Effect of Impurity on
Melting and Boiling points of a substance:
Melting Point:
Impurities decrease
the melting point of solids. That is, the solid melts before its melting point.
For example, melting point of ice is 0°C, at atmospheric pressure, but salt
addition to it decreases it melting point to -2°C to -25°C, depending on the amount
of salt added.
The impurity
molecules, here salt, causes disturbance in the solid lattice of ice, and cause
it melt before its melting point. The presence of impurity molecules between
the melted water molecules also prevents the solid from freezing again to its
solid state. That is why salt is sprinkled on roads to melt away ice after snow.
Boiling Point:
Impurities increase
the boiling point of liquids. That is, the liquid boils after their boiling
point. For example, boiling point of water is 100°C, at atmospheric pressure;
but addition of salt increases it. It becomes harder for the water molecules to
boil off, as vapours, due the presence of obstacle salt particles in their path.
