Intermolecular Forces: Boiling & Melting Points

Joanna Tatomir, Laura Foist
  • Author
    Joanna Tatomir

    Joanna holds a PhD in Biology from the University of Michigan and is currently working towards a degree in Veterinary Medicine at Michigan State University. She has taught a combination of ESL and STEM courses to secondary and university students.

  • Instructor
    Laura Foist

    Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

Learn about the relationship between intermolecular forces and melting point and intermolecular forces and boiling point. See different intermolecular forces. Updated: 11/23/2021

Table of Contents


Intermolecular Forces and Melting Point

What are Intermolecular Forces?

Atoms, molecules, and compounds are held together by a combination of intramolecular and intermolecular forces. Intramolecular forces refer to the bonds that hold atoms together in a molecule, such as covalent, ionic, and metallic bonding. By contrast, intermolecular forces refer to the attraction that occurs between molecules when they are near one another. Intermolecular forces can be characterized as either weak or strong, depending on how much attraction occurs between molecules. Weak intermolecular forces result in a greater distance between two or more molecules. Especially when compared to strong intermolecular forces, in which molecules possess a strong attraction to each other.

Intermolecular forces are especially important in many organic molecules, as these forces contribute to the function, characteristics, and properties associated with different substances. While a single intermolecular force might be considered weak in isolation, the sum of these intermolecular forces contributes to the strength of the substance as a whole. For example, intermolecular forces contribute to the double helix structure of DNA (deoxyribonucleic acid) by holding together the base pairs, forming a genetic material. Intermolecular forces provide the structure to DNA while at the same time being weak enough to be broken when DNA transcription occurs.

Types of Intermolecular Forces

There are three main types of intermolecular forces: Van der Waals dispersion forces (usually referred to as London dispersion forces), Van der Waals dipole-dipole interactions (more commonly known as dipole-dipole interactions), and hydrogen bonding. Some textbooks also identify a fourth intermolecular force, usually referred to as an ion-dipole interaction. This type of ionic interaction refers to the electrostatic attraction occurring between an ion and a polar molecule.

Each type of intermolecular force possesses differences in the strength of these attractive forces. The strength of intermolecular forces can be described as follows, from weakest to strongest: Van der Waals dispersion forces (London forces) < Van der Waals dipole-dipole Interactions < Hydrogen bonding < Ionic interactions (ion-dipole interactions). Therefore, looking at the types of intermolecular forces found in a compound can provide important insights into various properties of that substance, such as its melting and boiling point.

The general rule of thumb is that the melting point increases as the strength of the intermolecular force increases. Greater intermolecular strength, the more energy is needed to break the bonds found in a compound. As a result, compounds with strong intermolecular forces require more energy to break the bonds, which is required to melt a substance.

For example, ethanol ({eq}CH_3CH_2OH {/eq}) has a melting point of -114.1°C (-173.5°F), while ethane ({eq}C_2H_6 {/eq}) has a melting point of -182.8°C (-297°F). Ethanol has a higher melting point due to the presence of a hydroxyl group ({eq}OH {/eq}), which produces greater hydrogen bonding within ethanol when compared to ethane. Therefore, the presence of hydrogen bonding in ethanol causes it to have a higher melting point than ethane.

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  • 0:04 Intermolecular Forces
  • 1:35 Dispersion Forces
  • 2:53 Dipole-Dipole Interactions
  • 3:46 Hydrogen Bonding
  • 5:02 Ionic Bonds
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Intermolecular Forces Boiling Point

Intermolecular force strength also influences the boiling point of a substance. In general, increasing intermolecular force strength produces a concomitant increase in boiling point. Looking at the same example above, ethanol ({eq}CH_3CH_2OH {/eq}) has a boiling point of 78.37°C (173.1°F), while ethane ({eq}C_2H_6 {/eq}) has a boiling point of -89°C (-128.2°F). Once again, the presence of hydrogen bonding in ethanol due to the presence of a hydroxyl group ({eq}OH {/eq}) results in a higher boiling point than ethane.

Ethanol structure

determining intermolecular forces

Ethane structure

determining ntermolecular forces

Determining Intermolecular Forces

Determining the types of intermolecular forces in compounds can provide students and researchers with important insights into the physical properties of a substance. Intermolecular forces are determined in part by how the electrons are shared within the covalent bonds of different molecules. When electrons are shared equally within a covalent bond, a non-polar molecule is formed. By contrast, when the unequal sharing of electrons is present in a molecule, a polar molecule is formed.

Because intermolecular forces increase the bonding strength between two or more molecules, intermolecular forces can impact the melting and boiling points of compounds. In general, as intermolecular force strength increases, the melting and boiling points of a substance also increase.

As stated above, there are four different types of intermolecular forces- Van der Waals dispersion forces (London forces); Van der Waals dipole-dipole interactions; hydrogen bonding; and ionic interactions (ion-dipole interaction).

Van der Waals Dispersion Forces

Van der Waals dispersion forces are more commonly referred to as London dispersion forces. The London dispersion force represents the weakest of the four intermolecular forces. London dispersion forces typically occur between nonpolar molecules when a temporary dipole is formed due to changes in electron distribution. This produces a temporary dipole, or the presence of a partial positive and partial negative charge, in a molecule. For example, Van der Waals, or London, dispersion forces represent the only type of intermolecular force found in gasoline ({eq}C_8H_{18} {/eq}), since it is a completely nonpolar molecule.

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Frequently Asked Questions

Do high melting points indicate weak intermolecular forces?

When intermolecular forces occur between molecules, they typically help to increase the strength of attraction between these molecules. Higher melting points are, therefore, indicative of stronger intermolecular forces rather than weak intermolecular forces.

Which intermolecular force would affect the boiling point the most?

Hydrogen bonding, dipole-dipole interactions, and ionic interactions represent the three intermolecular forces that have the greatest impact upon the boiling point. As the strength of these three intermolecular forces increases, the boiling point also raises.

What intermolecular force would affect melting point the least?

London or Van der Waals dispersion forces, are considered the weakest intermolecular force and are made between electrons that are temporarily attracted to each other when temporary dipoles are formed in a molecule. Since this is the weakest intermolecular force, London dispersion forces would have the least effect upon the melting point.

Do intermolecular forces increase melting point?

As the strength of the intermolecular forces increases, more energy is needed to disrupt the attraction between these molecules. As a result, increasing intermolecular forces results in a concomitant increase in melting point.

What increases boiling point?

Intermolecular forces represent attract forces that occur between two or more molecules. As the strength of these intermolecular forces increases, the boiling point also rises.

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