States of matter

Welcome to your study notes on the States of Matter! This topic forms the absolute foundation of chemistry. By understanding how the tiny particles that make up our universe behave, you will be able to explain everything from why puddles dry up in the sun to how oxygen can be stored as a liquid in hospital tanks.

What you'll learn

How particles are arranged and move in solids, liquids, and gases.
The names of state interconversions and why they are classified as physical changes.
How to explain what happens to particles during changes of state.
How to use melting and boiling point data to predict the state of any substance at a given temperature.

1. The Three States of Matter

Everything around us is made of matter, and matter exists in three main states: solid, liquid, and gas. To describe and explain how these states behave, scientists use the kinetic particle theory. This theory states that all matter is made up of tiny, moving particles (atoms, molecules, or ions) that are held together by forces of attraction.

The differences between solids, liquids, and gases come down to three things:

Arrangement of the particles.
Movement of the particles.
Relative energy of the particles.

The arrangement and energy of particles in solids, liquids, and gases

Solids

In a solid, the forces of attraction between particles are very strong. This holds them closely together in a fixed, neat arrangement.

Arrangement: Regular pattern (often called a lattice structure) where particles are tightly packed and touching.
Movement: Particles cannot move from place to place; they can only vibrate about their fixed positions.
Relative Energy: Lowest energy of the three states.

Liquids

In a liquid, the forces of attraction are slightly weaker than in a solid, allowing the particles some freedom.

Arrangement: Randomly arranged but still close together, with most particles touching.
Movement: Particles can slide and move past one another. This is why liquids can flow and take the shape of the bottom of their container.
Relative Energy: Medium energy (higher than solids, lower than gases).

Gases

In a gas, the forces of attraction between particles are extremely weak, almost negligible.

Arrangement: Randomly arranged and spaced very far apart from each other.
Movement: Particles move rapidly and randomly in all directions, colliding with each other and the walls of their container.
Relative Energy: Highest energy of the three states.

Definition

Lattice

A regular, repeating three-dimensional arrangement of atoms, ions, or molecules in a metal or crystalline solid.

2. State Interconversions

Substances can transition from one state to another when we add or remove thermal (heat) energy. These transitions are called interconversions.

It is crucial to recognise that changing state is a physical change, not a chemical reaction.

Definition

Physical Change

A change in state, shape, or form that does not produce any new chemical substances. The chemical formula of the substance remains exactly the same, and the change is easy to reverse.

For example, when liquid water ( $H2O(l)\text{H}_2\text{O(l)}$ ) freezes into ice ( $H2O(s)\text{H}_2\text{O(s)}$ ), it is still chemically water. We can easily reverse this by warming it up. In contrast, a chemical change (or chemical reaction) creates entirely new substances and is usually very difficult to reverse (like baking a cake or rusting iron).

The diagram below shows the names of the changes of state between solids, liquids, and gases:

Names of interconversions between solid, liquid, and gas states

What happens to the particles during state changes?

To explain state changes, we must describe what happens to the energy, movement, and arrangement of the particles:

When we heat a substance (adding energy):

Melting (Solid to Liquid): Heating transfers thermal energy to the solid particles, which they store as kinetic energy. The particles vibrate more and more rapidly. Eventually, at the melting point, they gain enough energy to partially overcome the attractive forces holding them in their fixed lattice. The regular arrangement breaks down, and the particles begin to slide past one another.
Boiling (Liquid to Gas): Heating a liquid increases the kinetic energy of its particles, making them move faster. At the boiling point, the particles have gained enough energy to completely overcome the remaining forces of attraction between them. They break free from one another, spread far apart, and fly off as a gas.

Key Idea

Forces of attraction do not break down inside molecules!

When water boils, the forces of attraction between the water molecules are broken. The covalent bonds holding the hydrogen and oxygen atoms together inside each molecule do not break. You still have $H2O\text{H}_2\text{O}$ molecules!

When we cool a substance (removing energy):

Condensing (Gas to Liquid): As a gas cools, its particles lose kinetic energy and slow down. When they bump into each other, they don't have enough energy to bounce apart. The forces of attraction pull them together, forming a liquid.
Freezing (Liquid to Solid): As a liquid cools further, the particles lose even more kinetic energy and move slower. At the freezing point, the particles move slowly enough that the strong forces of attraction lock them into fixed, regular positions. They can now only vibrate.

