States of Matter

Matter is said to be in the solid state when it is rigid, that is, when it retains a definite shape and volume against the pull of gravity. Strong attractive forces exist among the particles that make up solids, causing them to position themselves close together in an orderly and definite arrangement in space. Their motion consists mainly of vibrating in a fixed position so the shape and the volume (amount of space they occupy) are maintained. The A piece of SEAgel sitting on soap bubbles. SEAgel (Safe Emulsion Agar gel) is a material claimed to be the first lighter-than-air solid. It is made from agar, a seaweed derivative used as a thickening agent in the food industry, and is biodegradable. SEAgel could be used as a thermal insulator instead of plastic foam or balsa wood. High density SEAgel could be used instead of plastic packaging.The substance is soluble in water above 122°F (50°C). Lawrence Livermore National Laboratory/Science Photo Library, National Audubon Society Collection/Photo Researchers, Inc. Reproduced by permission.
atoms, ions, or molecules cannot be pushed closer together; therefore, solids cannot be compressed.

Many solids exist in the form of crystals, which have simple geometric shapes, reflecting the regular spatial arrangement forms and shapes depending on the arrangement of the atoms, ions, or molecules of which they are made. This arrangement is called a lattice. Other solids, such as lumps of clay, seem to have no preferred shapes at all. They are said to be amorphous (without form). This is true because the individual crystals may be very tiny or because the substance consists of several kinds of crystals, randomly mixed together. Other solids, such as glass, contain no crystals at all.

When solids are cooled to lower temperatures, the orderly arrangement of their particles stays basically the same. The vibrations become slightly slower and weaker, however, causing the particles to move closer together, and the solid contracts slightly. But when solids are heated, the vibrations become faster and broader. This causes the particles to move slightly farther apart, and the solid expands a little. If heated enough, the particles will vibrate so vigorously that the rigid structure can no longer be maintained. The lattice begins to fall apart, first into clumps, and eventually into individual particles which can slip and slide past each other as they move about freely. At this point, the solid has become a liquid.

The temperature at which a solids loses its rigid form and turns into a liquid is called the melting point. Different substances have different melting points that are dependent on the sizes of the particles and the strength of the attractions between the particles. In general, heavier particles require more energy (higher temperatures) in order to vibrate vigorously enough to come apart. Also, the stronger the attractions between the particles, the more energy is required to break them apart and change the solid into a liquid. In both cases—heavier particles and stronger attractions—the melting point will be higher. Water serves as a good example. Liquid water freezes at the same temperature that ice melts, and the melting and freezing points are therefore identical. This is true for all substances. Ice melts at 32°F (0°C) which is uncharacteristically high for particles the weight of water molecules. This unusually high melting/freezing point is caused by the very strong attractive forces that exist between the molecules, making it very difficult for particles to move away from their neighbors and for the crystalline structure to collapse. Metals melt at much higher temperatures than ice. For example, copper is made into various shapes by melting it at 1,985°F (1,085°C), pouring it into molds, and cooling it. It is then usually purified further by electrolysis before it is commercially useful. Since pure substances have unique melting points which are usually quite easy to determine, chemists often use them as the first step in identifying unknown substances.

The amount of energy required to change a solid to a liquid varies from substance to substance. This energy is called the heat of fusion. Ice, for example, must absorb 80 calories per gram in order to melt into liquid water. Similarly, water releases 80 calories per gram of water to freeze into ice. Each of these changes occurs at the melting/freezing point of water, 32°F (0°C). In melting, since all the heat energy is used up in breaking the crystalline lattice, there is no change in temperature. However, once all the ice has melted, the absorbed energy causes the temperature of the liquid water to rise. This is generally true of the melting of all solids.