kinetic energy

Q: when a liquid evaporates,why that average kinetic energy of molecules in the liquid decrease?

Molecules in liquid are bound together by attractive intermolecular force. During evapoation, only those molecules that have sufficiently high kinetic energy (KE) can overcome the intermolecular force and escape as free molecules into thr air. Thus molecules that remain behind are of lower energies and this decrease the average KE of the molecules.
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Q:仲有,我想問一問點KE同PE係 state and tempature of substance 擔當住咩角色?

Temperature of a substance is attributed by the KE of molecules in the substance. The higher the temperature, the higher the KE of molecules will be.

Two substances at the temperature will have the same average KE for their molecules.

When a solid is heated, apart from the increase of KE of its molecules (this is observed as a temperature increase), the separation of molecules also increases, i.e. there is an INCREASE in potential enery (PE). The heat energy supplied to the substance is partly used to do work against the intermolecular force and this work done is changed to the PE of molecules.

It is because difference substances, because of different molecular structures, have different mangnitudes of intermolecular forces acting between their molecules. Thus, the change of molecular PE will be different in different substances even if the same change of temperature occurs. This is why different substances have different specific heat capacities. In fact, the specific heat capacity of a substance reflects the strength of intermolecular force and hence the PE change of its molecules.

Thus, on average, molecules in liquids have PE higher than those in solids, and gas molecules have the highest PE over solids and liquids.

When a liquid evaporates, there is no temperature change in that liquid, so the kinetic energy of the molecules should not be changed. Instead, since the molecules are further apart, so the potential energy among the molecules increases. Since the number of molecules in the liquid is decreasing, so the AVERAGE K.E. of the molecules in the liquid decreases.

KE is effected by :temp
PE is effected by:mass