Tip

Evaporating vs. Boiling

Boiling happens only at the boiling point, and bubbles of gas form throughout the entire liquid. Evaporation can happen at any temperature below the boiling point, and only occurs at the surface of the liquid.

3. Predicting States of Matter from Data

In your GCSE chemistry exams, you will often be given data about the melting point and boiling point of a substance and asked to predict what state it will be in at a specific temperature.

To solve these questions, keep these mathematical rules in mind:

If the given temperature ( $T$ ) is below the melting point ( $TmpT_{\text{mp}}$ ), the substance has not melted yet:

T_{\text{mp}} \implies \text{Solid}

If the temperature ( $T$ ) is between the melting point ( $TmpT_{\text{mp}}$ ) and the boiling point ( $TbpT_{\text{bp}}$ ), the substance has melted but not yet boiled:

T_{\text{mp}} \le T \le T_{\text{bp}} \implies \text{Liquid}

If the temperature ( $T$ ) is above the boiling point ( $TbpT_{\text{bp}}$ ), the substance has boiled:

T_{\text{bp}} \implies \text{Gas}

Let's look at a worked example of how to apply this logic step-by-step.

Example

Predicting physical states from temperature data

Question: Nitrogen has a melting point of $∘C-210\text{ }^\circ\text{C}$ and a boiling point of $∘C-196\text{ }^\circ\text{C}$ . Predict the physical state of nitrogen at:

$∘C-215\text{ }^\circ\text{C}$
$∘C-190\text{ }^\circ\text{C}$

Step-by-step Solution:

Identify your threshold values: Our benchmarks are the melting point ( $∘CT_{\text{mp}} = -210\text{ }^\circ\text{C}$ ) and the boiling point ( $∘CT_{\text{bp}} = -196\text{ }^\circ\text{C}$ ).
Evaluate the first temperature ( $∘C-215\text{ }^\circ\text{C}$ ): Compare $∘C-215\text{ }^\circ\text{C}$ to the melting point. Because negative numbers can be tricky, remember that $- 215$ is colder than $- 210$ .

-215\text{ }^\circ\text{C} < -210\text{ }^\circ\text{C} \implies T < T_{\text{mp}}

Since the temperature is below the melting point, the nitrogen is a solid.

Evaluate the second temperature ( $∘C-190\text{ }^\circ\text{C}$ ): Compare $∘C-190\text{ }^\circ\text{C}$ to both thresholds. The temperature $- 190$ is warmer than the boiling point of $- 196$ .

-190\text{ }^\circ\text{C} > -196\text{ }^\circ\text{C} \implies T > T_{\text{bp}}

Since the temperature is above the boiling point, the nitrogen is a gas.

Common Mistake

Negative number sign slip-ups

Many students make mistakes when comparing negative temperatures. For example, they might think $∘C-190\text{ }^\circ\text{C}$ is lower than $∘C-196\text{ }^\circ\text{C}$ because $190 < 196$ . Always draw a quick number line in the margin if you are unsure: the further left a number is, the colder (lower) the temperature!

Exam technique

In the exam

Specify arrangement AND movement: When asked to describe a solid, liquid, or gas, always give both its arrangement (regular/random) and its movement (vibrating/sliding/rapid). Missing one of these is a classic way to lose easy marks.
State changes are physical: If a question asks you to classify a change of state, state clearly that it is a physical change because no new substances are made, and it can be easily reversed.
Use the correct state symbols: Always use the lowercase letters in brackets: (s) for solid, (l) for liquid, (g) for gas, and (aq) for aqueous (dissolved in water).

Self review

Check yourself

Can you write down the state symbols for a solid, a liquid, a gas, and a substance dissolved in water?
Explain what happens to the kinetic energy and arrangement of particles when steam condenses into liquid water.
A substance has a melting point of $∘C44\text{ }^\circ\text{C}$ and a boiling point of $∘C280\text{ }^\circ\text{C}$ . What state is it in at room temperature ( $∘C25\text{ }^\circ\text{C}$ )?

