Changes of State in Chemistry – Melting, Freezing, Boiling and Condensation | O Level Chemistry (5070)
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In O Level Chemistry, students must understand how substances change from one state of matter to another. These changes are called changes of state and include processes such as melting, freezing, boiling, evaporation, and condensation.
Changes of state occur because of changes in temperature and energy of particles. When energy is added or removed, the movement of particles changes, causing the substance to change its physical state.
Understanding these processes helps explain many everyday phenomena such as ice melting, water boiling, and steam forming on cold surfaces.
What Are Changes of State?
A change of state is the process in which a substance changes from one physical form to another without changing its chemical composition.
The three main states of matter are:
• Solid
• Liquid
• Gas
When temperature changes, substances can move between these states.
Melting
Melting is the process where a solid changes into a liquid when heat is added.
When a solid is heated:
• Particles gain kinetic energy
• Particles vibrate more strongly
• Forces holding particles together become weaker
Eventually, the particles break free from their fixed positions and begin to move around each other, forming a liquid.
Example:
Ice melting to form water.
Freezing
Freezing is the process where a liquid changes into a solid when heat is removed.
During freezing:
• Particles lose energy
• Particle movement slows down
• Particles become arranged in a fixed structure
This causes the liquid to become a solid.
Example:
Water freezing to form ice.
Boiling
Boiling occurs when a liquid changes into a gas throughout the liquid.
When a liquid is heated:
• Particles gain energy
• Particle movement becomes faster
• Particles escape from the liquid and form gas bubbles
Boiling occurs at a fixed temperature known as the boiling point.
Example:
Water boiling to produce steam.
Evaporation
Evaporation is the process where a liquid changes into gas at the surface of the liquid.
Unlike boiling:
• Evaporation can occur at any temperature
• It happens slowly
• Only particles at the surface escape
Example:
Wet clothes drying in the sun.
Condensation
Condensation occurs when a gas changes into a liquid due to cooling.
During condensation:
• Gas particles lose energy
• Particle movement slows down
• Particles move closer together to form a liquid
Example:
Water droplets forming on a cold glass.
Energy Changes During Changes of State
Changes of state involve energy transfer.
When heat is added:
• Melting occurs
• Boiling occurs
• Evaporation occurs
When heat is removed:
• Freezing occurs
• Condensation occurs
These processes involve physical changes, meaning the substance itself does not change chemically.
Why Changes of State Are Important
Understanding changes of state helps explain:
• The water cycle
• Industrial processes involving heating and cooling
• Behaviour of gases and liquids
• Many laboratory experiments
It also helps students understand the kinetic particle theory, which explains how particles behave in different states of matter.
Quick Revision Summary
• Melting: Solid → Liquid
• Freezing: Liquid → Solid
• Boiling: Liquid → Gas
• Evaporation: Liquid → Gas at the surface
• Condensation: Gas → Liquid
Changes of state occur because of changes in particle energy and movement.
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