States of Matter: Particles, Changes and Diffusion
Particle Power: Solids, Liquids & Gases 🧊💧💨
Introduction
1. Introduction
Okay, real talk: this whole chapter is one idea wearing different outfits. Everything is made of tiny particles that are always moving. Whether something is a solid, a liquid or a gas just depends on how those particles are arranged and how much they jiggle about. Lock that in and melting, boiling, gas pressure and diffusion all click into place.
Here's the cheat code: at Core you state the properties, describe what the particles are doing, and describe what happens to a gas when you heat it or squeeze it. The one topic you also have to explain with particles is diffusion; explaining the others in particle terms is Extended work. This is your fast refresh, not the full textbook: one key idea per section, one quick worked example, and the exact move that scores the mark. Let's get into it. 💪
Here's the cheat code: at Core you state the properties, describe what the particles are doing, and describe what happens to a gas when you heat it or squeeze it. The one topic you also have to explain with particles is diffusion; explaining the others in particle terms is Extended work. This is your fast refresh, not the full textbook: one key idea per section, one quick worked example, and the exact move that scores the mark. Let's get into it. 💪
2. The Three States
The same particles can make a solid, a liquid or a gas. Only the arrangement and the movement change. Solid: neat rows, touching, only vibrating on the spot. Liquid: jumbled up, still touching, sliding past each other. Gas: far apart, flying around in all directions. That's it. Memorise the movement words and you're basically there: solids vibrate, liquids slide, gases fly.
Key idea🔑 Key idea: Solid = regular + vibrating. Liquid = random + sliding. Gas = far apart + fast. Always give the arrangement AND the movement. 🎯
Worked example
Worked Example: Describing the States
Worked Example: Arrangement + Movement 🔬
Sodium chloride is a solid at room temperature. Describe the particles in a solid, then say how a liquid and a gas differ.
- 1Solid: particles touching in neat rows, only vibrating on the spot.
- 2Liquid: still touching but jumbled, sliding past each other. Gas: far apart, moving fast in all directions.
Each state needs both parts: arrangement + movement. ✅
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3. Solid, Liquid or Gas?
Every property comes straight from the particles. Only a solid keeps its shape (particles locked in place). Solids and liquids keep their volume (particles touching). Only a gas can be squashed easily, because it has big gaps to close up, and only a gas has a really low density (same particles spread over way more space). So if something flows but you can't squash it smaller, that's a liquid.
Key idea🔑 Key idea: Keeps shape → solid. Keeps volume → solid or liquid. Squashes easily → gas. Big gaps = squashable + low density. 📦
Worked example
Worked Example: Which State Is It?
Worked Example: Use Both Clues 🕵️
A substance takes the shape of any container but always keeps the same volume. Which state is it?
- 1Takes the container's shape → not a solid (a solid keeps its own shape).
- 2Keeps the same volume → not a gas (a gas spreads to fill everything). Flows but fixed volume = liquid. 🎯
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4. Changes of State
A change of state is just a substance switching between solid, liquid and gas. No new substance, totally reversible. Learn them as pairs: melting (solid→liquid) and freezing (liquid→solid); boiling/evaporating (liquid→gas) and condensing (gas→liquid). Going "up" (heating) the particles gain energy and move more; going "down" (cooling) they lose energy and move less. Evaporating also makes a gas, but only from the surface and below the boiling point. That's the full list of five: melting, freezing, boiling, evaporating, condensing. The trick is always checking which direction the change goes before you name it.
Key idea🔑 Key idea: Five changes: melting, freezing, boiling, evaporating, condensing. Up (heating) = particles gain energy. Down (cooling) = particles lose energy. Check the direction before naming. 🔄
Worked example
Worked Example: Naming Changes
Worked Example: Which Way Does It Go? ➡️
For copper, change G turns liquid into gas, and change H turns liquid into solid. Name G and H.
- 1G is liquid → gas = evaporation (boiling).
- 2H is liquid → solid = freezing. (Don't mix it with melting, which is the opposite way.) ✅
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5. Predicting the State
Every pure substance has a fixed melting point and boiling point, and they chop the temperature scale into three zones. Below the melting point → solid. Between the two → liquid. Above the boiling point → gas. Just drop the given temperature onto that number line and read off the zone. Watch out for negative numbers, and always say the reason, not just the answer.
Key idea🔑 Key idea: Below melting = solid, between = liquid, above boiling = gas. Quote the comparison for the mark. 🌡️
Worked example
Worked Example: The State of Tin
Worked Example: Drop It on the Line 📏
Tin melts at 232 °C and boils at 2602 °C. What is its state at 175 °C?