What you'll learn

How particles are arranged and move in solids, liquids, and gases.
The names of state interconversions and why they are classified as physical changes.
How to explain what happens to particles during changes of state.
How to use melting and boiling point data to predict the state of any substance at a given temperature.

1. The Three States of Matter

The differences between solids, liquids, and gases come down to three things:

Arrangement of the particles.
Movement of the particles.
Relative energy of the particles.

The arrangement and energy of particles in solids, liquids, and gases

Solids

In a solid, the forces of attraction between particles are very strong. This holds them closely together in a fixed, neat arrangement.

Arrangement: Regular pattern (often called a lattice structure) where particles are tightly packed and touching.
Movement: Particles cannot move from place to place; they can only vibrate about their fixed positions.
Relative Energy: Lowest energy of the three states.

Liquids

In a liquid, the forces of attraction are slightly weaker than in a solid, allowing the particles some freedom.

Arrangement: Randomly arranged but still close together, with most particles touching.
Movement: Particles can slide and move past one another. This is why liquids can flow and take the shape of the bottom of their container.
Relative Energy: Medium energy (higher than solids, lower than gases).

Gases

In a gas, the forces of attraction between particles are extremely weak, almost negligible.

Arrangement: Randomly arranged and spaced very far apart from each other.
Movement: Particles move rapidly and randomly in all directions, colliding with each other and the walls of their container.
Relative Energy: Highest energy of the three states.

Definition

Lattice

A regular, repeating three-dimensional arrangement of atoms, ions, or molecules in a metal or crystalline solid.

2. State Interconversions

Substances can transition from one state to another when we add or remove thermal (heat) energy. These transitions are called interconversions.

It is crucial to recognise that changing state is a physical change, not a chemical reaction.

Definition

Physical Change

A change in state, shape, or form that does not produce any new chemical substances. The chemical formula of the substance remains exactly the same, and the change is easy to reverse.

The diagram below shows the names of the changes of state between solids, liquids, and gases:

Names of interconversions between solid, liquid, and gas states

What happens to the particles during state changes?

To explain state changes, we must describe what happens to the energy, movement, and arrangement of the particles:

When we heat a substance (adding energy):

Melting (Solid to Liquid): Heating transfers thermal energy to the solid particles, which they store as kinetic energy. The particles vibrate more and more rapidly. Eventually, at the melting point, they gain enough energy to partially overcome the attractive forces holding them in their fixed lattice. The regular arrangement breaks down, and the particles begin to slide past one another.
Boiling (Liquid to Gas): Heating a liquid increases the kinetic energy of its particles, making them move faster. At the boiling point, the particles have gained enough energy to completely overcome the remaining forces of attraction between them. They break free from one another, spread far apart, and fly off as a gas.

Key Idea

Forces of attraction do not break down inside molecules!

When we cool a substance (removing energy):

Condensing (Gas to Liquid): As a gas cools, its particles lose kinetic energy and slow down. When they bump into each other, they don't have enough energy to bounce apart. The forces of attraction pull them together, forming a liquid.
Freezing (Liquid to Solid): As a liquid cools further, the particles lose even more kinetic energy and move slower. At the freezing point, the particles move slowly enough that the strong forces of attraction lock them into fixed, regular positions. They can now only vibrate.

Tip

Evaporating vs. Boiling

3. Predicting States of Matter from Data

In your GCSE chemistry exams, you will often be given data about the melting point and boiling point of a substance and asked to predict what state it will be in at a specific temperature.

To solve these questions, keep these mathematical rules in mind:

If the given temperature ( $T$ ) is below the melting point ( $TmpT_{\text{mp}}$ ), the substance has not melted yet:

T_{\text{mp}} \implies \text{Solid}

If the temperature ( $T$ ) is between the melting point ( $TmpT_{\text{mp}}$ ) and the boiling point ( $TbpT_{\text{bp}}$ ), the substance has melted but not yet boiled:

T_{\text{mp}} \le T \le T_{\text{bp}} \implies \text{Liquid}

If the temperature ( $T$ ) is above the boiling point ( $TbpT_{\text{bp}}$ ), the substance has boiled:

T_{\text{bp}} \implies \text{Gas}

Let's look at a worked example of how to apply this logic step-by-step.

