Hypothesis: if a fixed amount of gas is heated,
then the volume will
because
the heat will cause the molecules of gas to move

If fixed amount of gas is heated then the volume will increase because the heat will cause the molecules of gas to move freely and increase the kinetic energy.

Explanation:

If fixed amount of gas is heated then the volume will increase because the heat will cause the molecules of gas to move freely and increase the kinetic energy.

According to the Charle's law

The volume of given amount of gas is directly proportional to the temperature at constant pressure and number of moles of gas.

Mathematical expression;

V ∝ T

V = kT

V/T = k

if volume is changed from V1 to V2 and temperature change from T1 to T2 then,

V1/T1 = k      V2/T2= k

V1/T1 = V2/T2