2.1 Changes of States of Matter

Introduction

Matter can exist in different states — solid, liquid, and gas. When matter changes from one state to another, this is called a state change or phase transition. These changes are part of everyday life: melting butter, boiling water, freezing chocolate, and drying clothes all involve state changes.

This section explains what happens during state changes using kinetic particle theory and the idea of internal energy.

Definition

State Change (Phase Transition): A state change (or phase transition) is the transition of matter from one state to another, such as:

• solid → liquid
• liquid → gas
• solid → gas (directly)

These transitions occur without a temperature change and can be understood through kinetic particle theory and internal energy.

In simple words: A state change is when matter turns from one form to another — like ice melting into water or water boiling into steam.

Main Idea

• Matter changes state when it is heated or cooled.
• During a state change, the temperature remains constant.
• The energy added or removed during a state change is used to change the potential energy of the particles, not their kinetic energy.
• This shift in potential energy causes the overcoming or generation of inter-particle forces.

Core Concept

Internal Energy is composed of two parts:

• Kinetic energy — related to temperature (motion of particles).
• Potential energy — related to particle positions and interactions.

During phase transitions:

• Temperature remains constant → kinetic energy stays the same.
• Potential energy changes significantly → inter-particle forces are overcome or generated.

Simple explanation: When you heat ice, the temperature rises until it reaches 0°C. Then the temperature stops rising even though you keep adding heat. That extra heat is used to break the forces holding the ice particles together, turning ice into water. Only after all the ice has melted does the temperature start rising again.

Detailed Explanation

1. Melting (Solid to Liquid)

What happens: When a solid melts, it becomes a liquid.

According to kinetic particle theory:

• In a solid, particles are tightly packed in a stable, well-ordered state and vibrate in fixed positions.
• When energy (usually heat) is added to the solid, it causes the particles to vibrate more strongly.
• When energy is added to a solid at the melting point, it increases its potential energy.
• This weakens the forces that hold the particles in place without increasing their kinetic energy, so the temperature stays the same.
• When the energy is strong enough to overcome the forces holding the particles in place, the solid melts into a liquid where the particles are in a stable state but can move freely around one another.

Key point: Temperature remains constant during melting because the added energy is used to overcome inter-particle forces, not to increase kinetic energy.

2. Boiling (Liquid to Gas)

What happens: When a liquid boils, it turns into a gas.

According to kinetic particle theory:

• In a liquid, particles are closely packed but can move past each other.
• As energy is added, particles move faster and eventually have enough energy to break free from the liquid's surface into the gas phase.
• The added energy increases the potential energy of the particles, allowing them to overcome the attractive forces holding them in the liquid.
• This occurs at the boiling point, where the temperature remains constant as the liquid turns into gas.

Key point: Temperature remains constant during boiling because the added energy is used to overcome inter-particle forces, not to increase kinetic energy.

3. Freezing (Liquid to Solid)

What happens: When a liquid freezes, it turns into a solid.

According to kinetic particle theory:

• In a liquid, particles have enough energy to move around each other.
• As the liquid loses energy, particle motion slows down, and the forces of attraction pull the particles into a fixed, orderly arrangement characteristic of a solid.
• Energy is released during freezing, decreasing the potential energy of the particles.
• The temperature remains constant during this process as the energy released is used to form the solid structure.

Key point: Temperature remains constant during freezing because the released energy is used to form the solid structure.

4. Condensation (Gas to Liquid)

What happens: When a gas condenses, it turns into a liquid.

According to kinetic particle theory:

• In a gas, particles move away rapidly and are far apart.
• As the gas loses energy, particle motion slows down, and attractive forces pull the particles closer together, forming a liquid.
• Energy is released during condensation, reducing the potential energy of the particles.
• The temperature remains constant as the gas turns into a liquid.

Key point: Temperature remains constant during condensation because the released energy is used to bring particles closer together.

5. Sublimation (Solid to Gas)

What happens: When a solid sublimates, it turns directly into a gas without passing through the liquid phase.

According to kinetic particle theory:

• In a solid, particles are tightly packed in fixed positions.
• Some molecules may have enough energy to break the intermolecular bonds and go directly into the vapour state.
• This happens when the intermolecular forces are relatively weak compared to the strength of the covalent bonds within the molecule.

Examples of substances that undergo sublimation:

• Carbon dioxide (dry ice)
• Camphor
• Naphthalene

Key point: Sublimation is a direct solid-to-gas transition without the liquid phase.

Summary Table

Intermediate Level

During a state change:

• Kinetic energy (temperature) remains constant.
• Potential energy changes significantly.
• The energy added or released is used to overcome or generate inter-particle forces.

Why temperature stays constant:

• The added energy does not increase the speed of the particles.
• Instead, it weakens the forces holding the particles together (or strengthens them, in the reverse process).
• Only after the state change is complete does the temperature start changing again.

Key distinction:

• Temperature = average kinetic energy of particles.
• State change = change in potential energy (arrangement of particles).

Important Points

• - A state change is the transition of matter from one state to another.
• - State changes occur without a temperature change.
• - During state changes, kinetic energy remains constant but potential energy changes significantly.
• - Melting — solid to liquid (energy absorbed).
• - Boiling — liquid to gas (energy absorbed).
• - Freezing — liquid to solid (energy released).
• - Condensation — gas to liquid (energy released).
• - Sublimation — solid to gas (energy absorbed).
• - Examples of sublimation: carbon dioxide, camphor, naphthalene.

Remember

• - State change = phase transition.
• - Temperature constant during state change.
• - Kinetic energy constant; potential energy changes.
• - Melting and boiling absorb energy.
• - Freezing and condensation release energy.
• - Sublimation = solid → gas directly.
• - Examples of sublimation: dry ice, camphor, naphthalene.

Common Mistakes

Quick Revision

A state change (or phase transition) is the transition of matter from one state to another, such as solid to liquid, liquid to gas, or solid to gas directly. These transitions occur without a temperature change and can be understood through kinetic particle theory and internal energy.

Internal energy is composed of:

• Kinetic energy (temperature-related)
• Potential energy (particle positions and interactions)

During phase transitions:

• Temperature remains constant (and hence kinetic energy)
• Potential energy significantly changes
• This shift in potential energy causes the overcoming or generation of inter-particle forces

Types of state changes:

• Melting (Solid → Liquid) — energy absorbed
• Boiling (Liquid → Gas) — energy absorbed
• Freezing (Liquid → Solid) — energy released
• Condensation (Gas → Liquid) — energy released
• Sublimation (Solid → Gas) — energy absorbed

60-Second Revision

• State change = transition between solid, liquid, and gas.
• Temperature remains constant during state changes.
• Kinetic energy constant; potential energy changes.
• Melting → solid to liquid (energy absorbed).
• Boiling → liquid to gas (energy absorbed).
• Freezing → liquid to solid (energy released).
• Condensation → gas to liquid (energy released).
• Sublimation → solid to gas directly (energy absorbed).
• Examples of sublimation: dry ice, camphor, naphthalene.
• ⚠️ Warning: temperature does not change during a state change — the energy goes into changing potential energy.