Write operational definitions that explain the properties of and differences among solids liquids and gasses?

Answer 1

Rough operational definitions are:

Solid is a body that at given conditions (temperature and pressure) has its own shape and volume;

Liquid is a body that at given conditions (temperature and pressure) has its own volume but not its own shape: it adapts to the container shape.

Gas is a body that at given conditions (pressure and temperature) assumes both volume and shape of its container.

In reality these operating definitions have several limitations and ambiguities.

More precisely we can determine for each material a set of critical conditions. If we fix the pressure (for example one atmosphere, that is 1100 Pa) this means to fix a set of critical temperatures; in the simplest case we can reduce them to two temperatures: Tm, (melting temperature) and Te (evaporation temperature).

The stable state of any material below Tm is a crystal, that is an ordered disposition of atoms or molecules at the nodes of a regular lattice. This means that this is the minimum energy state and the material evolve up to stabilize around it. Crystals are in a certain way the only true solids.

Sometimes however the transition to the crystal state is very slow. In the case of normal glass, it is solid, but it is not a crystal, so that it has to evolve towards a crystal. The time needed to a piece of glass to become a crystal is several thousand years, so that at normal experience it appears to be stable.

Nevertheless glass, as many other solids like many plastics, is not an ordered disposition of molecules: it is said to be an amorphous.

As an out of equilibrium solid, glass has not a precise melting temperature, but in a large temperature range shares some solid and some liquid properties, flowing much like a very viscous liquid, but not occupying all the available container.

A well known plastic, called PDMS, is a transparent solid (an amorphous again) on short times, but if left in a container for a long time (like one or two days) slowly flows like a liquid and assumes the container shape.

Thus, if a well defined transition and well defined differences exists between crystals and liquids, the same is not true if we compare amorphous solids (without an ordered disposition of the molecules) with liquids.

On the other extreme, a true gas, having all the physical and chemical gas properties, would require a very little interaction among the particles constituting it, but for occasional collisions.

If strong interactions appears, due to the high density or due to the fact that the gas particles are for example electrically charged, the gas start to manifest, in selected condition and more evidently for selected materials, a liquid like behavior.

Thus, in conclusion, if crystal and perfect gasses are really different states of matter, that are equilibrium states below Tm and above Te, liquids are somehow in between and even if in normal experience they have well defined properties too, appearing as stable states in between Tm and Ts, cases exist where the clear identification of the solid-liquid or of the gas-liquid separation is uncertain.