Boiling Point And Dipole Moment

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This article discusses about the relationship between boiling point and dipole moment. The relation between temperature, pressure and boiling point is commonly known.

We even know the effects of molecular forces and molecular size on boiling point. Now we shall see how dipole moment affects the boiling point. But first we will discuss in brief about what exactly dipole moment means and then continue our discussion further to see how it affects the boiling point.

What is dipole moment?

Dipole moment is a commonly used term in electromagnetism. It is a measure of separation of two charges that are opposite in polarities. These polarities are namely positive and negative.

An electric dipole is concerned with the separation of two polar opposite charges- positive and negative. There is an another type of dipole moment too called as magnetic dipole moment. A magnetic dipole can be defined as circulation of an alternating electric current. Dipole moment is a vector quantity.

Dipole moment formula

Now that we know the meaning of dipole moment, we can continue our discussion further with the formula of dipole moment. The magnitude of dipole moment depends on two things- the value of charge and the distance by which they are separated.

The formula for dipole moment is given in the section below-

Where,

Mu is the dipole moment

q is the magnitude of charge

r is the distance by which the charges are separated

Does Dipole moment increase boiling point?

When dipole moment increases, the attraction between the atoms increases. As the intermolecular force increases with higher dipole moment, it gets difficult to break these bonds.

If these bonds are difficult to break then it will take more energy to boil this substance. Hence we can say that with increase in dipole moment, the boiling point also increases. Energy is all it takes to boil the substance after all.

How does dipole moment affect boiling point?

Before answering this question we shall keep one thing in mind- if the attractive forces between the molecules are more then the boiling point will also be more.

When the dipole moment has greater value then the attractive forces between the molecules also increases. Due to this forces it gets difficult to break the bonds and boil the substance. Hence the boiling point also increases with the dipole moment.

Is an increase in boiling point due to a decrease in dipole moment?

No. The reason has already been discussed in the above section. Increase in dipole moment increases the attractive forces between the molecules.

When these attractive forces increase, the molecules get more difficult to separate. It needs higher energy to separate these molecules. This makes the boiling point of the substance higher. So increase in boiling point is definitely not a result of decrease in dipole moment.

Factors affecting boiling point at microscopic level

When we talk about microscopic level, we are talking about effects of atomic behaviour on boiling point. At macroscopic levels we know that its the pressure and temperature that plays a very vital role in boiling a substance.

Now let us have a look on the factors taking place at microscopic level that affect the boiling point.

These factors are given in the section below-

  • Molecular forces – Molecular forces directly affect the boiling point. If the molecular forces are high then the atoms will be attracted to each other such that they won’t be able to separate easily. A higher amount of energy will be required to boil the substance. The molecular forces include dipole dipole interactions, Van Der Waals force, ionic bond, H bonds. The highest amount of attraction occurs when there is an ionic bond.
  • Shape – The shape of compound also affects molecular size. For longer chains the forces of attraction become significant and higher. This way the boiling point also rises. The boiling point of the compound will increase if the compound has long chain but the boiling point will decrease when the compound has branching taking place.
  • Molecular weight – Molecular weight of the compound also directly affects the boiling point of the compound. Greater the molecular weight, greater will be the attractive forces between the atoms. Hence more energy will be needed to boil the substance. The weight of the substance increases when there are more atoms attached to each other to make one molecule. More atoms will result into higher attractive forces and higher attractive forces will lead to higher boiling point.

Effect of pressure on boiling point

We have a common misconception about boiling. We think that boiling takes place due to the presence of high temperature. But that is not true. In this section we shall see how pressure affects the boiling point of a substance.

We can boil water at 15 degrees celsius too only if the pressure of surroundings is very less or let us say in vacuum conditions. It is the vapour pressure of the liquid and the ambient pressure which decides the boiling point. When the values of ambient and vapour pressures become equal, it is then the substance will start boiling.

boiling point and dipole moment
Image: Boiling

Image credits: user:Markus Schweiss, Kochendes wasser02, CC BY-SA 3.0

Types of boiling

Concept of boiling may seem simple to us, yet it is still divided in to three types. These three types are discussed in the section below-

  • Nucleate boiling – This is the most common type of boiling. In this type, we can see bubbles of steam forming at the surface where heat transfer is taking place. This heat bubbles move upward and carry the heat with them. Transfer of heat through heat bubbles is a very efficient way to transferring heat.
  • Transition boiling – The heat transfer through bubbles does not increase indefinitely but comes at a constant value or critical heat flux value. After reaching this point the heat co efficient starts falling. The reason being a large part of vapour that makes an insulation film near the surface.
  • Film boiling– When we increase the temperature further, we can see the vapour layer covering the surface. This reduces the heat transfer due to convection.

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