Regelation is the fusion of two blocks of ice by pressure, or the "successive melting under pressure and freezing when pressure is relaxed at the interface of two blocks of ice." It is a phenomenon in which water refreezes to ice after it has been melted by pressure at a temperature below the freezing point of water. This pressure makes an ice skate form a film of water that freezes once again after the skater has passed. The effect of impurities on melting point A substance containing impurities usually melts at a lower temperature than the pure compound, and melts over a wide range of temperatures. In general, the smaller the range of melting temperatures, the higher the purity of the sample. Foreign substances in a crystalline solid disrupt the repeating pattern of forces that holds the solid together. Therefore, a smaller amount of energy is required to melt the part of the solid surrounding the impurity.

This explains the melting point depression (lowering) observed from impure solids. The more impure the solid is, the more its structure is disrupted, and the greater the variation in intermolecular forces in different areas of the solid. The effect: the melting temperature is lowered compared to the pure solid, and the solid melts over a wider range of temperatures. Impurities in a solution prevent the ordering necessary to form a crystal lattice. This is why salt / water solutions will not freeze until sometimes as low as -20 ^0 C.

The concepts involved with melting points are similar. In a perfect crystal with no impurities the melting point will occur at one temperature. However when impurities are introduced there is no longer a continuous crystal structure. Instead the solid is made up of different regions, some with more crystal imperfections then others. Not surprisingly the melting point will not be as sharp.

This will also cause the melting point to begin earlier then expected. The effect of impurities on boiling point The boiling point of a substance will also change with the presence of impurity. Impurity will cause the boiling point to rise. From a molecular view the non-volatile impurity molecules have a chance to occupy a certain number of surface positions in the liquid.

This reduces the number of liquid molecules on the surface that will be able to vaporize. This decrease in vapour pressure will mean that the temperature will have to be higher before the vapour pressure becomes equal to the external pressure and boiling can begin. The boiling point of water can also be changed by adding impurities in the water. Impurities include salt, sugar, and other dissolving molecules. Impurities increase the boiling point of water. A simple explanation of this is that the impurities dilute the concentration of water (the number of water molecules per unit volume decreases), and the number of molecules that can vaporize at any give temperature decreases.

The result is that a higher temperature is required to achieve the same vapour pressure. Concentrated sugar-water solutions that are used for making candies and caramel boil at temperatures exceeding 150 ^0 C.