How To Find Freezing Point Of a Solution: Detailed Explanations

In this article, we will know the detailed explanations of how to find freezing point of a solution and its various methods and facts.

The freezing point is an essential property of the substance; it is both a physical and chemical process that transforms the nature of the substance due to temperature changes. It converts from one form of substance to another, and there are various methods to measure the freezing point of a substance.

Let us first study the freezing point/freezing point depression.

Freezing point and freezing point depression

The freezing point is an important physical and chemical property of the substance.

  • It is a temperature point at which the elements in their liquid form transform into solid matter. It differs for different substances. Whereas freezing point depression is also a fundamental property of the solutions in which the added solutes decrease the normal freezing of the solvent used.
  • FPD depends mainly on the solute concentration and not on other properties.
  • For example, salt reduces the freezing point of water during cold temperatures so that c cannot transform it into ice.
  • We can use Blagden’s law to calculate this, which combines both Raoult’s law and the Clausius-Clapeyron equation.
how to find freezing point of a solution
 
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To study how to find FP of a solution using molality.

How to find the freezing point of a solution with molality?

To find the freezing point, first, we have to measure the freezing point depression of a solution; by using molality, we can calculate it using the formula shown below,

∆Tf = Kf  m 

Here,

∆T indicates the changes that occur at the freezing point.

m symbolizes the molality of the solution taken

Kf indicates the freezing point depression of the solvent that is always constant and is given according to the solvents used.

To understand it better, let us solve an example.

Problem 1

Consider in the Himachal region, and calcium chloride is spread on the roads covered with ice so that it melts the ice and clears the blocked roads. For this purpose, they have used a solution that contains 7.08 moles of CaCl2 in 0.6 kilograms of water?

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Solution: Given 7.08 moles of CaCl2 in 600g (0.6 Kilograms) of H2O

First let us find the freezing point depression using

∆Tf = Kf

TSolution = Twater – ∆Tf

m =  (Mol Solute / Kg Solvent) and K = 1.86°C / m

m = (7.08 mol CaCl) / (600 g of H2O)= 11.8 m

∆T =(11.8m) (1.86°C/m) = 21.94°C

TSolution = Twater – ∆Tf

TSolution = 0.0°C – 21.94°C

TSolution = -21.94°C

The freezing point of the newly obtained solution is -21.94°C.

What is the freezing point in Celsius and Fahrenheit?

Freezing point, in general, can be expressed in two temperature scales, one on the Fahrenheit scale and the other on the Celsius scale.

  • First, let us know regarding the Fahrenheit temperature scale. In this scale, the lower temperature is based on 32° for the freezing point of water and the higher 212° for the boiling point of water; the difference between the minor and major temperature points has 180 equal points.
  • The next one is the Celsius scale. In this scale, the lower temperaturetemperature is based on 0°C for the freezing point of water and the higher 100°C for the boiling point of water.
how to find freezing point of a solution
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Now it’s time to understand the freezing point of water in detail.

Why is the freezing point of water?

Generally, we know that water’s freezing and the melting point will be 0 °C on the Celsius scale or 32 °F on the Fahrenheit scale.

  • If there is supercooling of water, then the temperature decreases; one more reason for the decrease in freezing and melting point is due to the presence of impurities that leads to freezing point depression.
  • The nature of atoms present in water spread out during crystalline formation. This action makes the ice appear less dense than the water, making it float. This action of molecules during the freezing process causes pressure that reduces the freezing point of water.

To know how to plot the freezing point on the graph.

How to find a freezing point on a graph?

We can easily find the freezing point of any solution by plotting a graph of temperature vs time or through temperature vs molality.

  • After plotting the values, we can draw two straight lines on the upper and lower portion of the freezing point. The temperature that is obtained at the intersection of these two lines can be considered the freezing point of the given solution.
  • In this graph, we can find the freezing point depression as it is considered the difference in the temperature range of solvent and solution. ΔTf can give the freezing point depression on the plot.

To know the method to calculate the freezing point of an electrolyte solution.

How to calculate the freezing point of an Electrolyte solution?

The electrolyte solution is a type of solution that consists of elements such as ions, atoms or molecules.

  • In this solution, a loss and gain of electrons make it conductive. Can even consider these electrolytic solutions ionic solutions, but we cannot consider the electrolytes to be ions in some extreme cases.
  • To calculate the freezing point of an electrolytic solution, we can use the same formula ∆Tf = Kf m that is used to calculate the freezing point using molality. But before using the formula, we have to analyze the number of ions and calculate its molality. Later by using the Kf, we can measure the freezing point of the given electrolytic solution.

To understand it clearly, let us solve an example.

Problem 2

Measure the freezing point of a solution that contains 0.624 g of CaCl2 in 165g of H20?

Solution:

O.624 g of CaCl2   x (1mol of CaCl2) / (111g of CaCl2)=  0.0056212 mol CaCl2

First calculate the molality of calcium chloride solution

Molality =  (0.0056212 mol CaCl2) / (0.165Kg of H2O)= 0.034067 Mol/Kg

CaCl Ca2+ + 2 Cl

                                   0.3406 m 0.3406m + 0.0745m = 0.1085 m

∆Tf = Kf

∆Tf = (1.86°C/m) x 0.1085 m

∆Tf = 0.201°C

TSolution = Twater – ∆Tf

TSolution = 0.0°C – 0.201°C

TSolution = –0.201°C

The freezing point of the electrolytic solution is -0.201°C.

Frequently Asked Questions | FAQs

How to find freezing point of a solution and the boiling point of any solution?

Molality can be used to find the boiling and freezing point of any solution.

First, we have to multiply the total number of particles formed in the solution when it dissolves with the molality of the solution indicated by ‘m’. We will get the total concentration value of the dissolved particles in the solution by doing this. After comparing these values, the higher solution concentration leads to the high boiling point, whereas the lower value depicts the low freezing point.

Why does water have similar melting and freezing points?

The melting and freezing are two different physical processes.

For water, it is a liquid matter with the same temperature point for both the melting and freezing process. In the freezing process, the water atoms will lose energy, stop their movement, and come together in a firm arrangement leading to solid formation. Melting is the opposite of freezing. But both occur at the same temperature.

Give a simple definition for freezing point?

The freezing point is different for each substance.

It is a temperature point at which the liquid taken into consideration becomes solid. As seen in the case of melting point, pressure increases that gradually increases the freezing point. For some substances freezing point will be lesser than the melting point depending on the nature of the substances – for example, organic compounds such as fats, mixtures etc.

In what method can you convert Fahrenheit to Celsius?

The formula that is widely used to convert the temperature from Fahrenheit to Celsius is mentioned below,

To convert the Fahrenheit to Celsius scale, first, we have to subtract 32 from any given temperature value, and later we should multiply the obtained numbers by 5556 or by (5/9). It is the F° to C° conversion formula.

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