P/T constant at constant volume
For a fixed mass of gas at constant volume, pressure is directly proportional to absolute temperature: (i.e. = constant). Both conditions — fixed mass and constant volume — must be stated. Temperatures MUST be in kelvin. If doubles in kelvin at constant volume, doubles. This law is NOT on the Edexcel formula sheet, so recall it. Example: 100 kPa at 300 K becomes 150 kPa at 450 K.
Absolute zero = 0 K = −273 °C
Absolute zero is the lowest possible temperature, 0 K = −273 °C, where gas particles have the minimum possible kinetic energy and exert no pressure. The kelvin scale starts here. Convert with : 0 °C = 273 K, 27 °C = 300 K, −73 °C = 200 K. A 1 K step equals a 1 °C step; only the zero point differs. Always convert to kelvin before using .
Pressure from particle collisions
Gas pressure comes from particles colliding with the container walls; the total force per unit area is the pressure (). Heating at constant volume makes particles move faster (more kinetic energy), so wall collisions are more frequent AND each exerts a greater force. Both effects raise the pressure. A 3-mark "explain" needs all three: faster particles, more frequent collisions, greater force per collision.
Drawn from real examiner reports.
Convert to kelvin, never Celsius
needs temperatures in kelvin. Using °C gives a wrong answer with no credit: 27 °C and 54 °C look like a ratio of 2, but 300 K and 327 K give only 1.09. And ADD 273, never subtract — subtracting turns a positive temperature negative, which is impossible. Convert first, on its own line.
June 2024 Paper 2P Q8(a): most candidates converted correctly, but a subset subtracted 273 (giving a negative temperature) — the conversion step needs to be automatic. Paraphrased from the published examiner report.
State constant volume and fixed mass
Asked for the pressure–temperature relationship, you must also state the conditions: fixed mass of gas AND constant volume. Without them the statement is wrong — a gas free to expand obeys a different law. Safe phrasing: "for a fixed mass of gas at constant volume, pressure is directly proportional to absolute temperature".
June 2024 Paper 2P general: candidates who omitted stated conditions on law-based questions consistently lost one mark per question. Paraphrased.
Absolute zero: value AND minimum KE
Defining absolute zero as "the coldest temperature possible" is too vague for the mark scheme. Give both elements: (1) the value 0 K (= −273 °C), AND (2) the physical meaning — the temperature at which gas particles have the minimum possible kinetic energy. One element scores at most one mark; write both.
Say "kinetic energy", not just "energy"
When temperature rises, state that the particles' kinetic energy increases and they move faster — not merely that "energy increases". Dropping "kinetic" loses the specificity mark, because the temperature of a gas reflects the average kinetic energy of its particles.
June 2024 Paper 2P Q8(b)(i): virtually all candidates said the molecules' energy increases with temperature, and the majority correctly specified kinetic energy — those who wrote only "energy" risked the specificity mark. Paraphrased.
0 °C is not absolute zero
0 °C is the melting point of ice — particles are still moving fast — not absolute zero. Absolute zero is −273 °C (0 K), the point of minimum particle KE; the two differ by 273 degrees. Putting 0 °C at the origin of a P–T graph, or treating it as the true zero of temperature, is a fundamental error.
Constant volume vs constant pressure
This law is for a sealed rigid container: heating at constant volume raises the pressure. If instead the gas can expand at constant pressure (a free piston or balloon), heating raises the volume, not the pressure. Do not apply = constant to a case where the volume changes.
Show the kelvin conversion step
Lay out questions as: (1) write for each temperature; (2) state and rearrange the formula; (3) substitute the kelvin values; (4) evaluate with a unit. If the final arithmetic slips, steps 1–3 still earn marks.
Give the full kinetic-theory chain
For "explain why pressure rises when a gas is heated at constant volume", write the whole chain: faster particles → more frequent wall collisions → greater force per collision → higher pressure (). "Particles move faster" alone scores only 1 of 3.
Sense-check with the ratio
At constant volume the pressure ratio equals the kelvin temperature ratio. After calculating, check: if rose by ×1.5, should rise by ×1.5. A pressure that fell while the gas was heated, or a huge jump, signals a Celsius slip or a bad rearrangement.
Read the P–T graph in kelvin
Plot temperature in kelvin on the x-axis: the graph is a straight line through the origin (). Extrapolate it back to to locate absolute zero at 0 K (−273 °C). Using °C shifts the line off the origin and loses the intercept mark.
| Quantity | Symbol | Formula | Unit |
|---|---|---|---|
| Gas pressure–temperature law | , | Pa, K | |
| Kelvin conversion | K | ||
| Absolute zero | — | — | |
| Pressure (general) | Pa |
Where , = pressure before and after (Pa); , = absolute temperature before and after (K); = force (N); = area (m²).
Conditions for the gas law: fixed mass of gas AND constant volume. Both must be stated when asked.
Define absolute zero.
A sealed, rigid metal cylinder contains gas at a pressure of 100 000 Pa and a temperature of 27 °C. The cylinder is placed in an oven and heated until the temperature of the gas reaches 177 °C. The volume of the gas does not change.
Calculate the new pressure of the gas inside the cylinder.