Example

Predicting physical states from temperature data

Question: Nitrogen has a melting point of $∘C-210\text{ }^\circ\text{C}$ and a boiling point of $∘C-196\text{ }^\circ\text{C}$ . Predict the physical state of nitrogen at:

$∘C-215\text{ }^\circ\text{C}$
$∘C-190\text{ }^\circ\text{C}$

Step-by-step Solution:

Identify your threshold values: Our benchmarks are the melting point ( $∘CT_{\text{mp}} = -210\text{ }^\circ\text{C}$ ) and the boiling point ( $∘CT_{\text{bp}} = -196\text{ }^\circ\text{C}$ ).
Evaluate the first temperature ( $∘C-215\text{ }^\circ\text{C}$ ): Compare $∘C-215\text{ }^\circ\text{C}$ to the melting point. Because negative numbers can be tricky, remember that $- 215$ is colder than $- 210$ .

-215\text{ }^\circ\text{C} < -210\text{ }^\circ\text{C} \implies T < T_{\text{mp}}

Since the temperature is below the melting point, the nitrogen is a solid.

Evaluate the second temperature ( $∘C-190\text{ }^\circ\text{C}$ ): Compare $∘C-190\text{ }^\circ\text{C}$ to both thresholds. The temperature $- 190$ is warmer than the boiling point of $- 196$ .

-190\text{ }^\circ\text{C} > -196\text{ }^\circ\text{C} \implies T > T_{\text{bp}}

Since the temperature is above the boiling point, the nitrogen is a gas.

Common Mistake

Negative number sign slip-ups

Exam technique

In the exam

Specify arrangement AND movement: When asked to describe a solid, liquid, or gas, always give both its arrangement (regular/random) and its movement (vibrating/sliding/rapid). Missing one of these is a classic way to lose easy marks.
State changes are physical: If a question asks you to classify a change of state, state clearly that it is a physical change because no new substances are made, and it can be easily reversed.
Use the correct state symbols: Always use the lowercase letters in brackets: (s) for solid, (l) for liquid, (g) for gas, and (aq) for aqueous (dissolved in water).

Self review

Check yourself

Can you write down the state symbols for a solid, a liquid, a gas, and a substance dissolved in water?
Explain what happens to the kinetic energy and arrangement of particles when steam condenses into liquid water.
A substance has a melting point of $∘C44\text{ }^\circ\text{C}$ and a boiling point of $∘C280\text{ }^\circ\text{C}$ . What state is it in at room temperature ( $∘C25\text{ }^\circ\text{C}$ )?

What you'll learn

1. The Three States of Matter

Solids

Liquids

Gases

2. State Interconversions

What happens to the particles during state changes?

When we heat a substance (adding energy):

When we cool a substance (removing energy):

3. Predicting States of Matter from Data

Recap questions

Review States of matter by teaching Genie

Recap your knowledge with an interactive lesson

Remember key concepts with flashcards

States of matter

What you'll learn

1. The Three States of Matter

Solids

Liquids

Gases

2. State Interconversions

What happens to the particles during state changes?

When we heat a substance (adding energy):

When we cool a substance (removing energy):

3. Predicting States of Matter from Data

Recap questions

Review States of matter by teaching Genie

Recap your knowledge with an interactive lesson

Remember key concepts with flashcards

States of matter

What you'll learn

1. The Three States of Matter

Solids

Liquids

Gases

2. State Interconversions

What happens to the particles during state changes?

When we heat a substance (adding energy):

When we cool a substance (removing energy):

3. Predicting States of Matter from Data

Recap questions

Review States of matter by teaching Genie

Recap your knowledge with an interactive lesson

Remember key concepts with flashcards

States of matter Revision Guide

States of matter

What you'll learn

1. The Three States of Matter

Solids

Liquids

Gases

2. State Interconversions

What happens to the particles during state changes?

When we heat a substance (adding energy):

When we cool a substance (removing energy):

3. Predicting States of Matter from Data

Recap questions

Review States of matter by teaching Genie

Recap your knowledge with an interactive lesson

Remember key concepts with flashcards

States of matter Revision Guide