Question

Explain how salt can be used to both melt ice on the streets in the winter and freeze ice cream in an old-fashioned ice cream maker.

Answer 1

The common salt is used to melt ice on the streets in winter and also to freeze ice cream in old fashioner ice cream maker in the same way using only one concept. Tha econcept a colligative property

depression in freezing point.

The value of depression in freezing point is calculated as

delta Tf = i x Kf x m

i = van't Hoff factor = 2 in case of common salt NaCl

Kf = 1.86 K. Kg /m

m is molality of solute

Thus by adding common salt ot ice on streets , th efreeezing point of water is lowered. Thus at 0C the ice does not form/the formed ice starts melting. thus NaCl is commonly used to melt ice in cold countries during winters.

CaCl2 has i =3 , and higher molar mass, thus is more effective in lowering the melting point of ice and hence is more effective in clearing roads.

The same concept in making ice creams also.

Now we keep the milk in a mixture of ice san salt . due to depression in freezing point, th efreezing mixture of ice and salt is at much lower temperature than 0C. This temperature makes the milk frozen to ice cream.