Kinetic Molecular Theory and States of Matter
Quick Notes
- Kinetic Molecular Theory (KMT) explains how particles behave in solids, liquids, and gases.
- Solids (s): Particles vibrate in place, tightly packed — fixed shape and volume.
- Liquids (l): Particles slide past each other — fixed volume, no fixed shape.
- Gases (g): Particles move freely — no fixed shape or volume.
- Aqueous (aq): Substance dissolved in water.
- State symbols:
- (s) = solid
- (l) = liquid
- (g) = gas
- (aq) = aqueous solution
- Changes of state:
- Melting: Solid → Liquid
- Freezing: Liquid → Solid
- Vaporization: Liquid → Gas
- Condensation: Gas → Liquid
- Sublimation: Solid → Gas
- Deposition: Gas → Solid
Full Notes
Understanding how matter behaves starts with a simple idea: particles are always moving.
This is the basis of the Kinetic Molecular Theory, and it helps us explain everything from why ice melts to how steam rises from hot tea.
What Is the Kinetic Molecular Theory (KMT)?
The KMT is a model that helps us understand the behavior of particles in different states of matter:
- All matter is made of tiny particles (atoms, molecules, or ions).
- These particles are in constant motion.
- The temperature of a substance is related to the average kinetic energy of its particles.
- The state of matter depends on the balance between kinetic energy and intermolecular forces.
States of Matter
Solids (s)
- Particles packed closely in a fixed, orderly arrangement.
- Vibrate in place only.
- Strong forces hold them together — fixed shape and volume.
- Not compressible, do not flow.
Liquids (l)
- Particles close but move past one another.
- Take the shape of their container but have fixed volume.
- Weaker forces than solids.
- Slightly compressible.
Gases (g)
- Particles far apart, moving rapidly.
- High kinetic energy, weak forces.
- No fixed shape or volume — expand to fill container.
- Easily compressed.
Aqueous (aq)
- Substance dissolved in water, e.g. NaCl(aq).
State Symbols in Chemical Equations
We use state symbols to indicate the physical state of substances:
- (s) = solid
- (l) = liquid
- (g) = gas
- (aq) = aqueous solution
Example: NaCl dissolving NaCl (s) → Na+ (aq) + Cl− (aq)
Changes of State
Adding or removing energy changes a substance’s state without changing its identity.
These changes are physical — the substance stays the same, just in a different form.
For Example: Water is still H₂O whether it’s ice, liquid water, or steam.
| Change | Description | Direction |
|---|---|---|
| Melting | Solid becomes liquid | Solid → Liquid |
| Freezing | Liquid becomes solid | Liquid → Solid |
| Vaporization | Liquid becomes gas | Liquid → Gas |
| • Evaporation | Slow vaporization at surface | |
| • Boiling | Rapid vaporization throughout liquid | |
| Condensation | Gas becomes liquid | Gas → Liquid |
| Sublimation | Solid turns directly into gas | Solid → Gas |
| Deposition | Gas turns directly into solid | Gas → Solid |
Summary
- KMT explains particle behavior in solids, liquids, gases, and solutions.
- State depends on balance of kinetic energy and intermolecular forces.
- State symbols show physical form in equations.
- Changes of state are physical changes — substance remains the same.
Linked Questions
Why are some substances solid while others are fluid under standard conditions?
The physical state of a substance at standard conditions depends on the strength of the intermolecular or interparticle forces relative to the available thermal (kinetic) energy. Substances with strong forces — such as ionic, metallic, or covalent network solids — remain solid because particles are held tightly in fixed positions. Those with weaker forces, such as molecular substances with only London dispersion or dipole–dipole forces, have more particle mobility, making them liquid or gaseous at room temperature.
Why are some changes of state endothermic and some exothermic?
Changes of state involve breaking or forming intermolecular forces. When energy is absorbed to overcome attractions — as in melting, vaporisation, or sublimation — the process is endothermic. When particles come closer and new forces form — as in condensation, freezing, or deposition — energy is released, making the process exothermic. The direction of energy flow depends on whether the change increases or decreases potential energy within the substance.