Freezing Point Depression in Solutions and Colligative Properties

Are you a student who enjoys studying chemistry? Have you always wanted to find out the answer to the question of what is freezing point depression? Do you want to learn about the freezing point depression formula? Do you want to find out the answers to questions like what affects freezing point depression, and why does the freezing point depression occur?

If you have answered all of these questions with a resounding yes, then you are at the right place. Here, we are going to learn about every topic that chemistry students should be familiar with when it comes to the topic of the freezing point depression formula. We will also go through what affects freezing point depression, and why does the freezing point depression occur in the first place. So, let us begin with the basic freezing point depression definition.

Definition of Freezing Point Depression

Freezing point depression can be defined as the phenomenon of lowering the freezing point of any solvents after the addition of solutes. From this freezing point depression definition, it must be clear that this is a colligative property of solutions. In most cases, the freezing point formula is usually proportional to the molality of the solute that is added. With the help of this knowledge, one can also conclude that the freezing point formula is:

\[\Delta T_{f} = i \times kf \times m\]

In this freezing point depression formula, \[\Delta T_{f}\] is the freezing point depression, i is the Van’t Hoff factor, \[k_{f}\] is the cryoscopic constant, and m is the molality.

The depression of the freezing point formula can also be explained with the help of Raoult’s law. According to Raoult’s law, the vapour pressure of any pure solvent will decrease after the addition of a solute.

This further means that if the vapour pressure of a non-volatile solvent is zero, then the overall vapour pressure of the solution will be lesser than that of the pure solvent. To understand this law better, students can refer to the image given below.

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The below-given points will help in a brief introduction of the freezing point depression constant.

Now, let’s move on to discuss the reason why freezing point depression occurs in the first place. There are many reasons why the freezing points of solvents tend to depress upon the addition of a solute. Some of those reasons are mentioned below.

• At the freezing point of a solvent, there is an equilibrium that is present between the solid-state and liquid state of the solvent.

• This means that the vapour pressures of both the solid and liquid phases are equal.

• Once a nonvolatile solute is added to the solvent, the vapour pressure of the solution will be lower than the vapour pressure of the pure solvent.

• This entire procedure results in a condition in which the solid and the solution reach an equilibrium at lower temperatures.

All of this can also be understood with the help of the formula of depression in freezing point and the freezing point equation.

Freezing Point Examples

Before moving forward, let’s do a recap. Till now, we have learned the meaning of freezing point depression, why it occurs, and the freezing point depression equation. Moving forward, we will look at the examples, applications, and some interesting facts related to this topic.

For now, we will focus on helping students become more familiar with some freezing point examples. We have prepared a list of various freezing point depression examples. The freezing point depression examples are mentioned below.

• The freezing point of seawater is below zero Celsius. Seawater remains liquid at temperatures lower than that of the freezing point of pure water. This is due to the salts that are dissolved in the seawater.

• Another common example of this phenomenon can be observed in a solution of ethanol in water. The solution has a lower freezing point than pure water but a higher freezing point than pure ethanol.

We have also formulated a table that contains the normal freezing point and freezing point depression values of various solvents.

Uses of Freezing Point Depression

In this section, we will look at some of the uses of freezing point depression. We have prepared a list of all the major uses of freezing point depression. 

• In areas with lower temperatures, sodium chloride is spread over the roads. This is done in order to lower the freezing point of water. This also helps in preventing the build-up of ice.

• Calcium chloride is used instead of NaCl in places with temperatures below 18 degrees Celsius to melt the ice on the roads. This is because of the fact that calcium chloride dissociates into three ions. This causes greater depression in the freezing point of water.

• Ethylene glycol and water are generally used to make radiator fluids that are used in many automobiles. This helps in preventing the freezing of the radiator during the winter season.

• The freezing point depression formula can be used to determine the molar mass of a given solute.

• The same formula of freezing point depression can be used to estimate the degree to which a solute can dissociate in a solvent.

• This kind of measurement is known as cryoscopy ("cryo" meaning cold, "scopos" meaning observe; "observe the cold") and it relies on the knowledge of accurate measurement of the freezing point. 

• The concept of freezing-point depression is also employed as a purity analysis tool in differential scanning calorimetry. The results obtained are in mol%, but this method is known to be effective, giving results when other methods fail.

• This phenomenon is employed in the food industry (ice cream and dessert making) where salt/sugar is added to a freezing mixture to make ice cream.

• The measurements of freezing point depression (FPD) are also used in the dairy industry to ensure that only the required amount of water is added to the milk. Milk with an FPD of over 0.509 °C is said to be unadulterated milk.

Fun Facts about Freezing Point Depression

Did you know that many organisms can survive in freezing climates because their bodies tend to produce compounds like sorbitol and glycerol? The concept of freezing-point depression is biologically exploited by some organisms living in extreme cold to produce a large concentration of several compounds. There are many examples of organisms that produce antifreeze compounds, such as some arctic-living fish species called the rainbow smelt that produces glycerol and other molecules to survive in frozen-over estuaries during the winter months. In the case of other animals, such as the spring peeper frog (Pseudacris crucifer), the molality of the body fluids is momentarily increased in response to the cold temperatures of the winter months. At the same time, another effect of freezing temperatures in the peeper frog is a large-scale breakdown of hepatically stored glycogen and subsequent release of large amounts of glucose into the bloodstream.

The secretion of these compounds helps in decreasing the freezing point of the water in their bodies.

Also, have you ever wondered what exactly happens at the freezing point? According to experts, the freezing point increases with increased pressure. Once a supercooled liquid is brought to freezing, it results in the release of the heat of fusion. This increases the temperature to the freezing point quickly.

To know more about freezing point depression, log on to Vedantu and find out the expert views of the top mentors. Develop your basic concepts well and prepare yourself better to answer the questions aptly.