States

Discovering the three states

The world in which we live is made up of many objects and materials. All these materials can exist in different forms called states of matter:

The solid state
The liquid state
The gas state

A simple example is that of water, which we know well:

In the solid state: ice or snow
In the liquid state: rain, rivers, oceans
In the gas state: invisible water vapour in the air

These three states can be observed for many other substances such as glass, iron, copper, carbon dioxide, etc. Other, rarer states also exist, but we will see them later.

Key points

What are the three main states of matter?
Give an example of water in each state?

Understanding the properties of each state

Each state of matter (applies to water, metal, glass, air, etc.) has observable characteristics:

🧊

SOLID

Properties of a Solid

Fixed volume : always occupies the same space
Fixed shape : does not change shape (unless broken)

💧

LIQUID

Properties of a Liquid

Fixed volume : retains the same volume regardless of the container
No fixed shape : takes the shape of the container

💨

GAS

Properties of a Gas

No fixed volume : occupies all available space
No fixed shape : completely takes the shape of its container

State	Fixed volume	Fixed shape	Example
Solid			A wooden cube
Liquid			Water in a glass
Gas			Air in a balloon

Key points

Does a liquid have a fixed volume? A fixed shape?
Why do we say that a gas does not have a fixed volume?
Compare an ice cube and liquid water: what changes and what stays the same?

Explanation at the molecular level

To understand why the states are different, matter must be observed on a microscopic scale. All matter is made up of molecules. Their arrangement and motion determine the state of matter.

1. The solid state

Molecules are very close to each other.
They are strongly linked to each other, blocking any motion.
They form crystalline structures, highly organized and regular
Vibrate slightly but do not change position.

2. The liquid state

Molecules are close together but weakly linked
They are disorganized
Can slide over one another.

3. The gas state

Molecules are very far apart
No link between them
Move rapidly in all directions

Click on a state to observe the behavior of molecules

State	Distance between molecules	Bond strength	Organization	Motion
Solid
Liquid
Gas

Key points

What are the organization and compactness properties of molecules in the three states?
Why does a liquid keep its volume but change its shape?
Why does a gas occupy all available space?
Why can a gas be compressed, but not a liquid?

Going further: Rare states of matter

Besides the three common states, there are rarer states:

Plasma: gas whose molecules are so agitated that they split into charged particles (e.g. the Sun, lightning)
Bose-Einstein condensate: state obtained at temperatures close to absolute zero where molecules become almost motionless