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F3.2 · Explain states of matter using interparticle forces

Learn to explain states of matter using interparticle forces through clear examples and targeted practice.

Ontario Grade 11 Chemistry

Gases and Atmospheric Chemistry

Ontario Grade 11 Chemistry — study topic F3.2

A solid keeps its shape, while a liquid flows and takes the shape of its container. A gas spreads out to fill its container. These visible differences arise from how particles are arranged and move, and from how strongly they attract one another. This lesson uses a particle model to explain those differences. An interparticle force is an attraction between separate particles, such as atoms, molecules, or ions.

What you will learn

Describe how particles are arranged and move in solids, liquids, and gases.
Explain how attractions between particles help account for each state.
Use particle attractions to compare the states of familiar substances.

1. A particle-model bridge

Matter is made of particles. Depending on the substance, those particles may be atoms, molecules, or ions. A molecule is a group of atoms joined together. An ion is a charged particle. For this lesson, think of particles as tiny units that are always moving.

Particles can attract other particles. These attractions are called interparticle forces. They do not mean that the particles have become one particle. The attractions help keep particles near one another, while particle motion allows them to move.

The state a substance is in depends in part on the balance between the attractions among its particles and their motion. Stronger attractions make it harder for particles to separate. The particle model explains why solids, liquids, and gases behave differently.

Interparticle forces are attractions between separate particles.
The particles in a sample may be atoms, molecules, or ions.
State depends on particle arrangement and motion, as well as the attractions between particles.

2. Three states in the particle model

In a solid, particles are packed close together and remain near fixed positions. They can move around those positions, but they do not move freely past one another. Attractions hold the particles in an orderly arrangement. This helps a solid keep its own shape and volume.

In a liquid, particles are also close together. The attractions keep them near one another, but particles can move past neighbouring particles. A liquid therefore keeps much of its volume but flows to fit the shape of its container.

In a gas, particles are far apart compared with particles in a solid or liquid. They move freely through the available space. The attractions have little effect on the overall arrangement because particles are usually far from one another. A gas spreads to fill its container.

A state describes the form of a sample under particular conditions. The particle model does not mean that every solid, liquid, or gas has exactly the same particle arrangement. It gives a useful general explanation for their observable properties.

solid: particles close and fixed liquid: particles close and mobile gas: particles far apart and mobile

Solids keep their shape because particles stay near fixed positions.
Liquids flow because their close particles can move past one another.
Gases fill their container because particles move freely and are far apart.

3. Comparing attractions between particles

The kind and strength of attraction depend on the particles in a substance. In molecular substances, the attractions act between molecules. For example, water molecules attract one another strongly because of hydrogen bonding. Hydrogen bonding is a strong type of attraction between certain molecules that contain hydrogen bonded to oxygen, nitrogen, or fluorine.

Non-polar molecules, such as carbon dioxide molecules, attract one another through weaker attractions called dispersion forces. Dispersion forces are attractions that occur between particles, including non-polar molecules. The particles in a gas still attract one another, but the particles are far apart, so those attractions have little effect on their overall motion.

Ionic solids are made of positive and negative ions. Attractions between oppositely charged ions hold the particles in a solid arrangement. This is an interparticle attraction, but it is different from the attractions between molecules. The particle model can describe both kinds of substances without treating an ion as a molecule.

When comparing substances, consider what their particles are and how they attract. If attractions hold particles close together and limit their movement past one another, the material can be a solid or liquid. If particles are far apart and move freely, the material is a gas. The type of particles and their attractions help explain why different substances can be in different states under similar conditions.

Hydrogen bonding and dispersion forces are examples of attractions between molecules.
Oppositely charged ions attract in ionic solids.
Stronger attractions tend to keep particles close and make separation more difficult.

4. Applying the model carefully

Use evidence you can observe first: Does the sample keep its shape, flow, or spread to fill a container? Then connect that behaviour to a particle arrangement and movement. Finally, consider what attracts the particles. This order helps prevent a common mistake: naming a force without explaining how it relates to the state.

For example, water is liquid and carbon dioxide is a gas at room conditions. Water particles attract one another through hydrogen bonding. Carbon dioxide molecules are non-polar and have weaker dispersion attractions. The difference in attractions helps explain why the two substances have different states under those conditions.

This comparison is an explanation, not a rule that a particular force always means a particular state. The state depends on the substance and the conditions. Use the particle model to connect the strength of attractions with the observed arrangement and movement, rather than deciding a state from the name of a force alone.

