Re: Is shaving cream a solid, liquid, or gas?

Dear Mary Lou,

Terms like "solid","liquid" or "gas" are handy in a great number of situations, to put some order in the chaotic world that surrounds us. Their definitions are based on clear-cut associations in our mind: solid: stiff, hard, keeps its shape when squeezed; liquid: flowing, subject to gravity, forms a horizontal surface; gas: expands in all three dimensions, fills any void, is compressible. From this you see that there are questions that make either more or less sense, depending on what use you make of the answer. Many materials can be said to "fall on the border" between categories like liquid and solid. Clearly, shaving cream does not form a horizontal surface, it cannot be poured. It is thus not a typical liquid. But if you try to shape it into an object, you realize that it is not a good "solid" either. If you examine shaving cream under a microscope, you realize that it is not homogeneous, i.e. not the same throughout. It consists of tiny cells filled with gas. The "walls" of the cells, formed from special molecules, are rather stiff, but extremely thin, and in between there is a liquid "phase". Due to the microscopic dimensions of its structure, such a composite material is called a colloid. More information on colloids can be found in the Madsci Archive..

Best Regards

Werner Sieber