A student uses this apparatus to investigate how the volume of air changes with pressure - Edexcel - GCSE Physics - Question 3 - 2016 - Paper 1

For example, if we evaluate at a pressure of 98 kPa (substituting $p_2$):

1. Convert initial temperature to Kelvin, if necessary: $T_1 = 273 + 20$.
2. Substitute the values into the equation: $V_2 = \frac{98 \times 100 \times 277}{78 \times 283}$

Now evaluate the substituted expression: $V_2 = \frac{98 \times 100 \times 277}{78 \times 283} \approx 123 \: {m}^3$ This results in a new volume of approximately 123 m³, which we can round according to significant figures if required.

When the pressure decreases, as observed in the results table, the volume of the trapped air increases. Therefore, we can conclude that when pressure is reduced, the volume of air will expand.