How to make ice-cream? Put syrup and water into the small cup/bag Put ice and salt into the big cup/bag Put the smaller bag into the bigger bag Shake until the ingredients in the smaller bag freezes
Water At 0 °C, melting point of ice and freezing point of water The ice molecules are constantly escaping to water, and the water molecules are captured at the surface of ice. This equilibrium will be maintained as long as the temperature is kept at 0°C and there is no external interruption
Salt Salt decreases the freezing point Salt molecules disrupt the equilibrium Even though salt molecules are dissolved in water, they do not easily attached to the ice. This means that the number of water molecules able to captured by ice goes down, so the rate of freezing goes down as well At the same time, the rate of melting of ice is unchanged, so melting occur faster than freezing
Salt To return to equilibrium, the temperature must be lower to make the water molecules slow down and attached to the ice more. The higher the concentration of salt, the lower the freezing point drops.
What about sugar in the syrup? Temperature of the larger container has decreased: the inside decreases as well Sugar does not disperse into ice, only in water As water freezes out, the remaining sugar solution become more concentrated, making it harder for water to crystallize in its pure form. The temperature is further lowered, the equilibrium shifted and the freezing point has decreased
Ice-cream freezing curve Not all of the water is frozen! The rest remain as very concentrated sugar solution
Freezing-Point Depression ΔTf = Kfcm Kf : freezing point depression constant Cm : molar concentration