Particle model

"What is the equation for density?

Density = mass ÷ volume. ρ = m ÷ V. Density (ρ) in kg/m³, mass (m) in kg, volume (V) in m³."

"How does the particle model explain why solids are usually denser than gases?

In solids, particles are packed closely together — many particles in a small volume. In gases, particles are spread out with large gaps — few particles in the same volume. More particles per m³ → higher density."

"Describe the particle arrangement in a SOLID.

Particles are closely packed in a regular pattern. They vibrate about fixed positions — they can't move past each other."

"Describe the particle arrangement in a LIQUID.

Particles are still close together but arranged randomly (no regular pattern). They can move past each other — which is why liquids flow."

"Describe the particle arrangement in a GAS.

Particles are far apart, arranged randomly, and move quickly in all directions. They fill any container."

"Required practical 5: what does it investigate?

Determining the densities of regular solids, irregular solids, and liquids. Volume of regular shapes from dimensions (ruler/Vernier/micrometer); volume of irregular shapes by displacement (water in a measuring cylinder)."

"How do you measure the volume of an irregular solid?

Use a DISPLACEMENT method — submerge it in water in a measuring cylinder (or eureka can) and measure the volume of water pushed aside. (You can't use a ruler — the shape is irregular.)"

"Why are changes of state PHYSICAL, not chemical, changes?

Because no new substance is made — the material recovers its ORIGINAL properties if the change is reversed. (E.g. melt ice → water → freeze → ice again, same stuff.)"

"What happens to MASS when a substance changes state?

Mass is CONSERVED — it stays the same. No particles are created or destroyed, just rearranged. (Common trap: students think mass disappears when water boils away, but the steam still has the same mass as the water.)"

"Name the six changes of state.

Melting (solid→liquid), Freezing (liquid→solid), Boiling/Evaporating (liquid→gas), Condensing (gas→liquid), Sublimating (solid→gas directly, e.g. dry ice)."

"What is internal energy?

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

"What does heating a substance do to its internal energy?

Heating INCREASES the internal energy — the particles gain energy. This either: 1) raises the temperature, OR 2) causes a change of state. (Not both at the same time.)"

"What is the equation for specific heat capacity?

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

"Define specific heat capacity.

The amount of energy needed to raise the temperature of 1 kg of a substance by 1°C. Units: J/kg°C."

"What does a HIGH specific heat capacity mean?

The substance needs a LOT of energy to heat up by 1°C — and stores a lot of energy. (E.g. water has a high SHC — that's why it's used in central heating and takes ages to boil.)"

"What is latent heat?

The energy needed for a substance to change state — WITHOUT changing temperature. (The energy goes into breaking the bonds between particles, not into making them move faster.)"

"Define specific latent heat.

The amount of energy needed to change the state of 1 kg of a substance with NO change in temperature. Units: J/kg."

"What is the equation for energy needed for a change of state?

Energy = mass × specific latent heat. E = m × L. E in joules (J), m in kg, L in J/kg."

"What's the difference between specific latent heat of FUSION and VAPORISATION?

Fusion = energy to change SOLID to LIQUID (melting). Vaporisation = energy to change LIQUID to GAS (boiling). Vaporisation needs more energy (gas particles need much more separation)."

"On a heating/cooling graph, why are there FLAT sections?

Those are changes of state. During a change of state, energy goes into breaking particle bonds (latent heat), NOT into raising temperature — so the temperature stays constant while energy is being added."

"What's the difference between specific heat capacity and specific latent heat?

SHC: energy to change the TEMPERATURE of a substance (no state change). SLH: energy to change the STATE of a substance (no temperature change). Both apply to 1 kg."

"How are gas particles moving?

In constant RANDOM motion — in all directions, at a range of speeds."

"What does the TEMPERATURE of a gas represent?

The AVERAGE KINETIC ENERGY of its particles. Higher temperature → particles moving faster on average."

"What happens to gas pressure if temperature increases (at constant volume)?

Pressure INCREASES. Higher temperature → particles move faster → they hit the walls of the container more often AND harder → more force per unit area → higher pressure."

"How does a gas exert pressure on its container?

Gas particles collide with the container walls. Each collision exerts a tiny force at right angles to the wall. Pressure = total force per unit area from all these collisions."

"What is the equation for a gas at constant temperature?

Pressure × volume = constant. pV = constant. Pressure in pascals (Pa), volume in m³. (So p₁V₁ = p₂V₂.)"

"If you DOUBLE the volume of a gas (at constant temperature), what happens to its pressure?

Pressure HALVES. They are inversely proportional — pV stays constant. Bigger volume → particles spread out → fewer collisions with the walls per second."

"Why does squeezing a gas (decreasing its volume) INCREASE its pressure?

Particles are in a smaller space, so they hit the walls of the container MORE OFTEN. Same force per collision but more collisions per second → higher pressure. (Note: this assumes temperature is constant.)"

"What is 'work done' on a gas?

The transfer of energy by a force. When you push a piston in (compress a gas), you do work on the gas — transferring energy to it."

"What happens to a gas when work is done on it (e.g. compressing with a bike pump)?

The INTERNAL ENERGY of the gas increases — which usually causes the TEMPERATURE to rise. (That's why a bike pump gets warm when you pump quickly — you're doing work on the gas and it heats up.)"

"Why does a bicycle pump get warm when you use it?

You're doing WORK on the gas by compressing it. This increases its internal energy, raising its temperature. The warm gas heats the pump walls."