Cooling Curves

Introduction

A heating curve shows what happens as a substance gains energy. A cooling curve tells the story in reverse. If you let molten wax or hot stearic acid cool in a test tube while recording its temperature, the readings fall, pause, and then fall again. Those pauses are not errors in your measurements. They reveal the moments when the substance is freezing and giving out energy, and they tell you its freezing point.

Core explanation

What a cooling curve shows. A cooling curve plots temperature against time as a substance loses energy to its cooler surroundings. For a pure substance cooled from gas to solid, it is roughly the mirror image of a heating curve: sloping sections separated by flat plateaus.

Sections of a cooling curve (gas to solid):

Section Graph State(s) present What is happening --- --- --- --- 1 Slopes down Gas Gas cools, particles slow down 2 Flat at boiling point Gas and liquid Condensing 3 Slopes down Liquid Liquid cools 4 Flat at freezing point Liquid and solid Freezing 5 Slopes down Solid Solid cools

Why there are plateaus. As the substance changes state, particles come closer together and attractive forces form between them. This releases energy — condensing and freezing are exothermic . The energy released balances the energy being lost to the surroundings, so the temperature stays constant until the change is complete.

Same temperatures as heating. A pure substance condenses at its boiling point and freezes at its melting point. Water condenses at 100 °C and freezes at 0 °C. The freezing point and melting point are the same temperature, approached from opposite directions.

A classroom example. A common experiment uses a substance such as stearic acid, which freezes at about 69 °C. Its cooling curve falls steeply, then stays flat at about 69 °C while crystals form, then falls again towards room temperature. The slope usually becomes gentler as the substance nears room temperature, because the temperature difference driving the energy loss gets smaller.

Pure and impure. A pure substance gives a sharp, flat freezing plateau. An impure one freezes gradually over a range of temperatures, giving a sloping section instead.

Step-by-step reasoning

To interpret a cooling curve:

1. Check the axes: temperature against time. 2. Follow the line from left to right as the temperature falls. 3. Find each flat section. 4. The upper plateau is condensing, at the boiling point. 5. The lower plateau is freezing, at the freezing (melting) point.

Visual explanation

Picture a downhill path with two level resting places. The path descends, levels out at the boiling point where droplets form, descends again, then levels out at the freezing point where crystals grow, before descending once more. Particle diagrams along the path go from widely spaced circles to a jumbled liquid to a neat, regular solid lattice.

Real-world analogy

Imagine a leaking water tank that is also being topped up by a hose. Normally the level falls, but while the hose is running it exactly replaces the leak, so the level stays steady. During freezing, the energy released by particles joining the solid is the hose; the energy lost to the surroundings is the leak.

Real-world example

Metal casting depends on cooling curves. When molten metal is poured into a mould, it cools, then stays at its freezing point while crystals grow, and only then continues to cool. Engineers study these curves to control how the crystals form, which affects the strength of the finished casting.

Why?

Why does the temperature stay constant during freezing even though the surroundings are colder? As particles lock into place in the solid, attractions form and energy is released. This energy replaces what is lost to the surroundings, so the particles' average kinetic energy — and the temperature — stays the same until all the liquid has frozen.

Common misconception

"Freezing point and melting point are different temperatures." For a pure substance they are the same. Ice melts at 0 °C and water freezes at 0 °C; the only difference is the direction of the change.

Worked example

Question: A cooling curve for a liquid falls from 90 °C, stays flat at 54 °C for four minutes, then falls to 25 °C. State the freezing point, the melting point, and the states present during the flat section.

Reasoning: The flat section shows freezing. The melting point equals the freezing point for a pure substance. During freezing, both liquid and solid are present.

Answer: Freezing point 54 °C; melting point 54 °C; liquid and solid present together.

Quick check

1. What change of state is taking place on the lowest plateau of a cooling curve from gas to solid? Answer: Freezing (liquid to solid).

Exam focus

Label cooling curves carefully: temperature on the vertical axis and time on the horizontal, with the plateaus marked "condensing" and "freezing". Explain plateaus by saying energy is released as attractions form between particles. Remember that freezing point equals melting point for a pure substance.

Advanced insight

A very pure liquid cooled carefully can drop below its freezing point without solidifying, called supercooling. On a cooling curve this appears as a small dip below the plateau temperature. When crystals finally start to form, the energy released makes the temperature jump back up to the freezing point, where it then stays until freezing is complete.

Summary

A cooling curve plots temperature against time as a substance cools. Sloping sections show one state losing energy; flat sections show condensing and freezing, when energy released by forming attractions balances energy lost to the surroundings. A pure substance condenses at its boiling point and freezes at its melting point, giving sharp plateaus; impure substances freeze over a range.

Practice questions

1. Why does a cooling curve have a flat section during freezing? Answer: Energy is released as attractions form between particles, balancing the energy lost to the surroundings, so the temperature stays constant. 2. Water vapour is cooled to −10 °C. At what temperatures will plateaus appear on the cooling curve? Answer: At 100 °C (condensing) and 0 °C (freezing). 3. What states are present during the upper plateau of a cooling curve from gas to solid? Answer: Gas and liquid. 4. How would the freezing section of a cooling curve for an impure substance differ from that of a pure one? Answer: It would slope gradually over a range of temperatures instead of being a sharp, flat plateau.