Effect of Temperature on Matter
Solids
The temperature effect on heating a solid varies depending on the nature of the solid and the conditions required in bringing the change.Fusion or Melting
When we heat a solid, we add energy to the system increasing the vibration of the particles. Eventually these particles break free from their binding forces and fuse. Fusion is the change of state from solid to liquid. This is generally referred to as melting. The temperature at which the substance melts is called its melting point. The normal melting point of a crystal is the temperature at which it melts under one atmospheric pressure. A substance has a definite melting point, at a constant pressure. Increase in pressure usually raises the melting point. During melting, the temperature of a substance remains constant till the entire substance is converted into liquid due to the latent heat of fusion. The extra heat is used up in changing the state by overcoming the forces of attraction.
Remember :
The amount of heat required by one kilogram of solid to convert itself into liquid at atmospheric pressure at its melting point is known as latent heat of fusion.
Sublimation
Some solid substances when heated get converted directly to the gaseous or vapour state without first passing through the liquid state. When this vapour or gas is cooled, it gets converted directly into a solid, without passing through the liquid state. Such change of state of a solid directly into gas or vapour and back to the solid state, without passing through the liquid state, is called as sublimation. When a sublimable solid substance is heated, it is said to 'sublime' into a gaseous state; and when sublimable substances are cooled from their vapour state, the solid obtained is called the 'sublimate'. Some sublimable substances are: iodine, camphor, naphthalene, dry ice carbon dioxide) etc.
Experiment
Take some crystals of iodine in an evaporating dish. Cover it with a glass funnel, as shown in figure 2.3. Close the tip of the tail of the funnel with some cotton. Heat the dish gently. The space inside the funnel gets filled with purple vapours of iodine. Stop the heating. The purple vapours solidify to form beautiful purple crystals of iodine on the cooler sides of the funnel. These crystals are the sublimate of iodine.
