Interpret melting and freezing

Temperature and energy transfer are different

Temperature describes how hot or cold a sample is. We measure it here in degrees Celsius, written °C. For example, 23 °C is warmer than 18 °C. Heating transfers energy into the sample; cooling transfers energy out

1. 1Check the state separately A solid can warm up while remaining solid. A higher temperature does not by itself mean that it has melted
2. 2Recall the particle model Solid particles vibrate around positions. Liquid particles stay close but can move past neighbours. Neither state means motionless particles

Name the change from its starting state

Suppose a solid sample becomes a continuous liquid when heated. It changes from solid to liquid, so it melts. When that liquid loses energy and becomes solid again, it freezes. The same substance is present before and after

1. 1Melting Solid becomes liquid as energy is transferred in
2. 2Freezing Liquid becomes solid as energy is transferred out. Freezing does not have to happen at 0 °C; different substances have different transition temperatures

A solid warms from 16 °C to 21 °C and remains solid. Which change does the record show?

Compare temperature and state separately

One material, twelve model particles

Particles attract one another. In a solid they remain near positions; in a liquid they can change neighbours while staying close. The next model shows one hypothetical pure material: pure means one substance. Pressure, the push on the sample, is held fixed

1. 1Read the drawing Twelve equal circles represent particles, not drops. A and B track two of them. The dashed outline marks a small model region, not a container. Mixed states have solid on the left and liquid on the right, without a dividing wall
2. 2Read the controls Higher energy settings mean energy transferred in; lower settings mean energy transferred out. They are not joules or elapsed times. Snapshot changes show particle arrangements, not measured speeds
3. 3Keep the limits The values are idealised, not measurements or a scale drawing. The ordered solid is one example. Volume changes, heat loss and delayed freezing are not modelled

Keep snapshot 1. Predict what changes from energy setting 1 to 2, then from 2 to 6. Try both comparisons and read temperature separately from the solid/liquid counts

What happens from setting 2 to 6 in this model?

Energy can change the state without raising temperature

For this pure material at fixed pressure, melting occurs at 22 °C. While solid and liquid coexist, energy transferred in changes the arrangement against attractions rather than raising temperature. Coexist means both states are present together

1. 1Particles do not melt individually The same particles gain freedom to move past neighbours. They do not become little liquid drops, grow larger or disappear. Attractions still act in the liquid
2. 2After melting Once the sample is fully liquid, further energy transfer can raise its temperature. In the model this happens from settings 6 to 8
3. 3Predict the reverse From setting 6 down to 2, predict whether the solid portion grows and whether temperature changes. Energy is being transferred out, not destroyed

Lower energy from setting 6 to 4, then to 2. Next return to 4 and compare snapshots 1 and 5: follow A and B in the liquid and the small movements on the solid side

Which description fits the change from energy setting 6 down to 2?

A sealed pure sample melts and later freezes without forming a different substance. What changes at the particle level?

Separate the particles themselves from their arrangement and freedom of movement

Compare with the material's transition point

A different hypothetical pure material melts at 42 °C at fixed pressure. Consider settled samples where only solid and liquid are possible, with no delayed freezing. At 35 °C it is below the point, so it is solid. At 45 °C it is above the point, so it is liquid

1. 1Exactly at the point At 42 °C, temperature alone cannot tell how much is solid or liquid. The sample may be at the start, part-way through or at the end of the transition
2. 2One point, either direction Under these idealised conditions the same point applies to melting and freezing. The direction of energy transfer determines which state grows

A pure material melts at 42 °C at fixed pressure. A sample is recorded at exactly 42 °C, with no state observations. How much is liquid?

The same transition temperature can occur at different stages of melting

A pure sample is freezing at constant temperature. Does that mean no energy is transferred out?

Consider the changing particle arrangement, not only the thermometer

A thermometer reading needs a state observation

To investigate a transition, record the sample temperature and what is happening to its state together. A flat temperature record alone is not enough: a sample might simply have reached the room temperature without changing state

1. 1Record the conditions State the material and pressure conditions. Take repeated readings of the sample, not just the surrounding air, and note when solid and liquid are present
2. 2Compare model and experiment Real readings may vary through measurement uncertainty, uneven temperature or heat loss. A model's exact plateau is not a guarantee that every measured point will match
3. 3Practical work Using apparatus safely, making reliable observations and explaining uncertainty need supervised practical work and separate assessment. Choosing a proposed record does not demonstrate those skills

Which record best supports a claim about a sample's freezing temperature?

The record must link the measured temperature to liquid becoming solid

Use temperature, state and particles together

A temperature change is not the same as a state change. Melting and freezing rearrange the same particles; they do not turn individual particles into drops or stop all movement

1. 1Energy direction Melting transfers energy in; freezing transfers energy out. During a pure-substance transition at fixed pressure, temperature can remain constant while the phase proportions change
2. 2Evidence At the transition point, temperature alone does not give the amount in each state. Link temperature readings to state observations, and keep a model separate from real measurements