There are four common states of matter. Thinking about them raises some interesting points of insight regarding systems and the law of conservation energy.

R.C Smith

Considering a very simple – perhaps overly simple – example involving water, H2O, changing from solid to liquid and then to gas. Coinciding with these phase changes are a temperature range: from -40C – 0C, 0C-100C, and then finally 100C to 140C.

If one were to picture this in terms of the shape of a graph, we would see very clearly how the changes of state of water, from a solid when frozen to a gas when heated at boiling temperature, correlate with increases in temperature. With an increase in temperature of the water, the densities of the particles change and thus also the structure of the H2O molecules.

In total, there are four common states of matter according to the kinetic theory of matter: solids, liquids, gases, plasma. There is also a fifth state, which I will write about in the future, known as Bose-Einstein condensates. But in the particular example of this article, we are only considered three of the four phases (in order): solid, liquid and gas. The final phase, plasma matter, is not applicable here; although it is possible for water to enter a plasma state. In this case, the water (H2O) is split into hydrogen and oxygen, and the hydrogen and oxygen atoms experience ionisation (they turn into ions).


  1. i) Phase one (-40C – 0C): Solid

In relation to the temperature range described, at the point at phase one has heat been absorbed, the water is subject to -40C temperature and is therefore a solid. This is because, when water is subjected to temperatures below 0C, the freezing temperature, it will freeze; thus, turning into a solid. In this state, the arrangement of particles is in a regular pattern, as the particles are positioned closely together.

states of matter

Additionally, in a solid state, the water molecules have a relatively small amount of energy, which means that they are firmly constrained and held together in fixed positions. Understanding that the particles of matter experience forces of attraction, these inter-molecular forces are essentially strong enough to hold the particles together in fixed positions. In other words, in this solid state the water molecules do not possess enough energy to break from their bonds. Thus, what we see, is a solid (in this case in the form of ice) with a fixed volume.

But this notion of fixed position, as described above, can be deceiving. While the water molecules are understood to have a small amount of energy and are firmly constrained in this solid state, the kinetic theory of matter tells us that each particle is always in constant motion. In this case, the water particles are restricted but they are only restricted to a state of vibrational motion.

As an aside, one of the deeper intuitions here is that, due to the law of conservation of energy, the total energy of a closed system is constant. One of the implications of this law concerns how energy is never created or destroyed. It is simply transferred from one form to another, and this is a very important notion in physics (as in chemistry and other places). It helps describe, or at least offers a deeper intuition into our understanding, as to why the water particles in a solid state do not absolutely cease motion. They still contain energy – more precisely, kinetic energy – but do to the physics of water in a solid state, the only motion allowed is vibrational.  This vibrational motion is not enough to disrupt the structure of the solid.

But as we see when the temperature of the water increases from -40C to 0C, it approaches the melting point in which the water solid will experience significant change. With this change also follows the change in the structure of the particles.


  1. ii) Phase two (0C-100C): Liquid

In the second phase, where the water changes from solid to liquid, this correlates with an increase in the amount of heat absorbed. Additionally, we see the temperature of the water increase from 0C to 100C (the approximate boiling point), as it moves from melting point to boiling point.

It is important to notice the correlation between the gradual increase in the temperature of water and the gradual change of state. What we are witnessing is how, when the water solid is heated, the particles gain energy. In gaining energy, the rate of vibrational motion also increases. With this increase in vibrational motion, the structure of the water solid is gradually weakened; thus, one observes the expansion of the solid as it slowly changes to a liquid. This expansion is the result of how, in that solids have a notably fixed surface and volume (for reasons described in relation to phase one), when subject to heating the particle attraction decreases. The energy of the particles vibration motion intensifies, and this results in the particles moving farther apart. A consequence of this is an increase in volume and a decrease in density. Hence what one regularly observes when ice melts – there is, for a lack of better word, a dispersion, such as in the case of an ice cube being left of a desk at room temperature. As the water turns to liquid, it spreads across the desk.

This process continues as the water is gradually heated. In this case, where the temperature of the water increases from 0C to 100C, as the temperature climbs closer toward the boiling point, the heat transfers more and more energy until the particles break free from the inter-molecular forces holding them together. Finally, the water completely enters the liquid state, wherein the particles move further apart from one another with increased energy (than when in a solid).

Although there is increased distance from the particles, this is relative phrasing because they still remain close. The only difference is that they are now less restricted. Instead of touching and being fixed in position, the particles are allowed to pull apart and to move freely around each other.

Although the strength of inter-molecular forces decrease, which correlates with the molecules no longer being arranged in a regular pattern, forces of attraction remain and the molecules are now arranged randomly. Further, the intermolecular forces are such that the molecules remain close together, but they are no longer fixed and are less compressed.

states of matter

Notice, in reviewing the above image, the increase of space between the particles (now arranged randomly) in comparison when the water was a solid. This increase in the space between the particles correlates with a decrease in density from solid to liquid. To put it another way, the liquid is now diffuse.


iii) Phase three (100C-140C): Gas

In the third phase, the water, now a liquid, enters its boiling point. The process of boiling begins when the liquid begins to turn into a gas. At this point in the total time in which the temperature of the water has increased from -40C to 100C to 140C, we observe a significant increase in the amount of heat absorbed.

Interestingly, it is worth noting that the water remains at boiling point until significant heat absorption has been achieved, then a temperature increase is finally observed as the water begins to change from liquid state to gas.

In changing from liquid state to gas, the water liquid loses density. In other words, gases are much less dense than liquids, and so when the water changes from liquid to gas the water particles spread out even further (than when water changed from solid to liquid).

When the water liquid is heated, the increase in temperature at boiling point correlates with a continued increase in energy. Again, this refers to the law of conservation of energy. Hence, the water particles now vibrate more vigorously. The liquid then expands insofar that the particles further separate from each other. They gain so much energy that they can now break free from their attractive forces, and diffusion begins to occur more rapidly.

At this point, the particles can now move more quickly, as the forces of attraction become inconsistent among each particle. That is to say that the gas particles finally have sufficient energy to break free from the intermolecular forces that once held them together. Due to the lack of overwhelming particle attraction, the gas can spread out and fill its container.

states of matter

As illustrated in the image above, notice how the particles are more widely spaces than when a liquid or a solid. The particle arrangement also remains random, and because the gas particles can now move freely there is no order in the system. One could also say that, contrary to a solid, there is no fixed shape with a gas.

Although in this short article a very simple example was chosen in relation to H2O, it nevertheless offers incredible insight into common states of matter and how they relate to the law of conservation of energy. One might not immediately think of this incredible microworld that exists when simply looking at an ice cube melting on a table, or water boiling in a pot, but the nature of reality is incredibly rich in detail once we begin to look more closely.