Concepts of Physics

IIT JEE Physics (1978-2018: 41 Years) Topic-wise Complete Solutions

Temperature of ice-salt mixture

Introduction

Kulphi is a popular Indian dish item, especially in summers. The vendors freeze Kulphi in ice-salt mixture. Normally when two objects are mixed the temperature of the mixture is intermediate between the temperatures of the two objects. But that does not happen for ice-salt mixture. This experiment is to give a direct feel of this phenomena.

Apparatus

Ice in a vessel, Plastic glass, salt, digital thermometer/thermocouple, weighing machine, watch glass, a stick.

Procedure

  1. Take 100 g of ice in the plastic glass. Put a thermometer with mercury bulb well inside the ice.
  2. Measure the temperature of ice.
  3. Weigh 10 g of salt on the watch glass using the balance.
  4. Put this salt in the ice and mix with the stick. Measure the temperature.
  5. Put 10 g of more salt in ice and mix it. Measure the temperature. Repeat this to get the minimum temperature.
  6. At what ratio of ice and salt, the temperature of the mixture is minimum?

Discussion

Suppose a solid is kept at it's melting point in a closed vessel. There could be some liquid phase too. If no heat is taken from it or given to it, the masses of the liquid phase and the solid phase remain constant. However, the solid does melt at the surface and the liquid freezes there. So the two rates are equal.

Ice also has water layer at the surface and this two-way process goes on. If you put salt in this, the salt dissolves in the melted water and decreases it's freezing point. Thus the process of water freezing back is considerable slowed down. Thus ice melts faster.

Now melting needs latent heat. This heat is taken from the mixture of ice and the water which cools down below 0 degree Celsius.

Note

table to be made