Heat Science: Change of State

Fusion and solidification

The physical state, in which a substance exists at ordinary temperatures, can be changed by heating or cooling. When a solid is heated it melts into liquid and on being further heated the liquid passes into vapor. Conversely, when a vapor is cooled it condenses to form a liquid and a liquid on cooling solidifies. When a substance changes from the solid to the liquid state, the process is known as fusion or melting, and when it changes from the liquid to the solid state the process is called solidification or freezing.

Melting point: When a solid is heated its temperature gradually rises and at a certain temperature it begins to melt or turn into liquid. This temperature remains constant if the on it remains constant throughout the process of melting and is called the melting point and the heat absorbed during fusion is utilized for the change of state.

Freezing point: A liquid when gradually cooled begins to freeze or solidify at a particular temperature which is different for different substances. During freezing the though cooled will not fall in temperature. The temperature that remains constant throughout the process of solidification is called the freezing point and for each substance it slightly varies when the super incumbent pressure varies. In fact, the melting point of a solid is the same as the freezing point of the corresponding liquid.

Latent heat

Some heat is absorbed or liberated when a substance changes from one state to another. It may be observed that when heat is continuously given to a solid its temperature rises till at a certain temperature it begins to melt. If further heat is supplied, melting continues, the temperature remaining constant. The heat supplied at this stage does not produce a rise in temperature and the heat absorbed by the solid at the melting point is utilized break the potential energy of molecular binding in solids. Heat thus utilized brings about a change of state at constant temperature and Black called this as latent heat. When a liquid solidifies this heat is given out. The word latent means hidden; so latent heat means that heat for which there is no external manifestation, such as rise or fall of temperature.

Liquids of evaporation absorb similar heat at the boiling point and pass into vapor without change of temperature; latent heat is absorbed by the liquid to fight against the molecular binding forces in liquid which is almost absent in vapor molecules.

The latent heat of fusion of a solid may be defined as the quantity of heat required to change unit mass of the substance at its melting point from the solid to the liquid state without any change of temperature. The same quantity of heat is also given out by unit mass of substance at the same temperature in changing from the liquid state to the solid state without any change of temperature. The statement that latent heat of fusion of ice is 80 calories indicates that 1 gm of ice at 0°C requires 80 calories of heat to be converted to 1 gm of water at 0°C. Conversely, 80 calories of heat are given out when 1 gm of water at 0°C changes to 1 gm of ice at 0°C.

Sensible heat 

Sensible is that which brings about a change of temperature when given to a substance and is, therefore, accountable. But latent heat is only absorbed or liberated at the time of change of state without change of temperature.

According to the kinetic theory, temperature of a substance is measured by the kinetic energy of its molecules. Sensible heat increases the kinetic energy of its molecules. Sensible heat increases the kinetic energy of the molecules, while latent heat increases the potential energy of the molecules.

Laws of fusion

(1)   Every (crystalline) solid melts at a definite temperature. This temperature is called the melting point or fusion point of the solid. It is also the freezing point or solidification point of the corresponding liquid. When the pressure is one atmosphere, the melting point is called its normal melting point.

(2)   The rate at which fusion takes place is proportional to the rate of supply of heat, but temperature remains constant until the whole of the solid melts.

(3)   In order to melt unit mass of a solid into its liquid at the same temperature, a definite quantity of heat, known as the latent heat of fusion of the solid, must be given to it. This latent heat of solid varies very little with the pressure on its surface.

(4)   During fusion there is always a change in volume. For solids like paraffin wax, ghee, copper, etc. which expand on melting, an increase of pressure raises the melting point, while for solids like ice, iron, antimony, bismuth, which contract on melting, an increase of pressure lowers the melting point. 

Crystalline solids and amorphous solids

Crystalline solids have regular geometrical shape. Most solids belong this class. Ice, sugar, copper sulphate, common salt, metals, etc. are crystalline in structure. Only crystalline substances have a definite melting point.

    

Amorphous solids have no regularity of shape- they are structureless solids. Glass, pitch, etc. belong to this class. Amorphous substances change first from the solid to the plastic state, and then to the liquid state. The plastic state exists over a long range of temperature.