What changes is how the particles sit and move. Locked in place, they make a solid; sliding past each other, a liquid; flying apart, a gas. Temperature decides which, and every substance has its own switch points.
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0 · 100 °C for water
Solids keep their shape
Liquids keep their volume
Gases keep neither
One particle model, three states
THE SHORT ANSWER
It is all about how particles sit.
Shape, volume and squashiness follow from the particle picture.
THE PARTICLE MODEL
Fixed, sliding or free
In a solid the particles are in fixed positions and vibrate. In a liquid they stay in contact but move past one another. In a gas they are widely separated and move at random.
Glass is an amorphous solid: no regular crystal pattern, but it holds its shape. Old panes are uneven because of how they were made, not because they flowed.
Pick a substance to see where it melts and boils at normal air pressure. The bar shows how wide its liquid range is.
Figures from LibreTexts Chemistry, the University of Illinois physics Q&A (citing the CRC Handbook) and Chemistry World.
TWO NUMBERS WORTH KNOWING
Change the state, change everything.
One figure on how much room a gas takes, one on how pressure moves a boiling point.
1,000× or more
Volume increase when a liquid boils
Going from liquid to gas increases the volume by a factor of 1,000 or more, because the particles move far apart. That is also why gases are highly compressible and liquids are not.
LIQUID TO GAS · LIBRETEXTS
≈ 95°C
Water's boiling point in Denver
Lower air pressure lowers the boiling point. In Denver, Colorado, water boils at about 95 °C rather than 100 °C.
BOILING AT ALTITUDE · ILLINOIS
Melting and boiling points are always quoted at a stated pressure, usually one atmosphere. Change the pressure and the switch points move.
FOUR THINGS TO NOTICE
Kitchen science, no lab needed.
You can see the particle model at work with things already in the house. Ask an adult before using a hob or kettle.
Watch ice keep its shape
An ice cube sits in a glass as a cube. Its particles are fixed in place, so a solid keeps a definite shape and volume.
Pour water into two shapes
The same water fills a tall glass or a wide bowl. A liquid keeps its volume but takes the shape of its container.
Notice the steam spread
Steam from a kettle spreads out and fades into the room. A gas has no fixed shape or volume and fills the space it is in.
Add salt to water
Salt lowers the freezing point. Water with 29.2 g of salt per kilogram stays liquid down to about −1.85 °C, a little below plain water's 0 °C.
AI-generated image, not a photograph of a real person or place.
THE GLASS QUESTION
Not quite solid, not a liquid.
AI-generated image, not a photograph of a real person or place.
Glass forms when molten material cools fast and skips the regular crystal pattern. Chemists call it an amorphous solid. Quartz and window glass are both silicon dioxide, but quartz is an ordered crystal and glass is a random network.
The old story that cathedral windows are thicker at the bottom because glass flows does not hold up. A 2017 Penn State study of 13th-century glass at Westminster Abbey found that even nano-scale changes would take billions of years. Panes were uneven when made, and were often fitted thick edge down.
Questions
Questions people ask
What are the three states of matter?
Solid, liquid and gas. A solid has a definite shape and volume, a liquid has a definite volume but takes the shape of its container, and a gas has neither and fills its container. Plasma is sometimes added as a fourth state.
What is the particle model?
It explains the states by how particles are arranged and move. In solids they are in fixed positions and vibrate, in liquids they stay in contact but move past each other, and in gases they are widely separated and move at random.
What is it called when a liquid turns into a solid?
Freezing. The reverse, a solid turning into a liquid, is melting, and the melting point of a solid is the same temperature as the freezing point of its liquid: 0 °C for water at normal pressure. A solid can also turn straight into a gas without becoming a liquid first, which is called sublimation.
Is glass a liquid?
No. Glass is an amorphous solid: it has no long-range crystal order but holds its shape. Research on Westminster Abbey glass found room-temperature flow far too slow to change old windows.
Can something be both a liquid and a solid?
A thick mix of cornflour and water comes close. Move your fingers through it slowly and it flows like a liquid; punch or squeeze it and it behaves like a solid, because the tiny starch particles lock together and the water is pushed into the gaps between them. The Institute of Food Science and Technology calls it a non-Newtonian fluid: unlike water, it does not keep a constant viscosity when force is applied.
Why does water boil at a lower temperature up a mountain?
Air pressure is lower at altitude, and lower pressure lowers the boiling point. In Denver, Colorado, water boils at about 95 °C.
Can a metal be liquid at room temperature?
Yes. Mercury is liquid at room temperature, freezing at −39 °C. Gallium melts at about 30 °C, so it can melt on a hot day.
: Added FAQs on what it is called when a liquid turns into a solid (freezing, melting, sublimation) and on cornflour slime, which acts as both a liquid and a solid, and noted plasma as a fourth state. New sources: LibreTexts Chemistry and the Institute of Food Science and Technology.
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