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.


Evaporation, gradual change of state from liquid to gas that occurs at a liquid’s surface. Examples of evaporation include rainwater evaporating from warm pavement after a thunderstorm and wet paint drying as solvents in the paint evaporate. Fingernail polish also hardens as acetone evaporates from the liquid polish.

A liquid is made up of atoms or molecules (bound groups of atoms) that are in constant motion, traveling at different speeds. The average speed of these particles depends only on the liquid’s temperature. If the particles have enough energy, fast-moving particles striking other particles near the liquid’s surface may impart enough speed, and therefore enough kinetic energy (energy of motion) to cause the surface particle to leave the liquid and become gas atoms or molecules. The particle’s kinetic energy is directly related to its speed.

As particles with the most kinetic energy evaporate, the average kinetic energy of the remaining liquid decreases. In a similar way, if a basketball team loses its tallest players, the average height of the team is diminished. Because a liquid’s temperature is directly related to the average kinetic energy of its molecules, the liquid cools as it evaporates (see Heat Transfer).

A liquid’s surface area and temperature affect its rate of evaporation. Evaporation rates also depend on the type of liquid, since liquids made up of different molecules differ in the amount of attraction that exists between the molecules.

Evaporation is an important part of the earth’s water cycle, the continual movement of fresh water between the earth’s surface and its atmosphere. Evaporation also plays a key role in the function of plants and animals.