S1.1.2: The Kinetic Molecular Theory

Kinetic Molecular Theory (KMT) =

a model that explains the behaviour of matter by theorizing that all substances are made of tiny particles 

Matter is made of

particles (atoms, molecules and ions) 

Particles are

always moving (even in solids) 

Temperature is linked to

average kinetic energy of particles 

The spacing between particles and the strength of intermolecular forces help

determine whether a substance is a solid, liquid or gas. 

Using KMT: 

States of matter are described by arrangement + motion +forces 

Solids 

• Particles close together in a fixed pattern 

• Vibrate in place 

• No flow 

Liquid 

• Particles close together but disordered 

• Can slide past each other 

• • Not fixed   

Gas 

• Particles far apart 

• Partilces move rapidly and randomly 

• • Fill the container 

State of Matter:

A physical form of a substance determined by particle arrangement and energy. Either solid, liquid or gas. 

KMT says particles are

always in constant random motion. 

Higher temperature means

higher average kinetic energy. 

Intermolecular forces (IMFs):

attractions between particles (molecules/ions) that affect melting/boiling and state 

Solid -

(s) 

Liquid -

(l) 

Gas -

(g) 

Aqueous (dissolved in water) -

(aq) 

Change of State

happens when thermal energy is absorbed or released, changing particles arrangement and motion 

Energy is used to

change intermolecular attractions (linked to potential energy) so temperature stays constant while the phase change is happening. 

Thermal Energy

energy transfer due to a temperature change  

Chemically particles stay the

same during changes of state. 

Evaporation happens at

the surface

boiling happened throughout the

whole entire liquid 

When heating a substance 

 temperature usually rises but it plateau during melting/boiling because added energy does not always go into increasing particles motion (temperature)

KMT explains: When not changing a

state, energy that is added increases average kinetic energy so temperature rises 

KMT explains: During a phase change the addded energy increase potential energy by

weakening attractions, so temperature stays the same until the change is complete. 

Endothermic –

absorbs heat from surround 

• Melting 

• Sublimation 

• • Vaporization 

Exothermic –

release heat to surroundings 

• Freezing 

• Deposition 

• • Condenstaiton 

Solid --> Liquid:  

Melting

Liquid --> Gas:  

Vaporization

Gas --> Liquid:  

Condensation

Liquid --> Solid:  

 

Freezing

Gas --> Solid:  

Deposition

Solid --> Gas:  

Sublimation

Gas =

any substances in the gaseous state 

Vapor =

gases form of a substances that is usually a liquid  or solid at room temperature 

Writing Tip 

 

• Explanation: Cause + Effect – Temperature/Particle Motion and Energy 

• According to KMT.