AQA Chemistry 5.2.2.1–5.2.2.2 · Investigation
States of matter
You keep heating a solid, but the thermometer stops rising. Where is the energy going, and how do you find a melting point from messy readings?
Combined Science · Foundation tier
Start here · an exam scenario
A tube of white wax in a hot-water bath.
A student puts a boiling tube of solid stearic acid, a waxy substance, into a beaker of water at 90 °C. She stirs the wax with the thermometer and reads the temperature every 30 seconds for ten minutes. The water bath never stops heating the tube.
Commit first
While the wax is melting, what does the thermometer do?
In a solid, particles are packed in a regular arrangement and vibrate about fixed positions. In a liquid, they are still close together but randomly arranged, and they move past each other. In a gas, they are far apart and move quickly in all directions. Melting and boiling need energy, because the forces between the particles have to be overcome. The stronger those forces, the more energy it takes, and the higher the melting or boiling point. Run the investigation and watch the energy go.
Investigation · the heating curve
Heat it. Plot it. Find the melting point.
0 of 21 readings
Thermometer 22.0 °C · not heating yet
The wax is solid. Its particles vibrate about fixed positions.
Think again
“Something clearly happens on that flat part of the graph, so melting must be a chemical change.”
Spot the flaw
ExplainWhat is the liquid in the tube at the end?
Critique the method
Good practice, or a flaw?
Another student wrote up the same investigation. Decide which steps would give trustworthy results and which would not.
Tap a card, then tap the box it belongs in. Tap a placed card to take it back.
State symbols · AQA 5.2.2.2
(s), (l), (g) or (aq)?
In an equation each substance carries its state. (aq) means dissolved in water, and nothing else.
Tap a card, then tap the box it belongs in. Tap a placed card to take it back.
Command words in this lesson
- Predict
- Compare the temperature with the melting and boiling points.
- Identify
- Pick out the reading or feature. An anomaly does not fit the pattern.
- Plan
- Equipment, steps in order, what you measure and how often, and how you stay safe.
Key fact
While a pure substance melts or boils its temperature stays constant: the energy goes into overcoming the forces between particles. It is a physical change; the substance is unchanged.
Examiner tip
Key note · AQA 5.2.2.1–5.2.2.2
States of matter
- Solid: packed in a regular arrangement, vibrating about fixed positions.
- Liquid: close together, random arrangement, moving past each other.
- Gas: far apart, moving quickly in all directions.
- Melting and boiling need energy to overcome the forces between particles. Stronger forces, higher melting and boiling points.
- While a pure substance melts or boils, its temperature stays constant. It is a physical change.
- State symbols: (s) solid, (l) liquid, (g) gas, (aq) dissolved in water.
Photograph this card. Tomorrow, cover it and say each line first.
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Ask Mr Badmus AI
Not sure what a flat section of a graph means?
The investigation is simulated. Readings include small random scatter of about half a degree and one deliberately misread value, so that anomalies can be identified. Stearic acid's melting point is taken as 69 °C; the water bath is modelled at 90 °C, so the liquid levels off below it. Particle pictures are the simple sphere model.