Solid, Liquid and Gas

We explain what a solid, a liquid and a gas are, the specific characteristics of each one and how they go from one state to another.

What is a solid, a liquid and a gas?

All matter that exists in the known universe exists in a specific physical state, depending on its internal energy conditions (which determine the degree of cohesion of its particles) and the temperature and pressure conditions of the medium that surrounds it.

These different physical states are scientifically known as states of aggregation of matter and Three main states are recognized: the solid state, the liquid state and the gaseous state that is, solids, liquids and gases. There are other known states, such as plasma or Bose-Einstein condensate, but they are very rare in our daily lives, and some exist only in the laboratory.

Around us, all matter spontaneously presents one of these three main physical states (solid, liquid or gaseous) but, through the alteration of its physical properties such as temperature and pressure, It is possible to take it from one state to another always through the addition or subtraction of energy to its particles. This procedure is known as the phase change process or change of aggregation state.

On the other hand, Each state of matter has its own characteristics and physical properties depending on the nature of the substance that makes up the matter under study. However, we should never confuse the physical state of matter with its chemical composition: water (H₂O) it will retain its chemical formula even if it is in liquid, solid or gaseous (vapor) form.

The changes between one phase and another do not alter the atomic composition of the substance in any way (therefore, they are not chemical reactions), but they do serve to separate mixtures, for example, using the difference in physical properties between their components.

The solids

Solids are easily recognizable because actively oppose the change in shape and volume since its particles are very close together and very ordered, and have enormous cohesion that allows them very little mobility. Therefore, we cannot easily deform most solids. Solids always tend to retain their well-defined and invariable structure.

Some characteristics of solids:

• They present a concrete form, with well-defined limits, and tend to preserve it.
• They have a high density and their own volume.
• They can float or submerge when dropped into a liquid.
• They are more or less flexible or rigid, that is, they admit (or not) a certain margin of deformation before breaking.
• They are ductile, malleable or brittle, depending on whether they can be molded into threads, sheets or if they tend to break under continuous force.
• They are incompressible, resistant to fragmentation and do not flow.

Solids arise as a consequence of solidification or freezing processes of liquids or deposition (or reverse sublimation) of gases. Furthermore, they can be melted into liquids or sublimed into gases through the injection of energy and appropriate pressure conditions.

The liquids

Liquids can be understood as an intermediate state between the rigidity of solids and the volatility of gases. Its main feature is the ability to flow and the fact of not having its own shape, but of acquiring the shape of the container that contains them. This is because its atoms have much less cohesion than solids, without completely losing the bond between them.

Its main features are:

• They are fluid, that is, they can move on an inclined surface.
• They do not have a fixed shape or structure memory, but they do have their own and constant volume. Therefore, they are poorly compressible.
• They have surface tension (initial resistance to penetration) and capillarity.
• They generally expand when exposed to heat and contract when exposed to cold (except water).
• They have viscosity, that is, a certain degree of adhesion between their particles and resistance to movement.

Liquids arise as consequence of the fusion or melting of a solid or the condensation or liquefaction (extreme compression) of a gas. On the contrary, to bring them to a solid state they need to be frozen or solidified in some way, and to turn them into a gas it is enough to subject them to evaporation (or vaporization).

The gases

Finally, gases present the most volatile state of matter, in which the particles are more separated and dispersed than in the other two states, and they move much faster. The attraction between them is so low that gases tend to expand until they fill the spaces that contain them, and the space between their particles is such that they can be greatly compressed.

Its fundamental characteristics are:

• They have very little density, which allows for a lot of compression.
• They have no shape or volume of their own.
• Gravitational forces practically do not affect its particles, which move with immense freedom.
• Its density undergoes important changes with temperature and pressure.
• They are capable of flowing, just like liquids.

Gases are the product of different solid sublimation processes or evaporation of liquids, and can be led to the solid state through deposition, and to the liquid through condensation (or liquefaction).

References

• “States of aggregation of matter” on Wikipedia.
• “Solid” on Wikipedia.
• “Liquid” in Wikipedia.
• “Gas” in Wikipedia.
• “States of the matter” in the Government of the Canary Islands (Spain).
• “States of matter” (video) in Educational Science.