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Principle of Methods of Mixture
MELTING POINT
For pure substances i.e., crystalline substances such as ice, naphthalene, etc., the melting and freezing points coincide, whereas in the case of amorphous substances like iron, glass, wax, etc., fusion and solidification take place over a short range of temperature, so that their melting and freezing. points are not fixed with definiteness. Since the melting and freezing points vary with different substances, it is often used to differentiate substances. -
Principle of Methods of Mixture
Effect of pressure on melting point:
The melting point of a substance changes when the pressure changes. A theoretical relation between the increase in pressure and the change in the melting point known as the Clausius -Clapeyron relation can be derived on the basis of thermodynamics. It is
\( \frac{{\text{dp}}} {{\text{dt}}} = \frac{\text{L}} {{\text{T}\left( {\text{V}_2 - \text{V}_1 } \right)}} \)
where V2 and V1 are the volumes per unit mass of the liquid and solid respectively at temperature T, dp/dT the ratio of the change of pressure with the change of the freezing point and L the latent heat of fusion.
From this relation it can be readily deduced. that the melting point of a few substances, which increase in volume on solidification (or contract on melting) like ice, cast iron, antimony, etc., is lowered by increase of pressure, whereas the melting point of most substances, which contract on solidification (or expand on melting) like wax, copper, etc., is raised by increase of pressure.
Calculations show that in the case of ice the melting point is lowered by about 0.0073° C per atmosphere increase of pressure, whereas in the case of paraffin wax, the melting point increases by about 0.04° C per atmosphere. -
Principles of Methods of Mixture
Examples of the lowering of the melting point of ice with increase of pressure
A) Regelation
The phenomenon of melting of ice due to excess pressure and its resoldification after the removal of the excess pressure is called regelation.
Example:
When a piece of wire loaded at both ends with equal weights is slung across a block of ice as shown in the figure, it is found that the wire cuts its way through the ice without however dividing the block into two pieces. The reason is that due to the pressure the ice immediately below the wire melts which enables the wire to sink through; but once the pressure is released, the water above the wire solidifies into ice again. It may be noted that the latent heat required to melt the ice below the wire comes from that evolved by the water solidifying above it. Hence, a copper wire will cut through the ice more quickly than an iron wire, because copper is a better conductor of heat than iron.
Note: Skating is possible on snow due to the formation of water below the skates. Water is formed due to the increase of pressure and it acts as a lubricant. -
Principles of Methods of Mixture
B) Supercooling
Most liquids, if cooled in a pure state in a perfectly clean vessel, with least disturbance, can be lowered to a temperature much below the normal freezing point, without solidifying. This is known as supercooling or superfusion. Thus, if a test tube containning distilled water is cooled slowly without stirring the water, the temperature is found to fall 3 to 4 degrees below 0°C without any ice being formed. Water can be cooled in this way even up to -10°C with a little care, and upto 12°C when covered with a layer of oil. Naphthalene, sulphur, phosphorous, antimony, etc., have also been supercooled far below their normal solidification points.
The phenomenon of supercooling does not really go counter to the fact that a liquid solidifies at a definite temperature, which is its freezing point. It is essentially an unstable phenomenon, since the introduction of the smallest quantity of the solid or even of impurity as dust particles, or any mechanical disturbances, such as shaking, stirring, etc., is enough to start solidification.
When once the solidification has started, the liquid continues to solidify and its temperature rises to the normal freezing point with the latent heat evolved and remains there till the entire mass has solidified.