Solids, Liquids and Gases

What you'll learn

This topic examines the properties and behaviour of solids, liquids and gases using the kinetic particle model. You'll need to explain how particle arrangement and movement determine the characteristics of each state, perform density calculations, understand gas pressure and temperature relationships, and describe Brownian motion as evidence for particle theory.

Key terms and definitions

Density — mass per unit volume of a substance, measured in kg/m³ or g/cm³

Brownian motion — the random movement of particles suspended in a fluid (liquid or gas), caused by collisions with fast-moving molecules in the fluid

Diffusion — the net movement of particles from a region of higher concentration to a region of lower concentration, resulting from random particle motion

Absolute zero — the lowest possible temperature (0 K or -273°C), at which particles have minimum kinetic energy

Pressure — the force acting per unit area, measured in pascals (Pa) or N/m²

Kinetic theory — the model explaining how all matter consists of particles in constant random motion, with the amount of kinetic energy determining temperature

Kelvin scale — the absolute temperature scale starting at absolute zero, where temperature in kelvin = temperature in °C + 273

Core concepts

Particle arrangement in solids, liquids and gases

The three states of matter differ in particle arrangement, spacing and movement:

Solids:

• Particles arranged in a regular, fixed pattern (lattice structure)
• Particles very close together, touching neighbours
• Particles vibrate about fixed positions but cannot move from place to place
• Strong forces of attraction between particles hold them in place
• Fixed shape and volume
• High density (particles tightly packed)
• Cannot be compressed

Liquids:

• Particles have no fixed arrangement (random)
• Particles close together, mostly touching
• Particles can move past each other and flow
• Moderate forces of attraction between particles
• No fixed shape (take shape of container) but fixed volume
• High density (particles still close)
• Very difficult to compress

Gases:

• Particles have no fixed arrangement (random)
• Particles far apart, with large spaces between them
• Particles move rapidly in all directions
• Very weak forces of attraction between particles (negligible)
• No fixed shape or volume (fill any container)
• Low density (particles widely separated)
• Can be compressed easily

Density calculations and measurements

Density is calculated using the formula:

density = mass ÷ volume

ρ = m/V

Where:

• ρ (rho) = density in kg/m³ or g/cm³
• m = mass in kg or g
• V = volume in m³ or cm³

Unit conversions:

• 1 g/cm³ = 1000 kg/m³
• 1 cm³ = 1 × 10⁻⁶ m³
• 1 m³ = 1 × 10⁶ cm³

Measuring density of regular solids:

1. Measure mass using a balance
2. Measure dimensions using a ruler or caliper
3. Calculate volume using appropriate formula (e.g., V = l × w × h for cuboid)
4. Calculate density using ρ = m/V

Measuring density of irregular solids:

1. Measure mass using a balance
2. Fill displacement can with water to spout level
3. Lower object into water and collect displaced water
4. Measure volume of displaced water using measuring cylinder
5. Volume of object = volume of water displaced
6. Calculate density using ρ = m/V

Measuring density of liquids:

1. Measure mass of empty measuring cylinder
2. Pour liquid into cylinder and record volume
3. Measure mass of cylinder with liquid
4. Mass of liquid = total mass - mass of cylinder
5. Calculate density using ρ = m/V

Diffusion and Brownian motion

Diffusion occurs in both liquids and gases:

• Particles move from high to low concentration through random motion
• No energy input required (particles already moving)
• Faster at higher temperatures (particles have more kinetic energy)
• Faster in gases than liquids (particles move faster and have more space)
• Lighter particles diffuse faster than heavier particles

Common examples:

• Perfume spreading across a room
• Food colouring spreading through water
• Ammonia and hydrogen chloride vapours forming ammonium chloride ring in a tube (ammonia travels further as lighter molecules)

Brownian motion provides evidence for kinetic theory:

