Explain heating a fixed gas mass at constant volume
Use faster average motion and stronger, more frequent wall impacts under controlled conditions.
Mapped to the named 0654 section using teaching from the corresponding single-science course. A 0654-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.
Before you begin
- Describe gas-wall collisions and pressure.
- Relate greater temperature to greater average particle kinetic energy.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What causes pressure on a gas-container wall?
What you will learn
- Explain the qualitative effect of temperature on pressure at fixed gas mass and volume.
- Distinguish heating from adding particles or changing their size.
State the controls before the explanation
Consider a fixed mass of the same gas in a constant-volume written model. Raising its temperature increases average particle kinetic energy, so its particles move faster on average. They collide with a given wall area more frequently and with stronger effects on average. The resulting average wall force per unit area increases: the gas pressure rises.
The amount and volume are fixed in this comparison. It is not explained by adding particles, enlarging the particles or shrinking the container. Individual speeds remain varied, so 'every particle now has exactly the same speed' is not the intended account. Cooling under the same controls reverses the average-motion and collision changes and lowers pressure.
Qualitative reasoning has limits
A temperature rising from 20°C to 40°C does not justify a claim that pressure doubles by taking the ratio of the Celsius readings. This section's Core requirement is a qualitative collision explanation, and Celsius zero is not the absolute-temperature zero. Do not introduce an unsupported numerical pressure-temperature ratio into that explanation.
If a vessel leaks or its volume changes, the original controls no longer hold. Heating may still affect the gas, but temperature alone does not establish the same fixed-mass, fixed-volume comparison. The example is a written model, not an instruction to heat a sealed vessel; actual gas investigations require suitable apparatus and teacher supervision.
Sources for this lesson
Pause and explain
Heating a fixed gas mass at constant volume makes its pressure rise. Which mechanism fits the controls?
Worked example
A fixed gas mass is warmed from 20°C to 40°C at constant volume. Predict its pressure change and evaluate a claimed doubling.
Show the worked solution
- Higher temperature gives faster average particle motion.
- More frequent and stronger average wall impacts increase pressure.
- The ratio 40/20 of Celsius readings does not establish doubled pressure.
Answer Pressure increases; doubled pressure is not established by the Celsius ratio.
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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.
Explain increased pressure when a fixed gas mass is heated at constant volume.
Give me a hint
Start with temperature, then link average motion to wall collisions.
Compare my reasoning
- Gas mass and accessible volume remain unchanged.
- Higher temperature gives greater average kinetic motion.
- More frequent and stronger average wall impacts increase force per area and pressure.
Pressure rises because average wall impacts become more frequent and stronger.
Look for these in your work
- State the two controls.
- Give the full collision chain.
Compare heating and cooling the same fixed gas mass at constant volume. Must all particles have identical speeds in either case?
Give me a hint
Use average motion and reverse the collision account for cooling.
Compare my reasoning
- Heating increases average motion and pressure.
- Cooling decreases average motion and pressure under the same controls.
- Particles can still have varied individual speeds in both samples.
Heating raises pressure; cooling lowers it; individual speeds need not be identical.
Look for these in your work
- Apply the controls in both directions.
- Use an average rather than an identical-speed claim.
A gas vessel warms while leaking and expanding. A student applies the fixed-mass, fixed-volume explanation as proof of a particular pressure rise. Evaluate it.
Give me a hint
Check every condition rather than only the temperature change.
Compare my reasoning
- Leaking changes gas mass.
- Expansion changes accessible volume.
- The original controlled comparison does not establish this vessel's pressure outcome without more information.
The controls fail; the pressure outcome is not established by temperature alone.
Look for these in your work
- Identify both failed controls.
- Avoid an unsupported pressure prediction.
Common mistakes
- Keep both mass and volume fixed.
- Particles do not need to grow.
- Celsius ratios do not establish numerical gas-law ratios.
What can you explain now?
What happens qualitatively on cooling a fixed gas mass at constant volume?
Compare with the explanation
Pressure decreases.
Slower average motion gives less frequent and weaker wall impacts on average.
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Make it stick
Explain a fixed-volume temperature change and identify when the comparison's controls or a Celsius ratio fail.
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.