Four states of matter can be seen in everyday life: solid, liquid, gas, and – somewhat more exotically – plasma. As a tightly bound combination of oxygen and hydrogen atoms, a water molecule is nothing out of the ordinary. Liquid water, steam or ice are still just water. Yet, it is intriguing to see how the very same building blocks of matter are capable of producing such broadly distinct states.
As far as water is concerned, one phase can change into another really quite abruptly. At 99 °C, liquid water can subsist, whereas at 101 °C, water is in its gaseous state, as steam.
We look at these states by increasing order of energy and entropy.
The Solid Phase
In a solid, each atom is tied to a specific equilibrium position. Although it vibrates to and fro about this position, the atom is unlikely to move relative to its neighbouring atoms. This is the reason why a solid is rigid.
Solids have crystalline structures, consisting of a single crystal, as for gemstones, or a collection of microscopic crystals, as for the majority of metals. In a crystal, the atoms oscillate around equilibrium positions that are regularly ordered. You can think of the whole structure as a simple pattern being repeated endlessly and periodically throughout space.
Crystals are cleaved along lines of weakness between several atomic planes. One of their distinctive features is their symmetry.
Ice (solid water) has at least seven crystalline phases, with water molecules arranged differently in each case. In ice, the molecules are packed together in an hexagonal arrangement, macroscopically observable in the symmetrical beauty of snowflakes.
Steel consists of a myriad of small crystals only visible under a microscope lens. Arranging and mixing different structural phases allows the industrial production of steels with varying hardness and flexibility properties.
Diamond (solid carbon), the hardest known of all substances, are generally cleaved before being polished into gemstones.
Solids exist only at relatively low temperatures. The density of substances in their solid state is roughly equal to that of the same matter in liquid state.
Liquids are much denser than gases, because their molecules are much more closely packed together. They are also arranged in a much more orderly way, although only over a short distance.
Liquids exist at a higher temperature than solids. When a gas is cooled, the kinetic energy of the molecules falls until it reaches a point where collisions can no longer prevent the clumping together of molecules, and the gas turns into a liquid.
Although they are able to flow, liquids do not expand to fill the whole volume of any empty container unlike gases. If the volume of liquid is smaller than that of the container, the liquid usually tend to occupy the lower part of the container. But if it is greater, the liquid will overflow.
Unlike gases, liquids resist being compressed.
The Gas Phase
Gases occur at higher temperatures than liquids, where the effects of kinetic energy overcome those of binding energy, and they all behave roughly in the same manner. The disruptive effect of atomic collisions will form if the average kinetic energy per atom is greater than 10% of the binding energy per atom.
Gases are not very dense and able to flow, adopting the shape and volume of any empty container they are kept in. They also can be compressed quite easily unlike liquids. So, the density of a fixed mass of gas depends on the size of its container.
For example, consider the air inside your living-room. Typically, the average spacing between the air molecules is very small – 3.3 x 10-9 metre – and yet, it is much larger than the normal range of action for intermolecular forces. Thus, each gas molecule spends its life beyond the range of influence of its closest neighbours. The molecules move freely, only occasionally colliding with others when their paths cross. This is precisely the reason why a gas does expand to fill an empty vessel. As the gas molecules experience no strong force to hold them together, they are free to simply drift apart until they fill the given space more or less uniformly.
Larger molecules collide more frequently and their paths are more erratic, because each collision involves an exchange of momentum and energy. This random motion of gas molecules as a result of collisions, is invisible to the naked eye and is called the Brownian motion.
The descriptions above are generalisations. For example, a gas can be pressurised and made denser, so that it behaves more like a liquid. And putty, or caulk, can be thought of as either a soft solid or a thick liquid.
The precise phase depends on the combined influences of the binding energy – the mechanical work that needs done against the forces which hold a molecule together, to disassemble the molecule into its component parts – and the kinetic energy of the molecules. The binding energy is a measure of the attraction between molecules, which causes them to adhere together, while kinetic energy has the opposite influence.
Temperature also plays a significant role in changing states. As temperature increases, the average kinetic energy of the molecules rises, thus explaining why solids melt and liquids vaporise.
As the temperature rises, molecules achieve the freedom to move independently past one another, though they remain in close contact – the liquid phase. At higher temperatures still, molecules break loose and achieve a much more tenuous state – the gas phase.
But it doesn’t end here.
At even higher temperatures, molecules split into separate atoms, which in turn disintegrate into free electrons and nuclei – the plasma phase of matter. Plasmas are widespread in the Universe, in stars and the interstellar medium, but you may be more familiar with the phenomenon of lightning and plasma balls.