When it comes to states of matter, you’re probably familiar with the big three: solid, liquid, and gas. You may even be familiar with the more exotic (yet also more plentiful) plasma state. But those are just the states you see in everyday life. In extreme conditions like supercooled laboratory chambers or the cores of neutron stars, matter can take on any number of weird and wonderful states.
All matter is made of smaller particles. Take water, for instance. If you zoomed in on an ice cube, you’d see a latticework of individual water molecules. If you zoomed in on a water molecule, you’d see three individual atoms: two atoms of hydrogen and one atom of oxygen. Those atoms, in turn, are made of even smaller parts: Hydrogen is made up of one proton and one electron, and oxygen is made up of eight protons, eight neutrons, and eight electrons. Protons and neutrons are made up of even smaller particles called quarks. (Quarks and electrons are what’s known as “elementary particles,” which mean they’re as small as small gets.)
States of matter, then, come down to the arrangement of those particles. (You may have heard the term phase of matter, which is technically different, but similar enough that many people use the two terms interchangeably.) The water in your ice cube is in a solid state. In a solid, the molecules are bound to one another by molecular forces, which makes it hold its shape and keep its volume fixed.
If you were to melt that ice cube, you’d have water in a liquid state. Here, the molecular forces would be weaker, leaving the molecules to move around a bit more but still stay close to one another. Liquids change their shape to fit their container (which is why people say that cats are liquid), and, like solids, they can’t be compressed.
Keep heating that liquid water, and you’ll get a gas. Gas particles move around and spread out much more than liquid particles. If it’s in a container, the gas will spread to fill it; if it’s not, the gas will spread out forever. Gas has no definite volume or shape.
Plasma is where the water example breaks down (literally — the energy required would break the molecular bonds and you wouldn’t have H2O anymore). Plasma is a hot ionized gas made up of highly charged particles. However, plasma is not the same thing as gas, since that electrical charge puts the stuff under the influence of any electric or magnetic fields nearby. While you might not be as familiar with this state of matter as you are with the previous three, plasma is, in fact, the most plentiful state in the observable universe, making up 99 percent of all observable matter. Stars, the jets that blast out of black holes, and even parts of Earth’s atmosphere are made up of plasma. If you’ve ever looked at a neon sign, you’ve been up-close and personal with this state of matter.
The states above are considered the four fundamental states of matter since they’re easily observable in nature. But there are plenty of other “non-classical” states of matter beyond those.
A few you’re intimately familiar with. Glass, for example. The molecules within glass aren’t as organized as they would be in a solid, but still more organized than a liquid. That makes glass an amorphous solid. Another state of matter is one you might be interacting with right this second. The liquid crystal in your phone or computer monitor’s liquid crystal display (also known as LCD) toes the line between the solid symmetry of a crystal and the disorganized flow of a liquid — it’s made up of particles that line up together but still flow easily.
There are also states of matter that only exist in the most extreme conditions. When you cool atoms to near absolute zero, you can get what’s known as a Bose-Einstein condensate, where all of those individual atoms coalesce into one object (that is, something that can be described by a single quantum wavefunction). It’s possible that this stuff could form invisible stars that we’re mistaking for black holes. Cooling things to very low temperatures also gives rise to a lot of super stuff: superconductors conduct electricity without resistance, superfluids flow without friction, and supersolids flow without friction but keep their shape.
These are only a handful of the many states of matter we know about, and in fact, there are probably even more to discover. But the next time someone asks you “liquid or solid?” in a rousing game of 20 Questions, try not to burst a brain cell.