When ice melts, it goes from a solid straight to a liquid. Why isn’t there some sort of pasty intermediary state? If the states of matter are about the proximity of a body’s particles, then it’d make sense for the ice to get softer and softer until it turns to water, right?
In: Chemistry
Imagine a line of kids walking across a playground, all holding hands. When they are moving slowly, they are holding each others hands well, and are in a well formed line. This corresponds to a cold solid that is very rigid. As they move faster, they start to bounce around a bit more, still managing to hold onto each others’ hands. This corresponds to a hot solid, which is a little more flexible, but still solid. At some point as they get faster, they let go of each other’s hands. As soon as they do that, they start to move away from each other, and the formation is gone. This corresponds to a hot solid becoming a liquid.
The reason there is a strict boundary between solid and liquid is that (most) solids behave like kids holding hands: the molecules are moving slowly enough that they are able to directly hold tight to their nearest neighbor. They either are holding, or are not holding, onto their neighbor – there is no in-between where they are holding on a bit.
To take the analogy further, after the kids have let go of each others’ hands, they are only moving away from each other slowly. As they speed up, they start to move away from each other more quickly, losing formation even more. There really is a continuum with the kids, rather than just holding hands or not. This is true of liquids too. Once the molecules have let go of each other, if they are only a little bit above the melting point, they will still move slowly, and if they are very hot, they will move quickly. We see this as viscosity, or “runniness”, in liquids like syrup (liquid sugar), metals, and lava (liquid rock): each of them is much runnier when it is far above melting point. This is not to say that syrupy liquids are sort of solid. They are not: the molecules are not holding on tight to their nearest neighbors any more. They just aren’t moving around very much.
All of this is hard to see with water because liquid water is quite runny at all temperatures, and solid ice it quite rigid at all temperatures. It does get a little bit runnier at high temperatures, and more rigid at lower temperatures, just not enough to see without measuring equipment.
Finally, there are actually some things that behave in the way you described. As you heat them up and cool them down, they just change how “runny” they are. The best example of this is glass. As you heat up glass, it just slowly gets softer, never hitting a specific melting point. These substances are called amorphous solids, or sometimes just glassy solids. Some people actually like to say that solid glass is really a liquid because of how there is no strict transition between liquid and solid, but I don’t like that terminology.
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