How can a liquid, like paint, dry into a solid?

(Lansing State Journal, January 25, 1995)

Matter is commonly found in one of three forms: solid, liquid or gas. Liquids are materials that can flow, such as water, mercury, or maple sap.

Just because something is liquid does not mean it will stay a liquid. For example, if it gets cold enough, liquid water will turn into solid water, a form we call ice. Alternatively, if it is heated enough, liquid water will turn into gaseous water, steam.

These three states of matter can change from one to another, depending on how active their molecules are. It is not exactly correct to say that liquids dry into solids; something else is occurring.

Now, in a situation such as paint drying there are two phenomena occurring. First, a solvent is evaporating. That is to say that some liquid component is turning into a gas. Second, dissolved substances are coming together and precipitating. That means that their molecules are coming together to form large collections of solid material.

Let's use watercolor paint as an example. The paint is really a mixture of solvent (water) and solutes (pigments). When they are mixed, the pigments dissolve and stay suspended in the water. As the water evaporates, the pigments end up stuck to the material that was painted.

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