The States of Matter

Solid, liquid, gas — and plasma. One picture of moving particles explains all four.

High schoolIntro Gen ChemUni Year 1
⏱️ About 12 min

Ice, water, and steam are the same substance — H₂O — in three different outfits. Nothing about the molecules changes when ice melts; only how they move and how tightly they hold together. Get that one idea, and every state of matter, and every change between them, snaps into focus.

💡
The big idea: Matter is made of particles in constant motion. Whether a substance is a solid, liquid, or gas comes down to a tug-of-war between the energy of that motion and the forces pulling the particles together. Add energy and the particles break free; remove it and they lock back down.
🎯 By the end, you'll be able to
  • Describe the particle arrangement and motion in solids, liquids, and gases
  • Explain plasma as a fourth state and where it shows up
  • Connect changes of state to particles gaining or losing energy
  • Explain why a change of state does not change the substance or its mass
📎 Helpful to know first

It all comes down to particles in motion

Every material — this page, the air in the room, the chair you're sitting on — is built from tiny particles (atoms, ions, or molecules) that are always moving. What we call a state of matter is simply a description of how those particles are arranged and how freely they move.

Two things compete. The particles have kinetic energy (motion) that tends to scatter them, and attractive forces between them that tend to pull them together. Heat feeds the motion; cooling starves it. The winner of that contest decides the state.

🔑 The particle model in one line
Solid = particles locked in place, only vibrating. Liquid = particles touching but free to slide past each other. Gas = particles far apart, flying freely. Same particles throughout — only the arrangement and motion change.

Solids, liquids, and gases

  • Solid — particles are packed close in a fixed pattern and can only vibrate around their spots. A solid holds its own shape and volume. This is why a steel bar stays a bar.
  • Liquid — particles are still close and touching, but the bonds keep breaking and re-forming, so they flow and slide over one another. A liquid keeps its volume but takes the shape of its container.
  • Gas — particles have enough energy to break away entirely. They spread out to fill any container, so a gas has neither fixed shape nor fixed volume, and it can be squeezed (compressed) because of all the empty space between particles.
e⁻ e⁻ n=1 (2 e⁻) e⁻ e⁻ e⁻ e⁻ e⁻ e⁻ e⁻ e⁻ n=2 (8 e⁻) 10 p⁺ 10 n Neon · Ne · Z=10 · mass number 20
A single gas particle up close — a neon atom. Neon is a noble gas, so it flies around as lone atoms. In a gas, particles like this are far apart and moving fast; in solid (frozen) neon they would be locked together, though each atom is exactly the same.
✨ Plasma: the fourth state
Heat a gas hot enough and its atoms start losing electrons, becoming a soup of charged particles called a plasma. It's not exotic — the Sun, lightning, neon signs, and the inside of a fluorescent tube are all plasma. In fact, most of the visible matter in the universe is plasma, not solid, liquid, or gas.

Changes of state: adding and removing energy

Heating or cooling moves a substance between states by changing how much energy its particles have — but it never changes what the substance is. Water is water whether it's ice, liquid, or steam.

  • Melting (solid → liquid) and freezing (liquid → solid).
  • Vaporizing / boiling (liquid → gas) and condensing (gas → liquid).
  • Sublimation (solid → gas directly, like dry ice) and deposition (gas → solid, like frost forming).

Going 'up' the ladder (solid → liquid → gas) absorbs energy; going 'down' releases it.

⚠️ Gases still have mass
A gas feels like 'nothing,' but its particles are real matter with real mass. Air has a mass of roughly 1.2 grams per litre at room conditions — a typical classroom holds tens of kilograms of it. This trips people up constantly, so hold onto it: gases weigh something.
📝 Worked example: You seal a bottle of liquid water, weigh it, then freeze it solid and weigh it again. Does its mass change?
  1. Freezing is a change of state, not a change of substance — the same H₂O molecules are present before and after.
  2. The bottle is sealed, so no particles enter or leave.
  3. The particles simply slow down and lock into a fixed arrangement (ice). Their number and their mass are unchanged.
✓ No — the mass stays exactly the same. Changing state rearranges particles; it never creates or destroys them.
✏️ Practice: Air has a density of about 1.2 g/L at room conditions. What is the mass of the air filling a 2.0 L bottle? (mass = density × volume.)
g
Solution
  1. mass = density × volume.
  2. = 1.2 g/L × 2.0 L.
  3. = 2.4 g. Small, but definitely not zero — gases have mass.

Check your understanding

1. In which state are the particles close together but still free to slide past one another?
That's a liquid: particles stay touching (so volume is fixed) but move around each other (so it flows and takes the container's shape). In a solid they're locked; in a gas they're far apart.
2. A sealed balloon of helium is weighed. What is true about the gas inside?
Gases are matter, so they have mass — helium particles are real atoms. It's a common misconception that gases are 'weightless'; they simply have low density.
3. When ice melts into liquid water, what has changed?
Melting is a change of state, not of substance. The same H₂O molecules remain — they just gain energy and move more freely. Mass and molecule count are unchanged.
🔎 Related lessons
Next: Units & Dimensional Analysis Density Inside the Atom
✅ Key takeaways
  • A state of matter describes how particles are arranged and how freely they move.
  • Solid = locked and vibrating; liquid = touching but sliding; gas = far apart and free.
  • Plasma is a fourth state — a hot, charged particle soup (stars, lightning, neon signs).
  • Changes of state add or remove energy but never change the substance or its mass.
  • Gases are real matter and have mass, even though they feel like nothing.
➡️ To measure any of this — a gas's mass, a liquid's volume, a solid's size — you need reliable units and a dependable way to convert between them. That's the next lesson.
Want to test yourself on this? Try the Chemistry practice test →