A physical property is a characteristic of a substance that can be observed or measured without changing the identity of the substance. Silver is a shiny metal that conducts electricity very well. It can be molded into thin sheets, a property called malleability. Salt is dull and brittle and conducts electricity when it has been dissolved into water, which it does quite easily. Physical properties of matter include color, hardness, malleability, solubility, electrical conductivity, density, freezing points, melting points, and boiling points.  

Freezing point is the temperature at which a liquid changes to solid.  Melting point is the temperature at which a solid is converted to a liquid.  Melting point and freezing points thus occur at the same temperature because the change of state involves the same two states (liquid-solid; solid-liquid).  It is the temperature at which a solid changes to a liquid or a liquid changes to a solid.  Evaporation occurs when water is lost from a substance.  When wet clothes are hung out on a clothesline on a sunny day and after a few hours the clothes are dry, the water has evaporated from the surface of the clothes.  Heat is removed as warmer water molecules leave the liquid.  The remaining liquid water cools down as the heat leaves, a phenomenon known as evaporative cooling.  As the temperature of the water increases, the molecules gain energy and move faster and faster until they have enough energy to overcome the attractive forces between the molecules and become a gas.  The temperature at which the molecules of a liquid become gas is the boiling point.Have you ever put a clear glass pot lid over a boiling pot of water and turned it off?  At first the gas molecules hit the lid and either fall back down into the pot or collect on the pot lid.  Walk away from the pot and after a while when you get back to it, you will notice that there are water droplets that have formed on the pot lid.  As the temperature of the water dropped, the molecules lost energy and some still on the lid converted into liquid.  The water droplets have formed due to condensation.  Condensation occurs when water vapor is converted back to liquid.

Sublimation occurs when a solid is converted directly to a gas.  An example of sublimation is dry ice.  It sublimes at -78^oC and produces a gas or vapor.  Deposition occurs when a gas is converted directly to a solid.  An example of deposition occurs when you pass frozen foods under cold water.  The water immediately freezes on the surface of the food.

Figure 1. Pencil.

For the elements, color does not vary much from one element to the next. The vast majority of elements are colorless, silver, or gray. Some elements do have distinctive colors: sulfur and chlorine are yellow, copper is (of course) copper-colored, and elemental bromine is red.

Density can be a very useful parameter for identifying an element. Density is the mass of a substance divided by its volume and can be in units of g/mL, g/cm³, kg/L, etc.  Of the materials that exist as solids at room temperature, iodine has a very low density compared to zinc, chromium, and tin. Gold has a very high density, as does platinum.

Figure 2. Diamond ring.

Melting and boiling points are somewhat unique identifiers, especially of compounds. In addition to giving some idea as to the identity of the compound, important information can be obtained about the purity of the material.

Glossary