Particle Model of Matter & States

The idea that all matter is made of tiny particles, and their spacing, motion, and energy explain the properties and behaviour of solids, liquids, and gases

Particle model of matter

Particles closely packed in fixed positions, vibrating about mean positions; lowest energy of the three states

Solid (particle arrangement)

Particles close together but disordered, able to slide past one another; moderate energy, enough to overcome some attractive forces

Liquid (particle arrangement)

Particles far apart, moving randomly at high speed; highest kinetic energy of the three states

Gas (particle arrangement)

During melting or boiling, temperature stays constant because the energy supplied is used to change the particle arrangement (overcome attractive forces) rather than raise average kinetic energy

Melting/boiling and temperature

Expansion or contraction of a substance is due to a change in the average spacing between particles, not a change in the size of the particles themselves

Common trap: particle size vs spacing

Indicates temperature and average kinetic energy are increasing (while heating) or decreasing (while cooling); the substance stays in the same state

Heating curve - sloping section

Indicates a change of state is occurring at constant temperature; energy is being used to overcome or release attractive forces between particles rather than change their average speed

Heating curve - flat section

The energy absorbed or released during a change of state at constant temperature, used to overcome (or resulting from forming) attractive forces between particles rather than changing temperature

Latent heat

Mass per unit volume of a substance, calculated as density = mass / volume

Density (definition)

Mass should be expressed in kilograms (kg) and volume in cubic metres (m3) to obtain density in kg per cubic metre (kg m-3); grams and cm3 give density in g per cm3

Density formula units

A common error is converting mass to kilograms but leaving volume in cubic centimetres (or vice versa), or comparing two densities expressed in different units without converting them to match

Unit conversion trap for density

1 cm3 is equal to 1 x 10^-6 m3, a conversion needed when converting density from g/cm3-based calculations to kg/m3

Volume conversion (cm3 to m3)

Found using displacement: the volume of the solid equals the rise in liquid level in a measuring cylinder when the solid is fully submerged, combined with its mass measured on an electronic balance

Density of irregular solids (method)

Solids show only a small expansion on heating; this is exploited in bimetallic strips, where two different metals expanding at different rates cause the strip to bend

Thermal expansion in solids

Liquids expand more than solids when heated; this property is used in mercury thermometers, which rely on volume expansion of the liquid to indicate temperature

Thermal expansion in liquids

Gases show the greatest expansion of the three states; heating a sealed gas increases its pressure because particles collide with the container walls more frequently and forcefully

Thermal expansion in gases

Water reaches its maximum density at 4 degrees Celsius; between 0 and 4 degrees Celsius it expands as hydrogen bonds rearrange, a behaviour important for aquatic ecosystems

Anomalous expansion of water

The particles move faster and collide with the container walls more often and with greater force; the number of gas particles does not increase

Why a heated sealed gas has higher pressure

For most substances, heating increases volume while mass stays the same, so density decreases

Effect of heating on density (general)

Saying particles themselves expand is incorrect; it is the average spacing between particles that increases when a substance expands on heating

Misconception: 'particles expand'

Incorrect explanation for constant temperature during melting/boiling; energy is still being supplied, but it changes particle arrangement instead of average particle speed

Sentence trap: 'no heat is supplied' during phase change

A strip made of two different metals bonded together that bends when heated, because the two metals expand at different rates due to thermal expansion

Bimetallic strip