Final answer:
Energy is required to increase a liquid's surface area because of surface tension, which is due to cohesive forces between molecules, requiring work to expand the surface. Stronger intermolecular forces result in higher surface tension, as seen with water's hydrogen bonds and mercury's metallic bonding.
Step-by-step explanation:
Energy is required to increase the surface area of a liquid due to surface tension, which arises from the cohesive forces between molecules. These forces act to pull the molecules at the surface towards the interior, creating a net attraction that must be overcome when the surface area is increased. Molecules within a liquid are surrounded by other molecules and experience even attractive forces in all directions, leading to a net force of zero. In contrast, molecules at the surface lack neighboring molecules on the side exposed to air, creating a net inward force. It is this imbalance that imparts a tendency for the surface to contract and minimize its area.
The work required to disrupt these cohesive forces and expand the surface is the energy we refer to as surface tension. For instance, water has high surface tension due to its strong intermolecular hydrogen bonds, which means it requires more energy to increase its surface area compared to liquids like benzene, which have weaker intermolecular forces. Similarly, mercury displays very high surface tension due to metallic bonding. Therefore, the stronger the intermolecular forces within a liquid, the higher the energy needed to extend its surface area.