting when ice turns into water. This common example clearly illustrates how the structure and freedom of movement in a system change, leading to increased entropy.

Sublimation

Sublimation is a fascinating change directly from a solid to a gas. This is not as common as melting, but it’s most famously seen when dry ice turns directly into carbon dioxide gas.
During sublimation:

• Particles transition into a state with significantly more randomness.
• The order present in the solid state is completely broken down.
• The system experiences a significant increase in entropy.

Since the gaseous state is so much more chaotic than the solid state, the leap in disorder is quite substantial. The directness of sublimation highlights the sharp increase in entropy, emphasizing how this phase transition affects the molecular order.

Condensation

Condensation sees a phase change from a gas to a liquid. This is the opposite direction of the melting and sublimation processes in terms of entropy.
During condensation:

• Gas particles lose energy and become more ordered.
• Their freedom to move as they please is reduced.
• Consequently, the system's entropy decreases.

Condensation transforms the random, high-entropy state of a gas into the more organized state of a liquid. A familiar example of condensation can be observed when steam cools down to form water droplets on a cold surface, illustrating how the loss of energy leads to a decrease in entropy.