All matter exists in one of three states: SOLID, LIQUID or GAS.
SOLID:
Particles are closely packed in a REGULAR ARRANGEMENT (often a lattice).
Particles VIBRATE about fixed positions — they cannot move past each other.
Definite shape and definite volume.
Incompressible.
Example: ice, sodium chloride, iron.
LIQUID:
Particles are closely packed but in a RANDOM arrangement.
Particles can FLOW and move past each other (no fixed positions).
No definite shape — takes the shape of its container.
Definite volume.
Incompressible (like solids).
Example: water, mercury, liquid nitrogen.
GAS:
Particles are widely SPREAD OUT — large distances between them.
Particles move RAPIDLY in all directions — constantly colliding with each other and container walls.
No definite shape or volume — fills any container.
Highly compressible (particles can be pushed closer together).
Example: oxygen, steam, carbon dioxide.
Changes of State
CHANGES OF STATE occur when energy is added or removed:
SOLID → LIQUID: MELTING (energy added — particles gain enough energy to break free from fixed positions)
LIQUID → GAS: BOILING/EVAPORATION (energy added — particles gain enough energy to escape the liquid)
GAS → LIQUID: CONDENSATION (energy removed — particles slow down, intermolecular forces pull them back together)
LIQUID → SOLID: FREEZING/SOLIDIFICATION (energy removed — particles slow down enough to form fixed lattice)
SOLID → GAS: SUBLIMATION (some solids go directly from solid to gas, e.g. iodine, dry ice/CO₂)
These are PHYSICAL CHANGES — the chemical composition does not change. Water is H₂O whether it is ice, liquid water or steam.
MELTING POINT — temperature at which a solid becomes a liquid.
BOILING POINT — temperature at which a liquid becomes a gas.
At these temperatures, the substance exists in BOTH states simultaneously.
State Symbols in Chemical Equations
STATE SYMBOLS are added after formulas in chemical equations to show the physical state of each substance:
(s) — SOLID
(l) — LIQUID
(g) — GAS
(aq) — AQUEOUS SOLUTION (dissolved in water)
Examples:
2H₂(g) + O₂(g) → 2H₂O(l) — hydrogen gas + oxygen gas → liquid water
NaCl(s) → Na⁺(aq) + Cl⁻(aq) — solid NaCl dissolves in water
CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)
State symbols give useful information:
(aq) shows a substance is dissolved in water — important for reactions in solution.
(g) shows a gas is produced — important if testing for hydrogen, CO₂ etc.
They make equations more informative and precise.
⚠️ Common Mistake
Changes of state are PHYSICAL changes — the substance does not change chemically. H₂O is always H₂O whether it is ice, water or steam. (aq) means dissolved in WATER specifically — not just any liquid. A substance in ethanol or acetone would NOT be labelled (aq).
📌 Key Note
Solid: fixed positions, vibrate. Liquid: close but can flow. Gas: widely spread, rapid random motion. Melting (s→l), boiling (l→g), condensing (g→l), freezing (l→s). State symbols: (s) solid, (l) liquid, (g) gas, (aq) aqueous solution.
🎯 Matching Activity — Match the State Change to its Name
Match each change of state to the correct term. — drag the symbols on the right to match the component names on the left.
Melting
Drop here
Boiling / evaporation
Drop here
Condensation
Drop here
Freezing
Drop here
Sublimation
Drop here
Liquid → gas — energy added, particles escape the liquid surface
Gas → liquid — energy removed, particles slow and are pulled together
Solid → gas directly — e.g. iodine, dry ice (CO₂)
Solid → liquid — energy added, particles break free from fixed positions
Liquid → solid — energy removed, particles slow enough to form regular lattice
🎯 Test Yourself
Question 1 of 2
1. What state symbol is used for a substance dissolved in water?
2. Ice melts to form water. Is this a physical or chemical change? Explain.
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