Question

Heat is removed from a gas, and the substance becomes a liquid. How can you explain this?

Answer 1

When heat is removed from a gas, the thermal energy of the molecule gets reduced, which also reduces the kinetic energy, resulting in slow movement of the molecules and this results in change of state.

Complete answer:
In order to answer our question, we need to learn about thermal energy. Thermal energy is the energy of a body due to motion or movement of its atoms or molecules. As the temperature increases, thermal energy increases so the kinetic energy of the atoms and molecules also increases. As the movement of particles increases, the molecules move far apart from each other. Thus thermal energy is the measure of average kinetic energy of the particles of matter and responsible for the movement of particles. This behaviour of the movement of particles is also called thermal motion.
The two factors namely, pressure and temperature are the deciding factors during the inter-conversion of a state of a substance. Example for changing a gas to a liquid, although the pressure is increased to increase the intermolecular interactions but the inter conversion is not possible unless the thermal energy of the molecule is lowered by decreasing the temperature.
Removal of heat from a substance is called an exothermic reaction. The heat is decreasing, and hence the speed of molecules are decreasing, causing them to move slower and that is why their phase gets changed.

Note:
The change of phase by altering the thermal energy can also be defined on the basis of Charles Law, because as temperature is increasing, the volume also increases, so molecules will move faster and occupy more space. Hence liquid changes to gas.