Why are there states of matter?

We all learned in school that there are three states (or phases) of matter: solid, liquid, and gas. Later, you may have learned of a fourth state, plasma; and, physicists say there various other states that emerge in extreme conditions (e.g. Bose-Einstein condensate). But let’s keep it simple, and go with three for now.

While helping my 12-year-old son with his homework, this question came up, and I haven’t been able to find the answer:

Why are there states of matter at all?

In other words, it’s easy to imagine a physical world where there are no phase transitions. In this alternate world, all matter would change continuously with temperature change. The molecules of the substance would continuously increase in space from one another, with no sudden changes in properties or structure. At the coldest temperatures, everything would be extremely solid. As the temperature warmed up, the solid would become progressively and continuously less solid, and more “mushy,” let’s say. More “liquid like” but continuously, not in a sudden phase transition. And as this liquid-ish form of matter warmed up into what we know as the “gas” state of matter, it would gradually and continuously become more fog-like–but again, with no sudden phase transition.

I have searched all over the Internet, and I haven’t found this question asked or answered. (I ended up reading some advanced stuff about energy states, and curves crossing, but that doesn’t answer the question.) Does anyone know the answer, and if you do, can it be explained in a way that regular people can understand it?

3 thoughts on “Why are there states of matter?

  1. Here’s the simulation that usually gets pulled out https://phet.colorado.edu/en/simulation/states-of-matter

    One way to approach the problem is via scale. Do states of matter make sense for a lone particle or are they defined on a group level? If they are defined on a group level, how big a group do I need to talk about? All the particles in a glass or some of them?

    So yes, phase changes do happen as if on a continuum. However, the steepness of the continuum is dependent upon how fast changes propagate: in practice things propagate fairly quickly (for common temperatures particle velocities are pretty high). Melting iron phase changes along a large sample with a point heat source much slower than supercooled water freezing.

    Here’s a nice simulation to see this idea (via salt water freezing) https://www.explorelearning.com/index.cfm?method=cResource.dspDetail&ResourceID=426

    However, it seems like your question is more about the liquid state. Again, I find the pHet simulation works well here. In liquid phases rotational energy dominates over vibrational or translational. However, there will be times when the dominance isn’t as significant. At those times the issue of scale becomes substantial (as does the relative contributions of each form of motion).

    Good luck at the Calgary Ideas conference – good group.

  2. I think it has to do with energy states, which relates to conductivity in the material. If we imagine a laser dumping lots of energy in one discrete location, it jumps to the next phase while the surrounding material remains in the lower energy phase. With less extreme amounts of heat, the energy spreads out in the material so virtually all changes phase together. It takes a certain amount of energy to get water from -1 degree (Celsius) to 0 and a similar amount to get it to change to liquid. I think this is the energy it takes to heat the entire block of ice up together.

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