Interconversion of states of matter
Elements compound and mixture of Class 8
About Interconversion of states of matter
The change of matter from one form to another that occurs as a result of a change in external conditions like temperature or pressure is known as inter-conversion of states of matter. The process of phase transition does not produce any change in the mass or composition of the matter. The different processes involving phase transition are given different names on the basis of initial and final states of matter.
The process involving the change of matter from liquid to solid on cooling is called solidification if freezing occurs at normal atmospheric pressure is called freezing point of that liquid.On cooling, the potential energy of the liquid molecules is released in the form of heat energy. Consequently, the intermolecular distance decrease and intermolecular force of attraction increases. The molecules lose their translational movement, rotational movement and they only vibrate about their fixed mean position. Thus the molecules of the liquid attain the molecular arrangement of a solid.
The process by which some solid substances directly change into the vapour state on heating without passing through the intermediate liquid state is called sublimation. The solid obtained on cooling the vapour is called sublimate and the vapour formed is called sublime.Example: Camphor, Iodine
The process which involves the change in state of matter from solid to liquid by heating is called melting or fusion. The temperature at which a solid changes into a liquid on heating at normal atmospheric pressure is called the melting point of that solid.
The heat energy supplied to the solid is absorbed by its molecules. This helps the molecules to overcome the intermolecular force of attraction as a result of which the molecules move apart from each other. The molecules acquire translational motion in addition to vibration motion, so they become free to move and thus attain the molecular arrangement of a liquid.
The melting point of solids which expand on melting, increases with an increase in pressure while the melting point of solids which contract on melting decreases with an increase in pressure.
Evaporation and boiling
The slow process involving the change of matter from liquid to gas from the surface of the liquid is called evaporation.The temperature at which a liquid changes into a gas on heating at normal atmospheric pressure is called the boiling point of that liquid and the phenomenon is called boiling.
The heat energy which is supplied to the liquid is absorbed by the molecules of the liquid and helps the molecules to overcome the intermolecular forces of attractions almost completely. As a result, the molecules move far apart from each other and start moving in all directions at random. Thus the liquid attains the molecular arrangement of a gas.
The process which involves the change of matter from gas to liquid on cooling is called condensation.On cooling, the potential energy of the gas molecules is released in the form of heat energy. Consequently, the intermolecular distance between the gas molecule decreases and the intermolecular force increases. As a result, the molecules become less free to move about. Thus the gas attains the molecular arrangement of a liquid.
Since the gases are highly compressible, they can be liquefied by the application of pressure provided the temperature of the gas is equal to or below its critical temperature and this process is called liquefaction.
Critical temperature is the temperature above which the liquefaction of the gas is impossible whatever be the pressure applied on it.For every gas, there is a specific critical temperature. The gaseous state of matter below its critical temperature is called vapour. To score More in your class 8 refer NCERT solutions for class 8 .