Question

Why do nonpolar substances tend to boil at relatively low temperatures?

Short answer

Nonpolar substances boil at low temperatures due to their weak London dispersion forces.

Step-by-step solution

Understanding Boiling

To understand why nonpolar substances boil at low temperatures, recognize that boiling occurs when molecules gain enough energy to overcome the intermolecular forces holding them together in a liquid state and enter the gas phase.

Types of Intermolecular Forces

Intermolecular forces refer to the forces that hold molecules together. These can include hydrogen bonding, dipole-dipole interactions, and London dispersion forces. Nonpolar substances primarily exhibit London dispersion forces (also known as van der Waals forces), which are generally weaker compared to the other types of intermolecular forces.

London Dispersion Forces

London dispersion forces arise from temporary fluctuations in electron density within molecules, leading to temporary dipoles that attract neighboring molecules. Although present in all molecules, they are the only forces in nonpolar substances, and their strength depends on the size and shape of the molecules. However, they are usually weak, making it easier for molecules to separate.

Low Boiling Temperatures

Since nonpolar substances rely solely on relatively weak London dispersion forces, they require less energy to overcome these forces compared to substances with stronger types of intermolecular forces. As a result, nonpolar substances tend to boil at lower temperatures.

Conclusion

The weak nature of London dispersion forces in nonpolar substances allows them to transition into the gas phase with less energy input, resulting in lower boiling points compared to polar substances with stronger intermolecular forces.

Key concepts

Boiling Point

The boiling point of a substance is the temperature at which it transitions from a liquid to a gas. This process occurs when molecules in the liquid gain enough energy to overcome the intermolecular forces holding them together.
Different substances have varying boiling points, primarily influenced by the strength of their intermolecular forces. Stronger forces require more energy, resulting in higher boiling points. Conversely, weaker forces mean less energy is needed, leading to lower boiling points.
Understanding boiling point is crucial when studying nonpolar substances, as it explains why these substances often boil at lower temperatures compared to polar substances with stronger intermolecular interactions.

Intermolecular Forces

Intermolecular forces are the attractive forces that exist between molecules in a substance. These forces determine many physical properties, such as boiling and melting points.
There are several types of intermolecular forces:

• Hydrogen bonding
• Dipole-dipole forces
• London dispersion forces

Nonpolar substances predominantly rely on London dispersion forces. These forces are generally weaker than hydrogen bonding and dipole-dipole interactions, which are found in polar substances.
By understanding the different types of intermolecular forces, students can predict and explain the behavior of different substances in various physical states.

London Dispersion Forces

London dispersion forces are a type of intermolecular force that arise from temporary shifts in electron density, creating temporary dipoles within molecules. These temporary dipoles can induce similar shifts in neighboring molecules, resulting in an attraction.
While all molecules experience London dispersion forces, they are particularly significant in nonpolar substances, where they represent the primary form of intermolecular attraction. The strength of these forces is affected by the size and shape of the molecules involved; larger and more elongated molecules tend to have stronger dispersion forces.
Despite their presence, London dispersion forces are relatively weak, allowing nonpolar molecules to separate easily and boil at lower temperatures.

Nonpolar Molecules

Nonpolar molecules are characterized by an even distribution of electrical charge, resulting in no permanent dipole moment. These molecules usually have symmetrical electron clouds, and examples include methane (\( \text{CH}_4 \)) and carbon dioxide (\( \text{CO}_2 \)).
The lack of a permanent dipole means that nonpolar molecules cannot engage in hydrogen bonding or dipole-dipole interactions. Instead, they depend on London dispersion forces for intermolecular attraction.
Due to the relatively weak nature of these forces, nonpolar substances often have lower boiling points and melting points compared to polar substances. Understanding the structure and behavior of nonpolar molecules is essential in fields like chemistry and materials science, where predicting physical properties is important.