What Is Freezing Point Depression?

The freezing point is the temperature at which a crystalline substance solidifies. Different crystals have different freezing points. Under a certain pressure, the freezing point of any crystal is the same as its melting point. For the same crystal, the freezing point is related to the pressure. For solidified crystals, the freezing point decreases with increasing pressure; for solidified crystals, the solidifying point increases with increasing pressure. During the solidification process, the liquid turns into a solid, and at the same time it releases heat. So when the temperature of the substance is higher than the melting point, it will be in a liquid state; when it is lower than the melting point, it will be in a solid state. Amorphous material has no freezing point.

Melting point refers to the transformation of crystals from solid to liquid

Freezing point pressure

The melting point (freezing point, the same below) of a substance usually refers to the situation at an atmospheric pressure; if the pressure changes, the melting point also changes. There are two different cases of melting points as a function of pressure. For most substances, the melting process is a process of increasing volume. When the pressure increases, the melting point of these substances will increase. For substances like water, unlike most substances, the volume of ice melting into water will shrink. (The same is true for metallic bismuth, antimony, etc.), the melting point of ice decreases as the pressure increases.

Impurities are dissolved at the freezing point

If a small amount of other substances, or impurities, are dissolved in the liquid, even if the amount is small, the melting point (freezing point, the same below) of the substance will change greatly. For example, when salt is dissolved in water, the melting point (the temperature at which solid and liquid phases coexist and balance) will drop significantly. Sea water is salt-dissolved water. In winter, seawater freezes at a lower temperature than river water. The melting point of saturated brine can drop to about -22 ° C.

Example: In order to separate the mixture of alcohol and water, Zhang Xiao proposed a solution based on what he had learned: water and alcohol can be separated according to the different freezing points of water and alcohol. To this end, he used the freezer in his home refrigerator to do the following experiments: he put a mixture of water and alcohol in different proportions into the freezer, and used a thermometer to monitor the temperature of the mixed solution, and observed regularly. The observed phenomena are as follows:

V alcohol / mL	V water / mL	V alcohol / V water	Observe the phenomenon
12.5	12.5	1/2	No solidification found at -8 ° C
12.5	30	1/4	No solidification found at -8 ° C
12.5	75	1/6	-8 mixed liquid began to solidify
12.5	370	1 / 30	The mixture at -6 started to solidify, and the alcohol was not separated from the water.
12.5	750	1 / 60	-4 The mixed solution started to solidify, and the alcohol was not separated from the water

(1) Please take a closer look at the data and records in the table. Can Zhang Xiao separate water and alcohol from the mixture?
(2) What is the effect of adding alcohol to the freezing point of water?

Parsing:

(1) As can be seen from the last column of the experimental table, water and alcohol did not solidify at first, and when the volume ratio reached a certain value, the mixed liquid could solidify, but the water and alcohol were not separated. The solution cannot separate water and alcohol from the mixture.
(2) Under normal circumstances, the freezing point of water is 0 ° C. It is not difficult to see from the last column of the table that after adding alcohol to water, the freezing point is lower than 0 ° C, so adding alcohol will cause the freezing point of water to decrease.
So the answer is: (1) No; (2) The freezing point of water will become lower.

Therefore, the presence of impurities is an important factor affecting the freezing point. Conversely, the freezing point (melting point) may sometimes be used for separation of impurities.