• Robert Brown (1827) observed pollen grains moving randomly in water under microscope
• Smoke particles in air show similar random motion under illumination
• Motion caused by invisible air/water molecules colliding with visible particles
• Collisions from all sides are unequal, causing random zigzag movement
• Demonstrates that molecules are constantly moving randomly
• Motion increases at higher temperatures (molecules move faster)

Observing Brownian motion:

• Use smoke cell apparatus with glass cell containing smoke particles
• Illuminate with bright light from the side
• Observe through microscope
• See bright specks (smoke particles) moving randomly
• Cannot see air molecules (too small), only their effect on larger smoke particles

Changes of state and energy

Changes between states require energy transfer but temperature remains constant during the change:

Melting and freezing:

• Melting: solid → liquid (energy absorbed to overcome forces between particles)
• Freezing: liquid → solid (energy released as forces form between particles)
• Occurs at melting/freezing point

Boiling and condensing:

• Boiling: liquid → gas (energy absorbed to completely overcome forces)
• Condensing: gas → liquid (energy released)
• Boiling occurs at boiling point throughout the liquid

Evaporation:

• Occurs at liquid surface at any temperature below boiling point
• Faster-moving particles escape from surface
• Remaining liquid cools (lower average kinetic energy)
• Evaporation rate increases with: higher temperature, larger surface area, moving air (wind), lower humidity

Energy during state changes:

• Temperature constant during change (energy breaks/forms bonds, not increasing kinetic energy)
• All energy goes into changing particle arrangement and separation
• Once change complete, temperature rises/falls again

Gas pressure and the kinetic model

Gas pressure results from particle collisions:

• Gas particles move randomly in all directions at high speed
• Particles collide with container walls
• Each collision exerts a tiny force on the wall
• Pressure = total force from all collisions per unit area
• More frequent or harder collisions → higher pressure

Effect of temperature on gas pressure (constant volume):

• Increasing temperature → particles gain kinetic energy
• Particles move faster
• Collisions more frequent and harder
• Pressure increases
• Relationship: pressure ∝ absolute temperature (in kelvin)
• P₁/T₁ = P₂/T₂ (for fixed mass and volume)

Effect of volume on gas pressure (constant temperature):

• Decreasing volume → particles have less space
• Same number of particles in smaller volume
• Collisions more frequent (particles hit walls more often)
• Pressure increases
• Relationship: pressure ∝ 1/volume
• P₁V₁ = P₂V₂ (Boyle's law, for fixed mass and temperature)

The Kelvin temperature scale:

• Absolute temperature scale starting at absolute zero
• 0 K = -273°C (absolute zero)
• 273 K = 0°C
• Conversion: T(K) = T(°C) + 273
• At absolute zero, particles have minimum kinetic energy (still vibrate)
• Cannot reach temperatures below absolute zero
• Gas laws only work with kelvin temperatures

Increasing gas temperature at constant volume

When a fixed mass of gas is heated in a sealed container (constant volume):

1. Particles absorb thermal energy
2. Average kinetic energy of particles increases
3. Average particle speed increases
4. Particles collide with walls more frequently
5. Each collision transfers more momentum (particles moving faster)
6. Both frequency and force of collisions increase
7. Pressure increases

The mathematical relationship:

• Pressure directly proportional to absolute temperature
• P/T = constant (when volume and mass are constant)
• Doubling absolute temperature doubles pressure
• Must use kelvin scale for calculations

Worked examples

Example 1: Density calculation

Question: A rectangular metal block measures 5.0 cm × 2.0 cm × 3.0 cm and has a mass of 240 g. Calculate the density of the metal in both g/cm³ and kg/m³. [4 marks]

Solution:

Volume = length × width × height V = 5.0 × 2.0 × 3.0 = 30 cm³ [1 mark]

Density = mass ÷ volume ρ = 240 ÷ 30 = 8.0 g/cm³ [1 mark]

Convert to kg/m³: 8.0 g/cm³ × 1000 = 8000 kg/m³ [1 mark]

