TL;DR: The relative abundance of the four states of matter shows us that many of the things that we’re used to on Earth, like seeing a river of running water, are some of the rarest sites in the universe.
Solids, liquids, and gases are the three states of matter in traditional physics and chemistry, but more recently a fourth has been added. Plasma.
Plasma is made when a gas has been heated or electrified such that its atoms turn into ions.
It also looks really cool.
Plasma sounds pretty rare and exotic, but this is actually a bias that comes from where we live.
Stars, including our own Sun, are almost entirely made of plasma. Considering that stars make up most of the (ordinary) matter in the universe, it turns out that 99% of the ordinary matter in the universe is plasma.
Plasma isn’t rare or exotic at all. It’s just rare on Earth.
It naturally occurs in very hot fires and for a brief moment in lightening strikes. We’ve artificially created it in special devices like plasma globes, plasma TVs, Tesla coils, and in neon lights.
I think it’s pretty cool that our TVs now contain the fourth state of matter.
A unique perspective
The funny thing is that it’s actually what we’re used to on the Earth that is rare and exotic. It’s here that we see a mix of the other three states (gasses, liquids, and solids) all in one place.
One of these in particular is extraordinary rare in the universe. Liquids.
Most of the universe is cold outer space, far from the warmth of stars, where nebulae form giant gas clouds and everything else is frozen solid.
It’s only in a tiny zone between the two extremes, near stars but not too far away, that liquids can exist. Usually this is within the interior and sometimes on the surface of rocky planets and moons.
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A state of transition
Have a look at the chart below, which shows the state of each element at different temperatures. The green liquid state only exists for a short period until it gets too hot and it boils into a gas.
The state of each element at different temperatures.
The difference between states
But what is actually going on here? What’s the difference between states and why does matter change at all?
To a physicist, the difference between the traditional three states of solids, liquids, and gasses is the closeness of their atoms. Atoms in a solid are closely compacted together and in a gas they’re far apart. In liquids they are somewhere between.
Heat forces their atoms apart. Heat is another word for the vibration of atoms and the higher the temperature, the more the atoms will push each other apart.
Heat doesn’t affect all atoms equally, because some of them m have a strong attraction for each other.
Metals, like gold and uranium, tend to still be solid at high temperatures because their atoms are strongly bonded, and it takes a large amount of heat to melt them – that is, to force their atoms apart.
Likewise at room temperature water is a liquid but oxygen is a gas. They’re at the same temperature and so will be vibrating at the same rate, but water molecules have a moderate attraction to each other, and oxygen does not.
Working under pressure
The last major influence on state comes from pressure. The higher pressure atoms are under, the more they are forced together.
High pressure can force gasses to become liquids or liquids to become solids, which is very useful when you want to save space when transporting materials, like on a rocket.
If the Earth suddenly lost its atmosphere and everything on the surface was subject to the vacuum pressure of space, the temperature of the oceans (about 17C) would be enough for them to evaporate into gas, along with all the water in our bodies.
This is one of the reasons why direct exposure to outer space is deadly for astronauts.
During a test at the Johnson Space Flight Center in 1965, a leaky spacesuit exposed an astronaut to a vacuum. He passed out after a few seconds but when he was revived, he said that the last thing he remembered was the saliva on his tongue beginning to boil.
A narrow road between extremes
So liquids can only exist in a narrow band of between extremes of temperature and pressure, which is what makes them so rare in the universe.
This is part of why manned space travel is so difficult. We have to maintain a delicate balance of temperature and pressure in a tin can for long periods of time in space, with no leaks.
Liquids are the precursor of complex chemistry, as they let atoms mix with each other in a way that isn’t always possible with solids and gasses. So finding this balance is crucial in building the chemical complexity required for life to form, and this strict criteria is part of the reason why life in the universe may be so rare.