IGCSE Physics / Solids, liquids and gases

Practical: a temperature-time graph for a change of state

Edexcel International GCSE Physics (4PH1), chapter 19: Solids, liquids and gases, specification point 5.11P. Free lesson.

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A temperature-time graph shows how a substance cools or warms, including the flat sections where it changes state. The shape of the graph helps explain what happens to particle energy during freezing, melting and boiling.

How to record a cooling curve

Use stearic acid, a waxy solid, to record how temperature changes as a liquid cools and freezes. Put the thermometer in the middle of the acid without letting it touch the glass, so it measures the acid rather than the tube. Melt the stearic acid by warming its tube in hot water. Keep the tube in an empty beaker so the acid cools slowly and evenly. Use a clamp stand to hold the tube steady.

What the cooling curve shows

The temperature first falls as the liquid cools. It then stays at about 69 °C while the stearic acid freezes, before falling again once it is solid. The flat part is the change of state, not a mistake in the readings. Before freezing: liquid stearic acid cools. During freezing: the temperature stays constant at about 69 °C. After freezing: solid stearic acid cools.

Why temperature stays constant during freezing

During freezing, stearic acid changes from a liquid to a solid. Energy is still transferred to the surroundings, but it comes from bonds forming between the particles. The particles do not slow down, so their average kinetic energy stays the same. That is why the temperature stays constant. For a full explanation, state the change of state, where the energy comes from, and that average kinetic energy stays constant.

How to sketch the heating curve for water

A heating curve follows the same idea as a cooling curve, but the substance is heated steadily. Starting with ice at −20 °C, the temperature rises, stays flat while the ice melts at 0 °C, rises again, then stays flat while water boils at 100 °C. The flat sections show changes of state, when energy breaks bonds between particles rather than raising the temperature. There is ice and water together during melting at 0 °C. At 100 °C, energy turns water into steam during boiling. Boiling takes far longer than melting. The water line is less steep than the ice line because water needs…

How to improve the temperature readings

A person can make an error when reading a thermometer. A temperature probe connected to a data logger records the temperature at exactly the right times without human reading error. For more accurate readings, use a temperature probe and data logger.

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