Describe thermal expansion at constant pressure
Recognise increased dimensions or volume during heating without assuming every constraint allows it.
Exam scope and assessment notes
Mapped to the named 0973 section using teaching from the corresponding single-science course. A 0973-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade. This qualification reports two grades on the 9–1 scale; confirm the entry route for your zone and exam series.
Before you begin
- Distinguish temperature change from change of state.
- Identify whether pressure or volume is held constant.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which situation explicitly allows a volume change at constant pressure?
What you will learn
- Describe the usual qualitative expansion of solids, liquids and gases on heating at constant pressure.
- Distinguish free expansion from fixed-volume or restrained situations.
Describe the macroscopic change
Thermal expansion is an increase in a material's dimensions or volume as its temperature rises in the usual examples studied here. At constant pressure, a freely expanding solid can become longer and larger, a liquid sample can occupy a greater volume, and a fixed gas sample can expand into more space. Cooling commonly gives contraction under comparable conditions without a state change.
Specify the material, temperature range and constraints. The qualitative account is not a numerical expansion coefficient or an unrestricted rule for every substance in every range. A state change is a separate issue: melting is solid to liquid, while thermal expansion can occur while the material remains solid. The Supplement explains the usual relative magnitudes through particles.
Constraints change what is observed
A fixed mass of gas in a rigid constant-volume model cannot increase its volume even if its temperature rises. The pressure then rises instead under the collision account. That is different from constant-pressure expansion with room for the gas to occupy more space. Do not apply both fixed-volume and freely expanding descriptions to the same constrained sample without stating a different situation.
A solid clamped between supports may develop forces or stresses when its usual expansion is restrained. A sample's inability to extend freely does not establish that temperature has no effect. These are described situations, not instructions to heat sealed containers or restrained objects; practical work needs suitable teacher supervision and apparatus.
Sources for this lesson
Pause and explain
A solid grows slightly longer on heating but stays solid. Must it have melted?
Worked example
Compare a freely expanding metal bar warmed at constant external pressure with a fixed gas mass warmed in a rigid constant-volume written model.
Show the worked solution
- The free bar can increase its dimensions while remaining solid.
- The rigid gas volume cannot increase.
- Under the fixed-gas collision model its pressure rises instead of its volume.
Answer The free bar expands; the rigid-volume gas has a pressure rise rather than a volume increase.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Describe the usual heating effect for freely expanding solid, liquid and fixed gas samples at constant pressure with no state change.
Give me a hint
Describe dimensions or volume rather than inventing a phase change.
Compare my reasoning
- The solid can increase its dimensions.
- The liquid can occupy a larger volume.
- The fixed gas sample can expand into more available space while pressure is maintained.
Usual thermal expansion increases solid dimensions and liquid or gas volume under the stated conditions.
Look for these in your work
- Keep constant pressure explicit.
- Retain each stated state.
Compare a fixed gas sample heated at constant volume with one heated at constant pressure that has room to expand.
Give me a hint
Identify which quantity is permitted to change in each model.
Compare my reasoning
- At constant volume, the gas cannot expand into a larger volume.
- Higher temperature raises pressure through changed collisions in that fixed-volume model.
- At constant pressure with room available, the gas can increase its volume instead.
Fixed volume: pressure rises; fixed pressure with room: volume increases.
Look for these in your work
- Do not mix the controls.
- Describe the contrasting observed quantities.
A warmed metal bar cannot lengthen because both ends are restrained. A student concludes that thermal expansion is irrelevant. Explain a likely consequence without calculating an unsupported force.
Give me a hint
Restraint changes the mechanical consequence of the usual expansion.
Compare my reasoning
- The material would normally expand freely in this example.
- The supports restrain the length change.
- Forces or stresses can develop; their numerical size needs further information.
Restrained expansion can develop forces or stresses even without visible free extension.
Look for these in your work
- Use the stated constraint.
- Avoid inventing a force magnitude.
Common mistakes
- State the pressure and volume conditions.
- Expansion need not be melting.
- Restraint can produce forces rather than free extension.
What can you explain now?
Must a warmed rigid fixed-volume gas container allow the gas volume to increase?
Compare with the explanation
No.
Its specified volume is fixed; heating instead changes pressure under the fixed-mass model.
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Make it stick
Describe thermal expansion within a state and explain why fixed-volume or restrained systems have different consequences.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.