Question

Why is the motion of gas molecules called thermal motion?Is there any temperature at which molecular motion stops?

Answer 1

Thermal motion:
According to the kinetic theory of matter, the inter-molecular forces are lessened as matter changes from one state to another. The increase in temperature causes an increase in the kinetic energy and breaking of inter-molecular forces. In gases the kinetic energy due to temperature is high and the molecules/atoms are free to move around due to the thermal energy possessed in the matter. Hence the motion is due to kinetic energy obtained from rise in temperature. What this means is that motion in gases is directly affected by temperature, a rise in temperature leads to high kinetic energy while decrease in temperature results in low kinetic energy.
Is there any temperature at which molecular motion stops?
Yes, this is called the absolute zero temperature which is 0 kelvin or about -273 degrees Celsius. At this temperature almost all molecular and atomic motions hypothetically ceases. It is observed by extrapolating an ideal gas law graph.