2.2.2 Cooling Curve

Introduction

A cooling curve shows how the temperature of a substance changes as it is cooled over time. It is a graph of temperature (y-axis) against time (x-axis).

The cooling curve shows the reverse process of heating. It starts with the substance at a high temperature (gas) and shows what happens as it loses heat, condenses into a liquid, and finally freezes into a solid.

Definition

Cooling Curve: A cooling curve is a graph that shows the change in temperature of a substance as it is cooled over time. It consists of distinct regions showing:

• temperature falling as the substance loses heat in one state, and
• temperature remaining constant as the substance undergoes a phase change.

In simple words: A cooling curve is a graph showing what happens to the temperature of a substance when you cool it continuously.

Main Idea

• A cooling curve shows the reverse process as a substance is cooled.
• It consists of similar distinct regions as the heating curve, but in reverse order.
• The curve starts from the gas phase (higher temperature) and ends at the solid phase (lower temperature).
• During flat regions, the temperature remains constant as the substance changes state.

Core Concept

A cooling curve is divided into five main regions, corresponding to the five stages of cooling a substance from gas to solid:

• Gas Phase — temperature falls (gas cools down)
• Condensation — temperature constant (gas → liquid)
• Liquid Phase — temperature falls (liquid cools down)
• Freezing — temperature constant (liquid → solid)
• Solid Phase — temperature falls (solid cools down)

Key rule: During a phase change, temperature remains constant. The energy released goes into changing potential energy, not kinetic energy.

Detailed Explanation

Region 1: Gas Phase (Higher Temperature to E)

What happens: The temperature of the gas decreases as it loses heat.

Kinetic particle theory interpretation:

• The kinetic energy of the gas particles decreases.
• Particles move less rapidly.
• The temperature falls.

On the graph: A sloped line going downward from the starting temperature to point E (the condensation point).

Region 2: Condensation (E to D)

What happens: The temperature remains constant as the gas condenses into a liquid.

Kinetic particle theory interpretation:

• The lost heat energy is released as the intermolecular forces bring the particles closer together.
• This results in the latent heat of vaporization being released.
• The kinetic energy (and therefore temperature) remains constant.
• The temperature stays constant until all the gas has condensed.

On the graph: A flat horizontal line from E to D (at the condensation point).

Key point: The energy released during condensation is the latent heat of vaporization.

Region 3: Liquid Phase (D to C)

What happens: The temperature of the liquid decreases as it loses more heat.

Kinetic particle theory interpretation:

• The kinetic energy of the particles decreases.
• Particles move more slowly.
• The temperature falls.

On the graph: A sloped line going downward from D to C (from the condensation point to the freezing point).

Region 4: Freezing (C to B)

What happens: The temperature remains constant as the liquid freezes into a solid.

Kinetic particle theory interpretation:

• The lost heat energy is released as the intermolecular forces arrange the particles into a solid structure.
• This results in the latent heat of fusion being released.
• The kinetic energy (and therefore temperature) remains constant.
• The temperature stays constant until all the liquid has frozen.

On the graph: A flat horizontal line from C to B (at the freezing point).

Key point: The energy released during freezing is the latent heat of fusion.

Region 5: Solid Phase (B to A and Below)

What happens: The temperature of the solid decreases as it continues to lose heat.

Kinetic particle theory interpretation:

• The kinetic energy of the particles decreases.
• Particles vibrate less.
• The temperature falls.

On the graph: A sloped line going downward from B to A and below.

Summary Table

Intermediate Level

During a cooling curve:

• Sloped regions = temperature is falling.
• Flat regions = temperature is constant (phase change).

Why temperature stays constant during a phase change:

• The energy released is used to form intermolecular forces.
• This changes the potential energy, not the kinetic energy.
• Since temperature depends on kinetic energy, the temperature remains constant.

Key distinction:

• Sloped region → kinetic energy changes → temperature changes.
• Flat region → potential energy changes → temperature constant.

Important Points

• - A cooling curve shows temperature change over time as a substance is cooled.
• - It consists of sloped regions (temperature changing) and flat regions (temperature constant).
• - During sloped regions, kinetic energy changes (temperature changes).
• - During flat regions, potential energy changes (temperature constant).
• - Condensation occurs at the first flat region.
• - Freezing occurs at the second flat region.
• - The energy released during condensation is the latent heat of vaporization.
• - The energy released during freezing is the latent heat of fusion.

Remember

• - Cooling curve = temperature vs. time graph.
• - Sloped regions → temperature falls.
• - Flat regions → temperature constant (phase change).
• - Condensation = first flat region.
• - Freezing = second flat region.
• - Kinetic energy changes in sloped regions.
• - Potential energy changes in flat regions.
• - Latent heat of vaporization → condensation.
• - Latent heat of fusion → freezing.

Common Mistakes

Quick Revision

A cooling curve shows the reverse process as a substance is cooled. It consists of similar distinct regions, but in reverse order:

• Gas Phase (higher temperature to E) — temperature decreases as gas loses heat.
• Condensation (E to D) — temperature remains constant as gas condenses into liquid. Latent heat of vaporization is released.
• Liquid Phase (D to C) — temperature decreases as liquid loses heat.
• Freezing (C to B) — temperature remains constant as liquid freezes into solid. Latent heat of fusion is released.
• Solid Phase (B to A and below) — temperature decreases as solid loses heat.

Kinetic Theory Interpretation:

• In regions where temperature changes, kinetic energy decreases.
• In regions where temperature remains constant (phase changes), kinetic energy is constant but potential energy is decreasing.
• The energy released is used to form intermolecular forces.

60-Second Revision

• Cooling curve = temperature vs. time graph.
• Sloped regions → temperature falls (kinetic energy decreases).
• Flat regions → temperature constant (potential energy changes).
• Condensation = first flat region (latent heat of vaporization released).
• Freezing = second flat region (latent heat of fusion released).
• Kinetic energy constant during phase change.
• Potential energy changes during phase change.
• ⚠️ Warning: temperature does not change during a phase change — the energy released goes into forming intermolecular forces.