Freezing Point Depression

Temperature at which something becomes frozen The addition of solutes lowers the freezing point of solvents, which is referred to as depression.

It is a colligative feature of solutions that is typically proportional to the molality of the solute that has been added to the solution in question. According to the following formula, a decrease in the freezing point of a solution can be characterised as follows.

ΔTf = i*Kf*m

Where

The freezing point depression is denoted by Tf.

The Van’t Hoff factor is represented by the letter i.The cryoscopic constant is denoted by Kf.

The molality is represented by the letter m.

In all cases, the solute is the substance added/present in smaller amounts, while the solvent is the original substance present in larger quantities. Because the chemical potential of the solvent in the mixture is lower than that of the pure solvent, the resulting liquid solution or solid-solid mixture has a lower freezing point than the pure solvent or solid, the difference between the two being proportional to the natural logarithm of the mole fraction. Similarly, the chemical potential of the vapour above the solution is lower than that of a pure solvent, resulting in a rise in boiling point. At temperatures below 0 °C (32 °F), the freezing point of pure water, freezing-point depression causes sea water (a mixture of salt and other compounds in water) to remain liquid.

Van’t Hoff Factor

The Van’t Hoff factor explains how solutes affect the colligative characteristics of solutions.The ratio of the concentration of particles generated when a material is dissolved to the concentration of the substance by mass is known as the Van’t Hoff factor.

The Van’t Hoff factor describes the degree to which a substance associates or dissociates in a solution. When a non-electrolytic material is dissolved in water, for example, the value of I is usually 1. The value of I on the other hand, is equal to the total number of ions present in one formula unit of an ionic molecule when it forms a solution in water.

CaCl2, for example, has an optimum Van’t Hoff factor of 3 because it dissociates into one Ca2+ ion and two Cl– ions. However, some of these ions in the solution form associations with one another, resulting in a reduction in the overall number of particles in the solution.

This factor is named after Jacobus Henricus Van’t Hoff, a Dutch physical chemist who earned the first Nobel Prize in chemistry. It’s worth noting that for electrolytic solutions, the measured Van’t Hoff factor is usually lower than the expected value (due to the pairing of ions). The more the ion charge, the higher the divergence.

Colligative Properties

A solution’s colligative qualities come in a variety of forms. Vapour pressure is reduced, boiling point is raised, freezing point is lowered, and osmotic pressure is reduced.

1. Lowering of Vapour Pressure

The molecules of a pure solvent occupy the whole surface of the solvent. When a nonvolatile solute is added to a solvent, the surface now contains both solute and solvent molecules, reducing the proportion of the surface covered by solvent molecules. Because the vapour pressure of the solution is completely due to the solvent, the vapour pressure of the solution is found to be lower than that of the pure solvent at the same temperature.

2. Elevation in Boiling Point

The temperature at which the vapour pressure equals atmospheric pressure is known as the boiling point of a liquid. The vapour pressure of a solution decreases when a non-volatile liquid is added to a pure solvent, as we know. As a result, we must raise the temperature of the solution to make the vapour pressure equal to air pressure. Elevation in boiling point refers to the difference between the boiling point of a solution and the boiling point of a pure solvent.

3. Depression in Freezing Point

The temperature at which the vapour pressure of a substance’s liquid equals the vapour pressure of the equivalent solid is known as its freezing point. When a non-volatile solid is introduced to a solvent, the vapour pressure falls until it equals that of a solid solvent at a lower temperature, according to Raoult’s law. Depression in freezing point is the difference between the freezing point of a pure solvent and its solution.

4. Osmotic Pressure

Osmotic Pressure is the fourth factor to consider.

When a semipermeable membrane is placed between a solution and a solvent, solvent molecules enter the solution via the semipermeable membrane, increasing the volume of the solution. Only solvent molecules pass through the semi-permeable membrane, which prevents larger molecules like solute from passing through. Osmosis is the spontaneous movement of solvent molecules from a pure solvent to a solution or from a dilute to a concentrated solution over a semipermeable membrane.

If some extra pressure is supplied from the solution side, the passage of solvent molecules through the semipermeable membrane can be stopped. The osmotic pressure of the solution is the pressure that simply stops the flow of solvent.

Uses

A salt truck is used to manually spread salt.

The freezing-point depression phenomenon has a wide range of applications. A car’s radiator fluid is made up of a mix of water and ethylene glycol. In the winter, the freezing-point depression keeps radiators from freezing. This effect is used by road salting to lower the freezing point of the ice it is applied to. Lowering the freezing point allows street ice to melt at lower temperatures, preventing dangerous and slippery ice accumulation. Sodium chloride, which is commonly used, can lower the freezing point of water to around 21 °C (6 °F). When the temperature of the road surface drops below a certain level, NaCl becomes ineffective, and other salts, such as calcium chloride, magnesium chloride, or a combination of them, are used instead. Because these salts are corrosive to metals, particularly iron, safer media such as sodium formate, potassium formate, sodium acetate, and potassium acetate are used in airports instead.

Freezing-point Some species that live in extreme cold use depression. These organisms have evolved mechanisms for producing high concentrations of different substances such as sorbitol and glycerol. This increased solute concentration lowers the freezing point of the water within them, keeping the creature from freezing solid even when the water around them freezes or the air around them becomes extremely cold. Some arctic fish, such as the rainbow smelt, create antifreeze substances in order to survive in frozen-over estuaries throughout the winter. Other animals, such as the spring peeper frog (Pseudacris crucifer), briefly increase their molality in response to low temperatures. Freezing temperatures cause a large-scale breakdown of glycogen in the peeper frog’s liver and the subsequent release of huge amounts of glucose into the circulation. Conifers have rich cell sap, which acts as an antifreeze in the winter.

Conclusion

Application, in cold countries, frozen ice on roads and footpaths during the winter is melted by spraying sodium chloride or calcium chloride, which lowers the freezing point of water, which lowers the freezing point of water. (2) In cold countries, frozen ice on roads and footpaths during the winter is melted by spraying salts such as sodium chloride or calcium chloride, which lowers the freezing point of water. The use of salts is used in the de-icing of aircraft and airport runways.