Comparison of Solids, Liquids and Gases

Comparison of solids, liquids and gases (fluids) can be done in aspects like macroscopic description, mobility of molecules, density, molecular spacing, effect of shear stress, effect of normal stress, viscosity and compressibility is represented below in tabular form.

1 Macroscopic Description Solids hold their shape; no need for a container Liquids take the shape of the container and will stay in open container. Gases expand to fill a closed container
2 Mobility of Molecules Molecules of solids have low mobility because they are bound in a structure by strong intermolecular forces. Liquids typically flow easily even though there are strong intermolecular forces between molecules. Molecules of gases move around freely with little interaction except during collisions; this is why gases expand to fill their container.
3 Typical Density High; e.g., density of copper is 8960 kg/m3. Medium; e.g., density of water is 1000 kg/m3. Small; e.g., density of air at sea level is 1.2 kg/m3.
4 Molecular Spacing Molecules are very close together. Molecules are held close together by intermolecular forces. Large on average, molecules are far apart.
5 Effect of Shear Stress Produces deformation. Produces flow. Produces flow.
6 Effect of Normal stress Produces deformation that may associate with volume change; can cause failure. Produces deformation associated with volume change. Produces deformation associated with volume change.
7 Viscosity Not applicable. High; decreases as temperature increases. Low; increases as temperature increases.
8 Compressibility Solids are difficult to compress; bulk modulus of steel is 160×109 Pa. Liquids are difficult to compress; bulk modulus of liquid water is 2.2×109 Pa. Gases are easy to compress; bulk modulus of a gas at room conditions is about 1.0×105 Pa.
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