Particle Theory and States of Matter

Ever wondered why ice is hard but steam floats away? It's all about how particles are arranged and how much energy they have. In solids, particles are packed tightly in fixed positions, only vibrating on the spot like people standing shoulder-to-shoulder in a crowded lift.

Liquids have particles that are still close together but can slide around each other - imagine a busy dance floor where people move but stay relatively close. The particles have more energy than in solids, so the intermolecular forces holding them together are weaker.

Gases are completely different - particles zoom around randomly with loads of space between them, like footballs being kicked around an enormous pitch. They have high internal energy and very weak forces between particles. When you heat or cool materials, you can make them change state through processes like melting, boiling, freezing, and even sublimation (solid straight to gas).

Quick Tip: Remember that temperature stays constant during state changes - all the energy goes into breaking or forming bonds between particles!

Specific Heat Capacity

Different materials need different amounts of energy to heat up - that's why a metal spoon gets scorching hot in your tea whilst a wooden one stays cool. Specific heat capacity tells us exactly how much energy we need to raise 1kg of a material by 1°C.

The key formula you'll need is ΔE = mcΔθ, where ΔE is the change in thermal energy, m is mass, c is specific heat capacity, and Δθ is the temperature change. Water has a particularly high specific heat capacity $4200 J/kg°C$, which is why it takes ages to boil a kettle!

In the required practical, you'll heat a metal block with an electric heater whilst measuring the current, voltage, and temperature change. The tricky bit is that the block keeps heating up even after you switch off the heater, so you need to record the highest temperature it reaches.

Safety is crucial here - those heaters get dangerously hot, so never touch them directly. Always wrap your block in cotton wool for insulation to get accurate results.

Exam Tip: Energy from the heater equals current × time × voltage $E = ItV$. Use this to find the total energy, then rearrange the main formula to find specific heat capacity!

Specific Latent Heat

Here's something mind-blowing: when ice melts or water boils, the temperature stays exactly the same even though you're still adding energy. That energy isn't disappearing - it's being used to break the bonds between particles instead of making them move faster.

Specific latent heat is the energy needed to change 1kg of a material from one state to another without changing temperature. Every material has two types: latent heat of fusion $melting/freezing$ and latent heat of vaporisation $boiling/condensing$.

The formula is simpler than specific heat capacity: E = mL, where L is the specific latent heat. You can spot these energy changes on heating curves - they're the flat horizontal bits where temperature stays constant whilst the material changes state.

For water, the latent heat of vaporisation is absolutely massive $2,260,000 J/kg$ compared to fusion $334,000 J/kg$. This explains why steam burns are so much worse than boiling water - that steam releases enormous amounts of energy when it condenses on your skin.

Memory Trick: Think "L for state Lines" - latent heat happens along the horizontal lines on heating/cooling curves!

Multiple Energy Changes

Real-world heating often involves several energy changes happening one after another. Imagine heating ice-cold water until it becomes superheated steam - you'll need energy for warming, melting, more warming, boiling, and even more warming!

The secret is breaking it down into stages. First, calculate the energy needed to reach the melting or boiling point using ΔE = mcΔθ. Then work out the energy for the state change using E = mL. Finally, calculate any further temperature changes in the new state.

Each state has its own specific heat capacity, so make sure you use the right value. Water is 4200 J/kg°C, but steam is only 1859 J/kg°C. This means steam heats up much faster than liquid water with the same energy input.

These calculations might look scary with all those big numbers, but just tackle them step by step. Write down what's happening at each stage, use the right formula, then add up all the energies at the end.

Calculator Tip: When dealing with millions of joules, convert to scientific notation early (like 2.5 × 10⁶) to avoid mistakes with all those zeros!

Density

Density tells us how tightly packed the particles are in different materials - it's why a small piece of lead feels much heavier than a large piece of polystyrene. The formula ρ = m/V $density = mass ÷ volume$ is one you'll use constantly.

Solids are usually densest because their particles are crammed together in regular patterns. Liquids are slightly less dense as particles can move around a bit more. Gases have really low density because particles are spread out over huge distances.

Different materials have wildly different densities even in the same state. A cube of iron will be much heavier than an identical cube of aluminium because iron atoms are more closely packed together.

For regular shapes like cubes or spheres, you can calculate volume using geometry formulas. The units matter too - make sure you're consistent with kg/m³ or g/cm³ throughout your calculations.

Unit Conversion: Remember that 1 g/cm³ = 1000 kg/m³. Water's density of 1 g/cm³ becomes 1000 kg/m³!

Density Experiments

There are three main methods for measuring density, depending on what type of object you're investigating. For regular solids like cubes or spheres, simply measure dimensions with a ruler (or vernier callipers for better accuracy) and calculate the volume mathematically.

Irregular objects need the displacement method - this is where you use a eureka can filled to the spout with water. When you carefully lower your object in, the displaced water flows into a measuring cylinder, giving you the object's volume directly.

For liquids like water, measure a known volume (like 50 cm³) in a measuring cylinder, then find its mass using a balance. Remember to subtract the mass of the empty cylinder first to get just the liquid's mass.

Your results should match known values reasonably well - steel is around 7800 kg/m³, and water is exactly 1000 kg/m³. If your answers are way off, check your measurements and calculations for errors.

Practical Tip: Lower irregular objects into the displacement can really slowly to avoid splashing - you'll get much more accurate volume measurements this way!

Heat Transfer

Thermal energy always flows from hot to cold through three different methods: conduction, convection, and radiation. Each one works in completely different situations and involves different mechanisms.

Conduction happens in solids when vibrating particles bump into their neighbours, passing on kinetic energy like a Mexican wave through the material. Metals are brilliant conductors because they have delocalised electrons that can zoom around transferring energy quickly.

Convection only works in fluids (liquids and gases) where particles can actually move around. Hot, less dense regions rise whilst cooler, denser regions sink, creating convection currents - this is how your radiators heat your room and why hot air balloons float.

Radiation is completely different - it's electromagnetic waves (mainly infrared) that can travel through empty space. This is how the Sun's energy reaches Earth and why you feel warmth from a fire even when you're not touching it. Dark, rough surfaces are much better at absorbing and emitting radiation than shiny ones.

Real-world Connection: Your home uses all three methods - conduction through radiator pipes, convection currents warming the air, and radiant heat from the radiator surfaces!

Gas Pressure

Gas pressure comes from billions of tiny particles constantly smashing into container walls - it's like being inside a room where invisible tennis balls are flying everywhere! The formula pressure = force ÷ area tells us that more collisions or harder collisions mean higher pressure.

Temperature is actually a measure of average particle kinetic energy. Higher temperature means faster-moving particles that hit walls more often and with greater force, so pressure and temperature are directly proportional.

Here's the clever bit: pressure and volume are inversely proportional (when temperature stays constant). Squash a gas into half the volume and the pressure doubles because particles hit the walls twice as often. The relationship p₁V₁ = p₂V₂ lets you calculate these changes.

The coldest possible temperature is absolute zero $-273°C or 0 Kelvin$, where particles would theoretically stop moving completely and pressure would be zero. Obviously, this is impossible to reach in real life, but it's a useful concept for understanding gas behaviour.

Exam Strategy: Always check if temperature is constant when using p₁V₁ = p₂V₂. If temperature changes too, you'll need more complex gas law equations!