Explanation MELTING (Solid to liquid) In solids, we observe that the intermolecular forces are very strong and their separation distances are very small. We notice as if a definite place is reserved for each atom forming a rigid structure. However, the atoms and molecules will be vibrating about their mean positions without interchanging their positions. When a solid is heated, its molecules absorb the heat energy which increases their average kinetic energy and the molecules start vibrating vigorously as the heat supply is increased. When the temperature reaches a critical value the interatomic forces can no longer hold the molecules in position and the molecules starts moving from their fixed positions. The solid starts changing into a liquid form. It may be noted that until the entire solid is transformed into liquid, the temperature of the solid does not increase even though heat is being supplied. The heat energy supplied only increases the intermolecular spaces without increasing their kinetic energy. In other words, during the process change of state from solid to liquid the heat energy is stored in the in the form of potential energy. This process of change of state from solid to liquid is called melting. We need a definite quantity of heat to melt one Kg of a solid into a liquid. This heat is called latent heat of melting. The critical temperature at which the change of state from solid to liquid takes place is called the melting point (mp) and is different for different solids. FREEZING (SOLIDIFICATION): (Change of State from liquid to solid) If the temperature of a liquid is lowered, the jiggling motion of molecules of the liquid slows down, since their kinetic is reduced. The attractive forces pull the molecules closer. The molecules and atoms start occupying their fixed positions forming a rigid substance. This process of change of state from a liquid to a solid is known as freezing. It may be noted a given liquid changes into a solid only at a particular temperature known as the freezing point. For most of the substance, the melting point and freezing points are the same. For example, water freezes at 0oC and solid ice melts at 0oC. The process in which, the liquid on cooling to a specific temperature changes into a solid is also known as solidification.   VAPOURISATION (BOILING): (Change of state from liquid to gas) When a liquid is heated, the average kinetic energy of the molecules increases gradually. The intermolecular forces will be reduced. Finally, the molecules acquire sufficient kinetic energy and break away from each other and start moving to the surface, while molecules of low kinetic energy start moving down setting up convection currents. When the liquid attains a critical temperature, the molecules start leaving the surface and escape into the atmosphere in the form of steam or vapour. One can also observe vigorous motion of molecules within the liquid as well as bubbles of water vapour. The bubbles, due to decrease in density rise to the surface and burst and the vapour escapes into the air. As we continue to supply heat, the liquid completely changes into its vapour state. This change of state from solid to liquid is known as vapourisation. The process of vigorous motion of liquid molecules up and down is called boiling. The critical temperature at which this phenomenon of vapourisation of liquid takes place is called the boiling point of the liquid and is different for different liquids. Some liquids change into vapour state even at room temperature. This process is known as evaporation. There is no critical temperature for evaporation to occur. The wet clothes become dry because of the process of evaporation. Similarly, petrol evaporates from the container if its lid is not closed. CONDENSATION: (Change of state from gas to liquid) When a vapour is cooled, the molecules slowly lose their kinetic energy and therefore their motion slows down and the molecules get attracted to each other. The intermolecular forces of attraction between the molecule increases. If the temperature is lowered sufficiently, the chaotic movement of molecules in the vapour state may be brought to an orderly arrangement, as found in a liquid. When this happens, the vapour or gas changes into a liquid. This process of change of state from gas to liquid is known as condensation. LIQUID vapourisation GAS Condensation SUBLIMATION: (Change of state from solid to gas): Some solids, when heated changes directly into gaseous state without passing through a liquid state. This process is known as sublimation. Sublimation is the process in which the molecules of the solid acquire sufficient kinetic energy to change directly into a gaseous (vapour) and on cooling directly changes back to solid state without changing into liquid state. For example, solids such as camphor, iodine undergoes sublimation since they have negligible forces of attraction between their molecules. On heating, the molecules are able to overcome the intermolecular forces of attraction and break away from each other and escape from the surface of the solid in the form of vapour. Other solids which undergo sublimation are ammonium chloride, dry ice ( solid carbon dioxide), mercuric chloride, etc. Sublimation can also take place in the absence of heat. We observe naphthalene balls evaporate or sublime at room temperature, a process we can detect by the odour of the vapour. COMMENTARY: Sublimation Solid Gas (sublime) Condensation . Water can exist in three different states - Ice(solid), Water(liquid), and steam(gas). Water can change from one state to other by heating or cooling. Water can also exist in all the three states simultaneously. The temperature at which this happens is called triple point. The triple point is different for different materials. For water, it is 273.16 k. A molecule of water comprises of one oxygen atom surrounded by two hydrogen atoms. We see that water occupies a definite volume when we pour it into a vessel. Water also flows down a slope. We never see the molecules of water breaking apart even though they are in constant motion. This is because of forces of attraction between the water molecules. Suppose we heat the water, the jiggling motion of the molecules increases. If the heating continues further, there comes a point at which the forces of attraction between the molecules is not enough to hold them together. The molecules fly apart because of increased kinetic energy due to the increase in temperature. They form what is known as water vapour or steam. When the temperature reaches a critical value water starts boiling with increased molecular movement with formation of bubbles and water starts changing into to steam. This is known as the gaseous state of water. This happens at the boiling point of water, which is 100 0C. Let us see what happens when the water is cooled below the room temperature. We notice the jiggling motion of the molecules steadily decreases as the temperature is reduced. When the temperature reaches a certain critical value (0°C), called the freezing point, the molecules lock themselves into a new pattern forming what is called ice. The molecules of ice form into arrays of hexagonal shapes with large gaps in between them. For this reason, the density of ice is less than that of water and we observe that the ice floats on water. Though ice has a rigid crystalline form its atoms are not static, they vibrate about their mean positions. However, when heat is supplied, the atoms vibrate greater and greater until each molecule breaks loose and form water, which we call melting. When we heat these flakes of ice, their rigid structure breaks down and we see the molecules filling the gaps and the large volume of ice shrinks when it becomes water by heating.