Suppose that you have a pot of boiling water and a tablespoon of salt. The salt is considered a solid and the water a liquid. When you mix the salt into the water, the solid disappears and you are left with a liquid, apparently. Did the salt melt, or did the salt dissolve? When a substance melts, it changes phase from a solid to a liquid by the application of heat. In the above example, if the salt melted, then a portion of the salt-water liquid is composed of pure liquid salt. Alternatively, when a substance dissolves, a homogenous solution is formed that is composed of two or more substances. The pure phase of the dissolved substances within the solution can be solid or gas. Since you do not know the pure phase of the salt within the salt-water liquid, you do not know whether the salt has melted or dissolved.
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To decide the issue, you continue to mix more salt into the water. You will discover at a certain point that the solid phase of the salt no longer disappears as it is mixed into the water. Thus, the solid phase of the salt did not change, even after the application of heat from the water. The salt did not melt; it merely dissolved into the liquid water. At a certain point, the solution became saturated with salt, and additional crystals could no longer be dissolved into it. If you continue to boil off the water, you will be left with the original phase of salt as a solid. The actual melting point of table salt is 801 °C, well above the boiling point of water.
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In the example above, a solid (table salt) was dissolved into a liquid. A gas can also be dissolved into a liquid. The carbonated water that is present in a typical soft drink is a solution of carbon dioxide gas that is dissolved into water at high pressure. If you open a soda bottle with carbonated water in it at regular atmospheric pressure and leave it open, the carbon dioxide gas separates from the water (much the same way that the boiling water separated from the salt crystals) leaving only the water, and other flavorings, behind. At such a point, the soda is said to have gone “flat.”
