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.

Structure of Liquids

Liquids, like all substances, are made up of atoms or bonded groups of atoms called molecules. The physical state of any substance—whether the substance is a liquid, solid, or gas—depends on the arrangement of the molecules in the substance. The molecules in a liquid are arranged tighter and more orderly than in a gas, but less orderly than in a solid. In crystalline solids, such as table salt or sugar, the molecules or other particles are stacked in a precise order, forming a three-dimensional pattern that repeats throughout the crystal. Scientists say these substances have long-range order. This means the atoms or molecules in the solid are ordered throughout the solid. In gases, the atoms or molecules have no order at all but move freely, allowing gases to expand and fill a container.

Liquids have no long-range order, but their molecules have some order with respect to the nearest neighboring molecules. Every molecule in a liquid has the same number of nearest neighbors, and each of these neighboring molecules are roughly the same distance from one other. But the position of molecules that are further away from a particular molecule in a liquid becomes more and more random with increasing distance. Liquids, therefore, have short-range order, or an orderly pattern only at very small distances (a few molecule lengths) from a given molecule.

Certain liquids, called liquid crystals, can have some long-range order, though not as much as a solid. These liquids contain long, rodlike molecules that, in certain temperature ranges, line up parallel to each other to create a long-range order. Unlike molecules in a solid, however, molecules in a liquid crystal retain the ability to slide past one another. The optical properties of some liquid crystals change with the arrangement of their molecules. For example, digital watches and calculators use liquid crystals that appear opaque when they are aligned in one direction and transparent when they are aligned in another direction.

Order in the molecules of any substance depends on the forces of attraction and repulsion between the molecules. These forces, called intermolecular forces, arise from electrical charges on molecules that attract or repel the charges on other molecules. Intermolecular forces account for many of the physical properties of a liquid, such as its boiling point, freezing point, and surface tension. They are also the reason a liquid can form from a gas or freeze into a solid.

A liquid forms from a gas when a drop in temperature or an increase in pressure causes the atoms or molecules of the gas to move closer together and acquire a short-range order. As the temperature of a gas drops, its molecules lose energy. The loss of energy causes the molecules to move more slowly, allowing them to come into closer contact with one another. An increase in the pressure of a gas brings molecules closer together by forcing them into a smaller space. Pressure can be increased by moving a given volume of gas from a large container into a smaller container, or by adding more gas to the original container. When the gas molecules move near enough to one another, intermolecular forces of attraction take over. These forces bring the molecules even closer together and into a short-range order, and a liquid forms. Likewise, when molecules in a liquid lose energy and move closer to one another, these same forces cause the liquid to turn into a solid. The molecules attract one another so much, they lock into place next to their neighbors and lose the ability to slide past each other. The molecules form a definite, unmoving pattern, which makes the solid rigid.

Molecules that are close together, as in a liquid or solid, also exert a force of repulsion on each other. The outer electrons of each molecule repel the outer electrons of their neighboring molecules. This repulsion makes it difficult to force the molecules any closer to one another, which in turn makes it difficult to reduce the volume of a liquid or solid. This property makes brakes work in automobiles and trucks. The brake pedal is connected to a piston in a cylinder filled with brake fluid. The driver activates the brakes by pushing on the brake pedal, which in turn pushes the piston in the cylinder. The brake fluid in the cylinder cannot be compressed, so the piston moves the fluid from the cylinder through hollow steel tubes filled with fluid and into pistons connected to brake pads. The liquid pushes these pistons into the brake pads, making the pads rub against the rim of the wheels. This rubbing causes friction, slowing down the wheels.