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

B. Freezing Point

The freezing point of a substance is the temperature at which the liquid form of the substance becomes a solid. The molecules of a liquid arrange into a more ordered structure as the liquid freezes. The freezing point of a substance is essentially the same as its melting point—that is, the point at which a solid becomes a liquid.

When a liquid freezes to become a solid, its volume usually shrinks by approximately 10 percent as its molecules move closer together. In solid aluminum, for example, each atom has 12 neighboring atoms, each at a distance of 2.86 x 10-8 cm. In liquid aluminum, each atom has 10 or 11 neighboring atoms at a distance of 2.96 x 10-8 cm. Thus, the atoms are less tightly packed in the liquid, and the liquid must contract as it freezes. The exceptions to this rule are water and the liquid forms of gallium and bismuth. These substances expand upon freezing. The structure of their solid state is less dense than that of their liquid state near the freezing point. In ice, each water molecule is solidly packed into a lattice, surrounded by four molecules equally distant from each other. This structure is actually less dense than the molecular patterns that can occur in the liquid form of water, which is why ice floats on water.