Freezing Point Depression – Definition, Examples, And Uses
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.
(Image to be uploaded soon)
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.
FAQs on Freezing Point Depression
1. What is freezing point depression in chemistry?
Freezing point depression is a colligative property of solutions where the freezing point of a liquid solvent is lowered by the addition of a non-volatile solute. In simple terms, a solution will freeze at a lower temperature than the pure solvent from which it was made. This phenomenon depends on the concentration of solute particles, not their chemical identity.
2. What is the formula to calculate the depression in freezing point for a solution?
The formula to calculate the depression in freezing point, as per the CBSE syllabus for the 2025-26 session, is:
ΔTf = i × Kf × m
Where:
- ΔTf is the freezing point depression (the change in freezing temperature).
- i is the Van't Hoff factor, representing the number of particles the solute dissociates into in the solvent.
- Kf is the molal freezing point depression constant, or cryoscopic constant, which is specific to the solvent.
- m is the molality of the solution (moles of solute per kilogram of solvent).
3. What is the difference between a solvent's freezing point and the freezing point depression?
The key difference lies in their definitions. The freezing point is the specific temperature at which a substance changes from a liquid to a solid state (e.g., 0°C for pure water). In contrast, freezing point depression (ΔTf) is not a temperature itself, but the *amount by which* the freezing point is lowered. It represents the change or decrease in the freezing temperature due to the presence of a solute.
4. What are some common real-life examples of freezing point depression?
Freezing point depression has several important real-world applications. Some common examples include:
- De-icing Roads: Spreading salt (sodium chloride or calcium chloride) on icy roads lowers the freezing point of water, causing the ice to melt even when the ambient temperature is below 0°C.
- Antifreeze in Cars: The coolant in a car's radiator is a mixture of ethylene glycol and water. This solution has a much lower freezing point than pure water, preventing the engine from freezing and cracking in cold weather.
- Making Ice Cream: Salt is added to the ice surrounding the ice cream mixture. This lowers the freezing point of the ice, allowing the mixture to get cold enough to freeze into ice cream.
5. Why does adding a non-volatile solute to a solvent cause its freezing point to decrease?
This phenomenon occurs because the solute particles disrupt the formation of the solvent's crystal lattice structure. At the freezing point, a dynamic equilibrium exists where the rate of molecules leaving the liquid phase to join the solid phase equals the rate of molecules leaving the solid to join the liquid. Adding a solute lowers the vapour pressure of the solution. As a result, a lower temperature is required for the solution's vapour pressure to equal the vapour pressure of the pure solid solvent, which is the new equilibrium point for freezing.
6. What is the cryoscopic constant (K_f), and is it the same for all solvents?
The cryoscopic constant, or molal freezing point depression constant (Kf), represents how much the freezing point of a solvent will be depressed for every 1 molal concentration of a non-dissociating solute. It is an intrinsic property of the solvent itself and is different for every solvent. For example, the Kf for water is 1.86 K·kg/mol, while for benzene it is 5.12 K·kg/mol.
7. How is freezing point depression different from boiling point elevation?
Both freezing point depression and boiling point elevation are colligative properties that depend on solute concentration. The primary difference is the direction of the temperature change:
- Freezing Point Depression: The presence of a solute lowers the temperature at which the solvent freezes.
- Boiling Point Elevation: The presence of a solute raises the temperature at which the solvent boils.
Both effects are caused by the solute lowering the solvent's vapour pressure.
8. Why is calcium chloride (CaCl₂) often more effective for de-icing roads than sodium chloride (NaCl)?
The effectiveness of a salt for de-icing depends on the number of particles it dissociates into in water, which is measured by the Van't Hoff factor (i). Sodium chloride (NaCl) dissociates into two ions (Na⁺ and Cl⁻), giving it a theoretical Van't Hoff factor of 2. Calcium chloride (CaCl₂), however, dissociates into three ions (one Ca²⁺ and two Cl⁻), giving it a theoretical Van't Hoff factor of 3. Since freezing point depression is directly proportional to 'i', CaCl₂ produces more particles per mole and thus causes a greater depression in the freezing point, making it more effective at melting ice, especially at lower temperatures.
9. How do certain animals biologically use freezing point depression to survive in freezing conditions?
Some organisms living in extremely cold climates have evolved to use this principle as a natural survival mechanism. They produce high concentrations of certain compounds, like glycerol, sorbitol, or glucose, in their bodily fluids. These solutes act as a natural antifreeze, significantly lowering the freezing point of the water in their cells and blood. This prevents ice crystals from forming, which would otherwise rupture cell membranes and cause fatal damage, allowing them to survive sub-zero temperatures.
10. Is freezing considered an exothermic or endothermic process?
Freezing is an exothermic process. This is because for a liquid to transition into a more ordered solid-state, its molecules must release energy to slow down and arrange themselves into a fixed crystal lattice. This release of energy into the surroundings is in the form of heat, which is the definition of an exothermic process. Conversely, melting is an endothermic process as it requires energy input to break the bonds of the solid structure.
