Melting Point And Pressure:5 Facts You Should Know


There are so many questions like how pressure is related to melting point? Why does it affect melting point? We are going to answer all these.

The specific temperature at which a solid turns into liquid is known as its melting point. As a substance has more density in the solid state in comparison with the density in the liquid state,so in most of the cases it is observed that the melting point rises when temperature increases.

But water shows an exception to this. In case of water when pressure increases the melting point of the ice decreases. Heat should be supplied to a solid in order to liquify it. This heat is referred to the heat of fusion. The melting point of a solid shows that the change in gibbs free energy is zero.

But the values of change in enthalpy(∆H) and change in entropy(∆S) are increasing(∆G=0,∆H>0,∆S>0). The condition for a solid to get melted is its gibbs free energy at the solid phase should be greater than the gibbs free energy at the liquid phase. Thermodynamically,

∆S= ∆H/T where T signifies the temperature of the melting point

Melting point and pressure relation

There is a common relation between melting point and pressure,that is melting point is directly proportional to the pressure in case of most of the materials. A very common reason behind it is when pressure is applied to a solid material it makes it more dense and compact in nature.

Hence it needs more energy to be melted. It means that the temperature which is responsible for the melting of that solid also needs to be increased. That means the melting point of that material also increases.

Does melting point change with pressure and how?

There is a LeChatelier’s principle that deals with pressure which is the external physical state of the matter. It states that when an external physical state of a matter changes,it will have a tendency to put an end to the equilibrium of the system.

Then the system tries to get accustomed to the changes that are occurred. When a solid substance gets heated,its molecules go away from each other.

As the intermolecular space between the molecules increases due to the decrease of the intermolecular force of attraction,the solid turns into the liquid state. In case of ice,it gets melted into water when it is heated.

Hence LeChatelior’s principle can help us to understand how the effect of pressure on ice can modify the melting point. Equilibrium will be acquired by the ice water system at the lower point as the pressure on ice gets increased. We all know that the volume of ice is lower than the volume of ice. Hence the ice occupies less space than the water.

There is an inverse relation between the pressure and the melting point of ice which is known to all. So when the pressure increases on the ice,its volume decreases more. Hence the melting point of ice also decreases. It means that in the case of an ice water system the melting point is inversely proportional to the pressure.

Why does pressure affect melting point?

Through the help of a phase diagram we can describe the reason for pressure affecting the melting point of a solid. This diagram will help us in showing the dependance of the transition on temperature and pressure both.

melting point and pressure
Phase diagram of water from wikipedia

In the diagram the gaseous phase remains at the lower portion where the pressure remains low. Solid phase remains at the left portion of the diagram where temperature is low and the liquid remains in between the solid and the gaseous phase.

Triple point refers to a point at which solid,liquid and gaseous phases remain at equilibrium. Liquid phase remains stable at this point. Melting point of most of the solids are indicated by the solid green line in the diagram.

The dependance of melting point on the pressure is much lower than the dependance of boiling point on the pressure. This happens because the volume change in solid to liquid transition is very insignificant.

When the pressure gets high,the melting point also gets increased as most of the liquids have less density than the solids.

The dotted green line in the above phase diagram indicates the melting point of water. The case of water is an exception as water is more dense than ice. The increment in pressure results in a decrement in the melting point of ice.

Vapour pressure and melting point relationship

The melting point of a solid and its vapour pressure maintains a positive relationship in between them. It means that when vapour pressure increases the melting point of a solid also increases.

Relationship between atmospheric pressure and melting point

The relation between the atmospheric pressure and the melting point is positive. It means that increment of pressure on a solid brings an increment of melting point of that solid. We all know that when a solid turns into liquid it acquires more volume.

In the liquid state the intermolecular force of attraction between the molecules decreases hence the intermolecular space between the molecules increases. This in turn increases the volume.

So if pressure is applied to a solid it becomes harder for it to change into the liquid state as pressure makes the structure of the solid more compact and more dense. Hence it becomes tough to overcome the intermolecular attraction force and to get liquified.

That is the reason why the melting point gets increased when pressure is increased.

Relation between pressure and melting point of ice

The ice water system shows an exception to the pressure and melting point relationship. In the case of this system the slope between the melting point and pressure is negative as the melting point of ice decreases when pressure is applied to it.

Conclusion

In this article the common relationship between melting point and pressure and its exception both have been described in simple and understandable words.

Ankita Biswas

I am Ankita. I have done my B.Sc in physics honours and my M.Sc in Electronics. Currently I am working as physics teacher in a Higher Secondary School. I am very enthusiastic about high energy physics field. I love to write complicated physics concepts in understandable and simple words.

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