what is critical temperature .how is it related to intermolecular force of attraction?

Critical temperature of a gas is defined as the maximum temperature at which a gas can be liquefied that is the temperature above which it cannot be liquefied no matter how much high pressure is applied. The critical temperature of a gas is a measure of the strength of the intermolecular forces of attraction of the gas. Weaker are the intermolecular forces of attraction, more difficult it is to liquify that gas and hence lower is the critical temperature of that gas. As an example you can consider helium and hydrogen. Both these gases have weak intermolecular forces of attraction and hence are difficult to liquify. Consequently they have low critical temperature. On the other hand, carbon dioxide has much higher critical temperature. This is because the intermolecular forces of attraction between carbon dioxide molecules are very high and therefore it can be easily liquefied.