The freezing point may be defined as the temperature at which the liquid and solid states of a substance have the same vapor pressure. It is observed that the freezing point of a solution is always less than the freezing point of the pure solvent. This is termed as the “depression in freezing point of a solution.” Freezing point depression has very interesting and useful applications in everyday life, which are discussed in this article.
Explanation of the Depression in Freezing Point of a Solution
This observation can be explained based on the lowering of the vapor pressure of a solution. When we add a solute to the solvent, some of the solute molecules will take up spaces at the surface of the liquid. As we know, to vaporize, solvent molecules must be present at the surface of the solution. Now the presence of solute molecules at the surface decreases the surface area available to solvent molecules, and therefore reduces the solvent vaporization. Hence, the addition of solute molecules lowers the vapor pressure of the solution, which in turn lowers the freezing point of a solution, and this is known as “depression in the freezing point of a solution”. It is a colligative property, which is observed in solutions that result from the introduction of solute molecules. The freezing point of a solution is directly proportional to the molality of the solute. Hence, it is calculated by the following formula, which is described in the below image.
1. Science of Ice-Cream
When you have a craving to cool off, then you have the first thought of ice cream in your mind. This delicious treat comes in many different flavors and forms. But have you ever wondered how this delicious treat is created? When we make creamy ice cream, we control the size of the ice crystals in our ice cream. As we know that when we freeze the liquid, we eventually get large ice crystals. If the ice crystals are bigger, our ice cream gets harder and crunchier, and we really do not want this. In order to prevent this, our ice crystals should be small. This is done in two ways; first, by adding an emulsifier. Ice cream is a mixture of water and fats. Naturally, they both have a tendency to separate out and if they separate, the water molecules will hang out by themselves on one side and will be more likely to form ice crystals upon freezing. Here, the emulsifier keeps the water molecules and fats mixed together. The second way to control ice crystal formation is by changing how fast the water freezes because if we freeze our ice crystals quickly, then we get bigger and bigger crystals. This is done by adding salt to ice, which lowers its freezing temperature because of depression in the freezing point of a solution phenomenon.
2. Salting Icy Roads
You might have observed that the application of salt results in the melting of ice, and this is the common procedure to de-ice the snow-covered roads. Sodium chloride and calcium chloride are the most common forms of salt, which are used to de-ice the roads. The main reason for pouring salt on an icy road is that a solution of water and dissolved salt has a lower freezing point than pure water because of depression in the freezing point of a solution. Water normally freezes at 32°F, but when we add salt, that temperature drops. The higher the concentration of salt, the lower its overall freezing point.
3. Natural Antifreeze: Survival in Cold Weather
Animals living in cold climates have natural antifreeze proteins (AFPs), which bind to ice crystals in their blood to prevent crystallization. AFPs are extremely efficient at inhibiting ice recrystallization in frozen solutions. This is an important function of the proteins in freeze-tolerant organisms. AFPs have been found in various organisms, such as marine fishes, freeze-avoiding insects, and other terrestrial arthropods. Organisms also produce relatively inert molecules, such as glycerol and other polyhydric alcohols, such as sorbitol and ribitol, which helps in decreasing the freezing point of the water in their bodies.
4. Radiator fluids in Automobiles
Radiator fluid might be an antifreeze coolant, which is used in automobiles to prevent freezing up of water in extremely cold climates. Antifreeze lowers the temperature of water in the car radiator so that it won’t freeze, even when the temperature reaches below 0°C.
5. Vodka: Why it does not freeze in the freezer?
You must have known that vodka can be stored in the freezer forever without getting frozen. So why does vodka does not freeze? The answer is very simple, it does not freeze because of the extremely low freezing point of alcohol. Vodka primarily contains water and ethanol. Pure water freezes at 0°C, and pure ethanol freezes at -114°C. Vodka does freezes but not at the temperature of the ordinary freezer. This is because it contains enough alcohol, which lowers the freezing point of water below -17°C of the ordinary freezer. This is the freezing point depression phenomenon, which accounts for the lowering of the freezing temperature of vodka.
6. Ocean Water
The whole ocean water is not frozen or solid, but the water in between the ocean is still liquid. This is again based on the freezing point depression phenomenon. As ocean water is salty, the temperature falls below the freezing point of pure water, which is responsible for the liquid state of water, even when the temperature is below 0°C.