Matter & Energy

Matter is composed of atoms or groups of atoms called molecules. The arrangement of particles in a material depends on the physical state of the substance. In a solid, particles form a compact structure that resists flow. Particles in a liquid have more energy than those in a solid. They can flow past one another, but they remain close. Particles in a gas have the most energy. They move rapidly and are separated from one another by relatively large distances.

Physical Properties of Liquids

D. Surface Tension

Liquids behave as though they have a delicate skin on their surface. This property is called surface tension. In rain droplets, surface tension acts like a thin balloon, holding the water molecules together in each droplet. Water-strider bugs take advantage of surface tension by flitting across the surfaces of ponds without falling through the surface.

Surface tension results from the intermolecular forces of attraction in a liquid. A water molecule deep inside a droplet experiences attractive forces in all directions from other molecules in the drop. The sum of these forces is zero, leaving no net force on the molecule. A molecule that is close to the surface, however, has more neighboring molecules inside the drop than it has near the surface. The forces pulling the molecule toward the center of the drop are stronger than those at the surface, so the molecule sticks to the drop instead of falling away.

Intermolecular forces of attraction make liquids pull together and minimize their surface area. Liquids do this because, like all matter, they seek to minimize the amount of energy they require to maintain their molecular structure. A liquid requires the least amount of energy when it has the smallest possible surface area. For small amounts of liquid in air, such as raindrops, the sphere is the shape with the smallest surface area. Gravity, another force acting on raindrops, stretches the droplets so that they are not exactly round. To overcome the attraction between molecules in a liquid and increase the liquid’s surface area takes energy. For instance, a moving car can transfer energy to a droplet of rain by hitting it, making it break apart or deform against the car into a shape that has more surface area.