P3: Particle Model

State the equation for density.

ρ = m / V (density in kg/m³ = mass in kg ÷ volume in m³)

Calculate the density of an object with mass 0.5 kg and volume 0.002 m³.

ρ = 0.5 / 0.002 = 250 kg/m³

Why are solids denser than gases?

In solids, particles are closely packed together; in gases, particles are widely spaced so the same mass occupies a much larger volume.

Describe the arrangement and motion of particles in a solid.

Particles are arranged in a regular lattice, close together, and vibrate about fixed positions.

Describe the arrangement and motion of particles in a liquid.

Particles are close together but in a random arrangement; they move around each other.

Describe the arrangement and motion of particles in a gas.

Particles are far apart in a random arrangement; they move rapidly in all directions.

When a substance changes state, what is conserved?

Mass is conserved.

Is a change of state a physical or chemical change? Why?

Physical - the material recovers its original properties if the change is reversed.

Name the six changes of state.

Melting, freezing, boiling/evaporating, condensing, and sublimating (solid directly to gas or vice versa).

RP5: Describe Required Practical 5 - measuring density.

Use a ruler/Vernier callipers/micrometer to measure dimensions of regular objects to find volume, and a displacement technique (eureka can) for irregular objects; measure mass with a balance; calculate ρ = m/V.

What is internal energy?

The total kinetic energy and potential energy of all the particles (atoms and molecules) that make up a system.

What happens to internal energy when a substance is heated?

The energy stored in the system increases - either the temperature rises or a change of state occurs.

State the equation for specific heat capacity.

ΔE = m × c × Δθ (change in thermal energy in J = mass in kg × specific heat capacity in J/kg°C × temperature change in °C)

Define specific heat capacity.

The amount of energy required to raise the temperature of 1 kg of the substance by 1 °C.

Calculate the energy needed to heat 2 kg of water (c = 4200 J/kg°C) by 10 °C.

ΔE = 2 × 4200 × 10 = 84 000 J

Why does temperature stay constant during a change of state?

Energy supplied changes the potential energy of particles (breaking/forming bonds) but not their kinetic energy, so temperature doesn't change.

What is latent heat?

The energy needed for a substance to change state without a change in temperature.

State the equation for specific latent heat.

E = m × L (energy in J = mass in kg × specific latent heat in J/kg)

Define specific latent heat.

The amount of energy required to change the state of 1 kg of a substance with no change in temperature.

What is the specific latent heat of fusion?

The latent heat for a solid-to-liquid (or liquid-to-solid) change of state.

What is the specific latent heat of vaporisation?

The latent heat for a liquid-to-gas (or gas-to-liquid) change of state.

On a heating graph, what does a horizontal section indicate?

A change of state is occurring - temperature is constant as energy goes into breaking bonds.

How do you distinguish between specific heat capacity and specific latent heat?

SHC is for temperature change with no change of state; SLH is for a change of state with no temperature change.

How does temperature affect the pressure of a gas at constant volume?

Increasing temperature increases pressure; gas molecules move faster, hitting the walls more frequently and with more force.

How does the motion of molecules relate to the temperature of a gas?

Temperature is related to the average kinetic energy of the molecules; higher temperature means higher average KE.

State the relationship between pressure and volume of a gas at constant temperature.

p × V = constant (pressure × volume = constant for a fixed mass at constant temperature).

What happens to the pressure of a gas when its volume is increased at constant temperature?

Pressure decreases - molecules hit the walls less frequently.

Calculate the new pressure if a gas at 200 Pa occupies 0.5 m³ and is compressed to 0.25 m³.

p₁V₁ = p₂V₂ → 200 × 0.5 = p₂ × 0.25 → p₂ = 400 Pa

What force does the pressure of a gas produce on a container wall?

A net force at right angles (normal) to the wall.

What happens to the temperature of a gas when work is done on it (e.g. in a bicycle pump)?

Doing work on the gas increases its internal energy, causing its temperature to rise.

Explain why a bicycle pump gets warm when used.

Work is done compressing the gas; this increases internal energy of the gas, raising its temperature; the pump casing conducts this heat.