Freezing Point And Boiling Point: Detailed Facts


We will be discussing about freezing point and boiling point in this article. Boiling is something which is related to a phase change from liquid to gaseous and freezing is related to a phase change from liquid to solid.

We know the basic information about water that it freezes at zero degrees celsius and boils at 100 degrees celsius at 1 atm pressure. This is the ambient pressure at sea level at normal conditions. We shall read more about freezing point and boiling point in this article.

What is boiling point?

Boiling point comes with a very common misconception that it is dependent on temperature. Whereas it is the pressure which decides the boiling point.

The boiling point can be defined as the value of temperature at which the vapour pressure of liquid becomes equal in magnitude with the pressure of surroundings. The value of boiling point can vary from place to place depending upon the places.

What is freezing point?

The temperature at which the liquid molecules start coming closer to exhibit properties of solid is called as freezing point. When the pressure increases, the freezing point also keeps increasing.

The freezing point is always lesser than boiling point in case of mixtures and organic compounds. Freezing refers to the state of a substance in which its molecules exhibit behaviour of solids. That is the molecules get closer to each other and make a compact and hard substance.

 freezing point and boiling point
Image: Boiling

Image Credits: user:Markus SchweissKochendes wasser02CC BY-SA 3.0

Is freezing point always zero?

The freezing point of different substances is different. For example alcohol freezes at minus temperatures and not at zero degrees.

The zero degrees temperature refers to the freezing point of water at sea level. When the surrounding temperature drops below zero degrees, then the water will start turning to ice. But this is not case everywhere as this happens only at 1 atm pressure. For different pressure values the freezing point value will also be different.

Is boiling point always hundred?

Hundred degrees celsius is the boiling point of liquid water. Other substances will boil at different temperatures. But even liquid water can boil at different temperatures let us find how.

The boiling point is a pressure dependent property. It changes with ambient pressure. So if we consider a sample of liquid water kept in vacuum conditions, the water will start boiling at 15 degrees celsius. The boiling water will not even feel hot due to the less temperature.

How to find freezing point from boiling point?

Molality, a property of solution that brings a relationship between freezing point and boiling point. It can be defined as the measure of number of moles of solute being mixed with a solvent weighing one kilogram.

Note that molarity is not same as molality. To find the freezing point from boiling point, we use the following relations

[latex]\Delta T_{b}=k_{b}.m.i[/latex]

and

[latex]\Delta T_{f}=k_{f}.m.i[/latex]

where,

kb and kf are boiling point elevation constant and freezing point depression constant respectively.

What is the relationship between freezing point and boiling point?

The relationship between boiling point and freexing point is already discussed in the above section. Although boiling point is related to phase change from liquid to steam and freezing is related to phasr change from liquid to solid but they both are related to each other.

Molality is a common property that draws a line to connect freezing point and boiling point. Note that it is molality and not molarity. We can define molality as the number of moles present in a solvent weighing 1 kilogram. Formulae to calculate freezing point and boiling point using molality are already discussed in the section above. We shall discuss about molality and molarity in detail in the next sections.

What is molality?

We came across a term called as molality in above section where we discussed in brief about the meaning of molality. We will read more about molality in this section. 

When we consider a solvent of one kilogram and count the number of moles of solute present in it, we are actually  calculating the molality of the solution. Revising the basics, a solution consists of solute and solvent where solvent is the base substance in which another substance (lesser in amount) called solute is added to it. This makes the solution. Mathematically, molality can be given as-

Molality, m = mol/kg

Where,

m is the molality of the solution

Mol is the number of moles of solute

Kg is the weight of solvent present in the solution in kgs

What happens at absolute zero?

Absolute zero is a condition where the system temperature becomes zero kelvin or -273 degrees celsius. Any thermodynamic system will have its lowest energy state at absolute zero.

At this temperature all the movements of the molecules freeze and no kinetic energy takes place. The enthalpy and entropy both have their minimum value at absolute zero temperature. No movement of particles take place. In Fahrenheit -459.67 degrees Fahrenheit.

How does freezing take place?

When a liquid starts turning into solid then it can be said that freezing is taking place. This happens when the kinetic energy of molecules become lesser in magnitude.

When the average kinetic energy becomes less, the net heat generated becomes less as a result of which the temeprature of the system goes down. Hence when freezing takes place, the temperature of the substance goes down.

Are melting point and freezing point the same?

Melting can be referred as the phase transformation of solid to liquid. And we already know that freezing refers to turning of liquid to solid.

Both of these things happen at same temperature. When the temperature is increased beyond the melting point, then the substance will change its state to liquid and when the temperature is decreased below the freezing point the substance will turn into solid. Hence we can say that freezing and melting points are equilibrium temperatures at which both solid and liquid properties are exhibited by the substance.

What happens to density of a substance during freezing?

It is a well established fact that liquids have lesser densities than solids and we know that when a substance undergoes freezing it starts turning into solid.

As the substance starts converting from liquid to solid, we can say that the density of the substance increases. The molecules come closer to each other increasing the density of the substance. However due to the chemical composition and orientation of atoms in water, the density of water decreases when it gets converted to ice. This is a notable exception.

Click to read more on Boiling Point And Dipole Moment.

Abhishek

I, Abhishek Khambhata, have pursued Btech in Mechanical Engineering. Throughout four years of my engineering, I have designed and flown unmanned aerial vehicles. My forte is fluid mechanics and thermal engineering. My fourth year project was based on performance enhancement of unmanned aerial vehicles using solar technology. I would like to connect with like minded people.

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