SOLID → GAS (directly): SUBLIMATION (energy supplied)
GAS → SOLID (directly): DEPOSITION (energy removed)
MASS IS CONSERVED:
When a substance changes state, no particles are created or destroyed — they simply rearrange.
The total mass before = total mass after.
Example: 100 g of ice melts to give exactly 100 g of liquid water.
Physical vs Chemical Changes
Changes of state are PHYSICAL CHANGES — NOT chemical changes.
Key difference:
PHYSICAL change: the substance keeps its chemical identity — REVERSIBLE.
CHEMICAL change: new substances are formed — usually irreversible.
Why changes of state are physical:
No new chemical bonds are formed or broken between different types of molecule.
Water freezing: H₂O molecules are still H₂O — just rearranged in a lattice.
Water evaporating: H₂O molecules still exist as individual molecules in the gas.
The change can be REVERSED:
Ice melts → water → refreezes → ice again. Same substance throughout.
Contrast with chemical change:
Burning wood: wood + oxygen → CO₂ + water. Original material cannot be recovered.
Particle Explanation of Changes of State
SOLID:
Particles in fixed positions in a regular lattice.
Vibrate in place — cannot flow.
Strong intermolecular forces hold them together.
LIQUID:
Particles close together but able to move past each other.
No fixed arrangement — can flow and take the shape of the container.
Weaker intermolecular forces than solid.
GAS:
Particles far apart — mostly empty space.
Move rapidly in all directions — random motion.
Very weak intermolecular forces (effectively none).
Fill any container.
DURING MELTING:
Energy supplied → particles vibrate more → forces between particles overcome → lattice breaks down → particles begin to flow.
DURING BOILING:
Energy supplied → particles gain enough energy to completely overcome intermolecular forces → escape from liquid surface into gas phase.
⚠️ Common Mistake
Changes of state are PHYSICAL changes — not chemical. The substance remains the same chemical compound throughout. Mass is ALWAYS conserved in a change of state — the number of particles doesn't change, only their arrangement.
📌 Key Note
Solid → liquid: melting. Liquid → gas: boiling/evaporation. Reverse: freezing, condensation. Sublimation: solid → gas directly. All are physical changes — reversible, mass conserved. Particles rearrange but are not created or destroyed.
🎯 Matching Activity — Change of State Match
Match each change of state to its name and direction. — drag the symbols on the right to match the component names on the left.
Melting
Drop here
Freezing
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Evaporation/Boiling
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Condensation
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Sublimation
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Solid → gas directly — e.g. dry ice (solid CO₂) at room temperature
Solid → liquid — energy supplied, particles gain enough energy to break from lattice
Gas → liquid — energy removed, particles slow and intermolecular forces pull them together
Liquid → gas — energy supplied, particles escape intermolecular forces
Liquid → solid — energy removed, particles slow down and form fixed lattice
🎯 Test Yourself
Question 1 of 2
1. 200 g of water is placed in a freezer and completely freezes. What is the mass of the ice formed?
2. Why is melting a physical change rather than a chemical change?
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