Gases and the Absolute (Kelvin) Temperature Scale

This lesson explains Gases and the Absolute (Kelvin) Temperature Scale for Cambridge IGCSE Physics 0625. It separates the Core requirements from the additional Supplement work for Extended candidates. Pay close attention to equation choice, unit conversion and the final sense check. After the example, use the related practice questions to check what you can do independently.

Core: why does gas pressure change?

Gas particles move randomly and collide with the walls of their container. These collisions produce a force on the walls. Pressure is force per unit area.

For a fixed mass of gas:

• Higher temperature at constant volume: particles move faster, collide with the walls more often and produce a greater pressure.
• Lower temperature at constant volume: particles move more slowly, so pressure decreases.
• Smaller volume at constant temperature: particles have less distance to travel between wall collisions, so collisions with the walls occur more frequently and pressure increases.
• Larger volume at constant temperature: wall collisions occur less frequently and pressure decreases.

At constant temperature, do not say the particles hit the wall with greater average speed when the gas is compressed. The key change is collision frequency.

Core: how do you convert Celsius to kelvin?

The official relationship is:

$T\text{ (in K)} = \theta\text{ (in °C)} + 273$

where $T$ is the temperature in kelvin and $\theta$ is the temperature in degrees Celsius.

Examples:

• $27\,°\text{C} = 27 + 273 = 300\,\text{K}$
• $0\,°\text{C} = 273\,\text{K}$
• $-5\,°\text{C} = -5 + 273 = 268\,\text{K}$
• $300\,\text{K} = 300 - 273 = 27\,°\text{C}$

A temperature interval has the same numerical size in kelvin and degrees Celsius. A rise of 10 °C is a rise of 10 K.

Core original question

A sealed rigid container holds a fixed mass of gas. The gas is heated.

Explain why the gas pressure increases. [3]

A complete answer is:

The particles gain kinetic energy and move faster. They collide with the container walls more frequently, producing a greater force on the walls. Because the wall area is unchanged, the pressure increases.

Original marking guidance

• particles move faster or have greater kinetic energy
• collisions with the walls are more frequent
• greater force per unit area or greater pressure

Supplement: what does pV = constant mean?

For a fixed mass of gas at constant temperature:

$pV = \text{constant}$

For two states of the same gas:

$p_1V_1 = p_2V_2$

If the volume is halved at constant temperature, the pressure doubles. A pressure-volume graph for this relationship is a curve rather than a straight line.

The equation only applies when:

• the mass of gas is fixed
• the temperature is constant
• pressure and volume units are used consistently between the two states

Supplement worked question

A fixed mass of gas occupies 12 litres at 200 kPa. It expands at constant temperature until its pressure is 100 kPa. Calculate the new volume. [3]

$p_1V_1 = p_2V_2$

$200 \times 12 = 100 \times V_2$

$V_2 = \frac{2400}{100} = 24\text{ litres}$

Both pressure values use kPa and both volumes use litres, so the paired units are consistent.

Original marking guidance

• one mark for selecting $p_1V_1 = p_2V_2$
• one mark for correct substitution and rearrangement
• one mark for 24 litres with the unit

Common mistakes

• Telling Core students to skip the Kelvin conversion. It is Core content in the 2026 to 2028 syllabus.
• Using Celsius in place of the absolute scale. Convert using $T = \theta + 273$ when a kelvin value is required.
• Adding 273 in a $pV = \text{constant}$ question that gives no temperature change. The equation already assumes constant temperature.
• Mixing kPa with Pa or litres with cubic metres between states. Make matching units before substituting.
• Saying compression makes particles faster at constant temperature. The collision frequency changes; the average particle speed does not increase when temperature is fixed.

Exam technique

For a pressure-volume calculation, write a small state table before substituting:

State	Pressure	Volume
1	$p_1$	$V_1$
2	$p_2$	$V_2$

For a particle explanation, use a cause-and-effect chain: temperature or volume change, particle-motion or collision change, force-on-wall change, pressure change.

How this is examined

Core Papers 1 and 3 can test qualitative pressure changes and Celsius-kelvin conversion. Extended Papers 2 and 4 can test those requirements plus $pV = \text{constant}$ and its graph. Practical papers may use a pressure gauge or require careful control of temperature, but no separate gas-law experiment is prescribed on this page.