States of Matter

The change from solid to liquid is a physical rather than chemical change because no chemical bonds have been broken. The individual particles—atoms, ions, or molecules—that made up the solid are the same individual particles that make up the liquid. What does change is the arrangement of the particles. In the liquid, the particles are at a higher temperature, having more energy than in the solid, and this allows them to move away from their nearest neighbors. The attractions between liquid particles, though less than those of solids, is still fairly strong. This keeps the particles close to each other and touching, even though they can around past one another. They cannot be pushed closer together, and so, like solids, liquids maintain their volume and cannot be compressed. Because their particles move freely around, liquids can flow, and they will assume the shape of any container.

Like solids, the particles of liquids are close to each other; therefore, the amount of space occupied by liquids is quite close to that of their corresponding solids. However, because of the disorderly arrangement, the empty space between the liquid particles is usually slightly greater than that between the particles of the solid. Therefore, liquids usually have a slightly larger volume-that is, they are less dense-than solids. A very unusual exception to this is the case of ice melting to form water, when the volume actually decreases. The crystalline lattice of ice has a cage-like structure of H₂O molecules with big, open spaces in the middle of the cages. When the ice melts and the crystal breaks down, the cages collapse and the molecules move closer together, taking up less space. Consequently, a given weight of water occupies more volume as ice than as liquid. In other words, ice is less dense than water. Therefore, ice floats on liquid water. Also, a full, closed container of water will break as it freezes because the ice must expand. A water pipe may break if it freezes in winter because of this unusual property of water.