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Solids, liquids, and gases are the three states of matter in traditional physics, but more recently a fourth has been included. Plasma.

It happens when a gas has been electrified or heated to temperatures so extreme that electrons are stripped from its atoms. You end up with a cloud of ions that are sensitive to electromagnetic fields and can conduct electricity.

It also looks really cool.

Air is turned into plasma by lightning, Tesla coils, and plasma globes. But the same process takes place on stars like the Sun, where temperatures are so high that almost all of the material in the star has become plasma.

Plasma sounds exotic, but because it makes up most of the matter in stars, and stars make up most of the ordinary matter in the universe, this means that 99% of the ordinary matter in the universe is plasma.

It is our world, where we see a mix of gasses, liquids, and solids all in one place that is special in the universe – particularly because of liquids.

Aside from stars, the rest of the universe is cold outer space, where nebulae form giant gas clouds and everything else is frozen solid.

It’s only in a tiny remainder between the two extremes, near stars but not too far away, that liquids can exist. Usually this is within the interior and on the surface of planets and moons. They’re like a brief, transitionary phase between solids and gasses, but are crucial in building the complexity required for life to form.

Liquids have to exist between these two extremes because temperature is crucial to what state something is in. Have a look at the chart below, shows the state of each element at different temperatures.

The state of each element at different temperatures.

To a physicist, temperature is the same thing as the vibration of atoms.

The coldest possible temperature is where atoms cease to vibrate, and to increase heat is to increase the vibration of atoms.

Things change from solid to liquid to gas as they get hotter as their vibrating atoms force each other apart, pushing the material to expand.

But not all atoms and molecules are equally susceptible to the influence of heat. Most of them have some degree of attraction to each other and it’s this attraction that holds the substance together.

The reason why carbon dioxide is a gas but water is a liquid at room temperature is that water molecules have a slight charge, and carbon dioxide does not. The charge attracts water molecules to each other, which makes them stick together despite the vibrations caused by heat. Many chemicals have similar quirks which determine what state they most easily become. Ionic molecules like salt often have an even stronger attraction to their own molecules, and easily form solids even at high temperatures.

But an email about states wouldn’t be complete without also mentioning pressure.

The higher pressure a material is under, the more its atoms are forced together despite their temperature. We can force gasses to become liquids, and liquids to become solids, something that can become really useful when transporting materials like rocket fuel.

But the concept works both ways. If the Earth suddenly lost its atmosphere, it would lose all of its pressure. Water, which is liquid at room temperature and standard atmosphere of pressure, becomes a gas if pressure drastically falls. If the Earth lost its atmosphere, the oceans would evaporate into gas, along with all the water in our bodies.

This is one of the reasons why direct exposure to outer space (or any very low-pressure environment, also known as a ‘vacuum’) 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, and when he was revived, he said that the last thing he remembered was the saliva on his tongue beginning to boil.

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Ben McCarthy

Ben McCarthy

Ben is the Founder of Discover Earth and the author of the Big Ideas Network.