A Level Chemistry OCR Practice Exam

Which of the following best describes van der Waals' forces?

1. Forces that exist only between charged ions

2. Forces characterized by strong covalent interactions

3. Weak interactions between molecules due to temporary dipoles

4. Forces that are present in ionic compounds only

The correct answer is: Weak interactions between molecules due to temporary dipoles

The correct description of van der Waals' forces is that they are weak interactions between molecules due to temporary dipoles. This phenomenon occurs because of the movement of electrons around the nucleus of an atom, which can create temporary regions of positive and negative charge within a molecule. These temporary dipoles induce corresponding dipoles in neighboring molecules, leading to an attractive force between them. Van der Waals' forces are significant in various physical properties of substances, particularly in nonpolar molecules where other forces, like hydrogen bonding or ionic interactions, are absent. They play a crucial role in molecular interactions in gases, liquids, and even in the solid state, influencing boiling points, melting points, and solubility. Other options focus on specific types of interactions. Some refer exclusively to stronger forces, which do not accurately describe van der Waals' forces as they involve no permanent charge separation or strong bonds typical in ionic and covalent interactions.