Start with observable behaviour, then describe particles and their attractions.
Compare substances only under stated or shared conditions.
A force type helps explain a state but does not, by itself, label the state.

Particle model at a glance

State	Particle spacing and movement	How attractions relate to behaviour
Solid	Close together; stay near fixed positions	Attractions keep particles in an arrangement that holds its shape.
Liquid	Close together; move past one another	Attractions keep particles near one another while allowing flow.
Gas	Far apart; move freely through the container	Attractions have little effect on the particles’ overall arrangement.

Worked example

Explaining two different states

At room conditions, water is a liquid and carbon dioxide is a gas. Use particle attractions to explain the difference.

Identify the particles
Water and carbon dioxide are molecular substances, so compare the attractions between their molecules.
Compare attractions
Water molecules form hydrogen bonds with one another. Carbon dioxide molecules are non-polar, so their molecules attract mainly through weaker dispersion forces.
Connect attractions to behaviour
The stronger attractions between water molecules help keep them close together while still allowing them to move past one another. This fits the behaviour of a liquid. The weaker attractions between carbon dioxide molecules have less effect on their motion, so the molecules remain far apart and move freely. This fits the behaviour of a gas.

Answer: Hydrogen bonding helps keep water molecules close together, so water is liquid at room conditions. Weaker dispersion attractions between carbon dioxide molecules are consistent with carbon dioxide being a gas under the same conditions.

Check: The explanation connects the observed states to both the type of particle attraction and the particles’ spacing and movement. It does not claim that attraction type alone determines state under every condition.

Common mistakes and how to avoid them

Saying that particles in a solid do not move.

Correction: Particles in a solid move around fixed positions; they do not freely pass one another.

Saying that there are no attractions between gas particles.

Correction: Gas particles still attract one another. Their large separations mean the attractions have little effect on their overall arrangement.

Treating ions in an ionic solid as molecules.

Correction: Ionic solids contain positive and negative ions. Describe the attractions between oppositely charged ions, not attractions between molecules.

Claiming that one named force always means a substance must be a particular state.

Correction: Use the force to explain particle spacing and movement under the stated conditions. Do not use force type alone to label the state.

Lesson summary

Solids, liquids, and gases differ in particle arrangement and movement.
Interparticle forces are attractions between separate particles.
Attractions help explain why particles stay close in solids and liquids, while gas particles are far apart and move freely.
Identify the particles, compare their attractions, and link those attractions to observable behaviour under the conditions given.

Check your understanding

Question 1

Which description best fits particles in a liquid?

They are close together and can move past one another.
They are far apart and fill the container by moving freely.
They remain at fixed positions and cannot move.
They have no attractions to other particles.

Show answer and explanation

They are close together and can move past one another.

Liquid particles stay close together but can move past one another, which allows a liquid to flow.

Question 2

Why can a gas spread to fill its container?

Its particles are far apart and move freely.
Its particles stay near fixed positions.
Its particles are locked into a close arrangement.
Its particles have no motion.

Show answer and explanation

Its particles are far apart and move freely.

Gas particles move freely through the available space and are far apart, so the gas spreads through its container.

Question 3

At room conditions, water is liquid and carbon dioxide is gas. Which comparison supports this observation?

Water molecules form hydrogen bonds, while carbon dioxide molecules have weaker dispersion attractions.
Water molecules have no attractions, while carbon dioxide molecules are held in a solid arrangement.
Both substances have identical attractions, so their states cannot differ.
Carbon dioxide molecules form hydrogen bonds, while water molecules have only dispersion attractions.

Show answer and explanation

Water molecules form hydrogen bonds, while carbon dioxide molecules have weaker dispersion attractions.

Water molecules form hydrogen bonds. Non-polar carbon dioxide molecules attract mainly through weaker dispersion forces. This comparison helps explain their different states at room conditions.

Key terms

Interparticle force: An attraction between separate particles, such as atoms, molecules, or ions.
Molecule: A group of atoms joined together.
Ion: A charged particle.
Hydrogen bonding: A strong type of attraction between certain molecules containing hydrogen bonded to oxygen, nitrogen, or fluorine.
Dispersion force: An attraction between particles, including non-polar molecules.

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Published by DoAssignment. This AI-assisted lesson follows Ontario Grade 11 Chemistry (SCH3U), expectation F3.2. It is a study resource, not an official curriculum publication.

Before publication, the draft is checked for structure, mathematical or chemical notation, calculations, course boundaries, and readability, and then requires administrator approval. Errors can still occur, so corrections are welcomed.

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