(Alternative: convert mass to kg: 240 g = 0.24 kg convert volume to m³: 30 cm³ = 30 × 10⁻⁶ m³ = 3.0 × 10⁻⁵ m³ ρ = 0.24 ÷ (3.0 × 10⁻⁵) = 8000 kg/m³) [1 mark for correct method]

Example 2: Gas pressure and temperature

Question: A sealed container holds gas at a pressure of 100 kPa at a temperature of 27°C. The container is heated to 127°C. Calculate the new pressure, assuming the volume remains constant. [3 marks]

Solution:

Convert temperatures to kelvin: T₁ = 27 + 273 = 300 K T₂ = 127 + 273 = 400 K [1 mark]

Use P₁/T₁ = P₂/T₂ 100/300 = P₂/400 [1 mark]

P₂ = (100 × 400)/300 = 133 kPa (or 133.3 kPa) [1 mark]

Example 3: Explaining Brownian motion

Question: Describe what is observed when viewing smoke particles in air under a microscope, and explain this observation in terms of particle motion. [4 marks]

Solution:

Observation: Bright specks of light (smoke particles) are seen moving randomly in a zigzag pattern [1 mark]

Explanation:

• Air molecules are in constant random motion [1 mark]
• Air molecules (too small to see) collide with smoke particles [1 mark]
• Unequal bombardment from different directions causes the smoke particles to change direction randomly [1 mark]

Common mistakes and how to avoid them

• Confusing evaporation and boiling: Evaporation occurs at the surface at any temperature; boiling occurs throughout the liquid at a specific boiling point. Don't say liquids "evaporate" at their boiling point — they boil.

• Wrong temperature scale in gas calculations: Always convert to kelvin (add 273) before using gas law equations. Using Celsius gives incorrect answers. Remember: absolute temperature only.

• Stating particles "stop moving" at absolute zero: Particles have minimum kinetic energy at 0 K but still vibrate. They never become completely stationary.

• Forgetting units or using wrong units: Density can be g/cm³ or kg/m³ — convert correctly (×1000). Pressure in Pa or kPa. Always show units in calculations and final answers.

• Explaining Brownian motion incorrectly: The visible particles (smoke/pollen) don't move on their own. Their random motion is caused by invisible molecule collisions. Don't say "particles collide with each other" — specify air/water molecules colliding with larger visible particles.

• Saying temperature increases during state changes: Temperature remains constant during melting, boiling, freezing or condensing. Energy input changes particle arrangement, not kinetic energy (which determines temperature).

Exam technique for "Solids, Liquids and Gases"

• "Explain" questions require cause and effect: Link particle behaviour to observable properties. For gas pressure questions, describe particle motion → collision frequency/force → pressure change. Each linked step typically earns one mark.

• Density calculations need clear working: Show formula, substitution and answer with units. Rearrange ρ = m/V correctly: m = ρV or V = m/ρ. Check units match (don't mix g with kg or cm³ with m³).

• Describe vs Explain command words: "Describe" means state what happens (observations, patterns). "Explain" means give reasons why using particle theory. Read the command word carefully — explanations earn more marks but require more detail.

• Diagram questions on particle arrangement: Draw particles (circles) touching in solids/liquids, widely spaced in gases. Show regular pattern for solids, random arrangement for liquids/gases. Label if asked. Typically 2-3 marks for accurate representation.

Quick revision summary

Solids have particles in fixed positions vibrating in a regular lattice; liquids have particles close but able to move past each other; gases have widely-spaced, fast-moving particles. Density = mass/volume, measured in g/cm³ or kg/m³. Brownian motion (random zigzag movement of smoke particles) provides evidence that molecules move randomly. Gas pressure results from particle collisions with container walls; pressure increases with temperature (P∝T in kelvin) and decreases with volume (PV = constant). State changes occur at constant temperature as energy overcomes particle forces.