- 1Compare 175 °C with the melting point: 175 is below 232 °C.
- 2Below the melting point = it hasn't melted yet = solid. Reason: "175 °C is below the melting point of 232 °C". ✅
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6. Heating Curves & the Flat Bits
Heat a solid steadily and graph temperature against time and you get a staircase: slopes (warming up) and flat parts (changing state). The lower flat part is melting (solid and liquid both present) and its temperature is the melting point; the higher flat part is boiling and its temperature is the boiling point. On a flat part the temperature stays constant until the change of state is finished, then it climbs again. A cooling curve runs downhill the same way, with flat parts for condensing and freezing. At Core you describe the curve; explaining why the temperature stays constant is Extended work.
Key idea🔑 Key idea: Flat part = changing state. The temperature of the flat part is the melting (freezing) or boiling (condensing) point. Slopes = warming or cooling. ⏸️
Worked example
Worked Example: Reading a Cooling Curve
Worked Example: What Happens on the Flat 🔓
A pure liquid is cooled steadily until it is all solid. The cooling curve has one flat region, G. What is happening during G, and what does the temperature of G tell you?
- 1Liquid is turning to solid, so during G the substance is freezing; liquid and solid are both present and the temperature stays constant until it has all frozen.
- 2The temperature of G is the freezing point, which for a pure substance is the same as the melting point. 🔓
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7. Evaporation vs Boiling
These both turn a liquid into a gas, so people mix them up, but they're different. Evaporation happens only at the surface and at any temperature below boiling (that's why a puddle dries on a cool day). Boiling happens all through the liquid and only at the boiling point (bubbles inside the liquid). For the mark, give both the place and the temperature.
Key idea🔑 Key idea: Evaporation = surface, any temp below boiling. Boiling = all through, only at the boiling point. State both. 💨
Worked example
Worked Example: Spot the Difference
Worked Example: Place + Temperature 🌡️
A student says evaporation and boiling are the same because both make a gas. Give a correct difference.
- 1Evaporation: only at the surface, at any temperature below boiling.
- 2Boiling: all through the liquid, only at the boiling point. Give both features for each. ✅
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8. Gas Volume: Heat & Squeeze
A gas changes its volume a lot when its temperature or pressure changes, and at Core you describe the two effects. Picture gas in a sealed syringe with a plunger that slides freely. Heat the gas → bigger volume (at constant pressure): the plunger moves out. Press harder on the gas → smaller volume (at constant temperature): the plunger moves in. The classic mix-up is the direction, so remember: temperature and volume go the same way, pressure and volume go opposite ways.
Key idea🔑 Key idea: Heat = bigger volume. More pressure = smaller volume. Temp & volume same way; pressure & volume opposite. 🎈
Worked example
Worked Example: Two Changes
Worked Example: Same Way / Opposite Way 🔁
A fixed mass of gas is heated at constant pressure, then (separately) the pressure is increased at constant temperature. What happens to the volume each time?
- 1Temperature up at constant pressure → volume increases (a free plunger would move out).
- 2Pressure up at constant temperature → volume decreases (the plunger moves in). ✅
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9. Heating a Sealed, Rigid Container
What if the gas can't change its volume? In a sealed, rigid container (a steel gas cylinder, an aerosol can) the volume is fixed. Heat it and three things are true: the volume stays the same, the particles move faster, and the pressure inside increases. Cooling does the reverse. That's why aerosol cans say "do not heat". A balloon can stretch, so heating increases its volume; a steel can can't, so heating increases its pressure.
Key idea🔑 Key idea: Can the volume change? Yes (balloon, free plunger) → volume increases. No (rigid, sealed) → volume stays the same, particles move faster, pressure increases. 🥊
Worked example
Worked Example: Heating a Sealed Can
Worked Example: Rigid Means Pressure 🔥
A rigid container of gas is sealed and heated. Which happen? 1: volume decreases. 2: molecules move faster. 3: pressure increases.
- 1Rigid container → volume can't change → statement 1 is wrong.
- 2Heating makes the molecules move faster (2 ✓), and in a fixed volume the pressure rises (3 ✓). Answer: 2 and 3. 🔥
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10. Diffusion
Diffusion is particles spreading out by themselves, from where there are lots of them to where there are fewer, just because they're always moving randomly. No stirring needed. It's fastest in gases (particles fast + lots of space), slower in liquids, and basically zero in solids. Bonus: warming things up makes diffusion faster, because the particles move faster.
Key idea🔑 Key idea: Diffusion = spreading by random motion, high → low concentration. Faster in gases than liquids, and faster when warm. 🌫️
Worked example
Worked Example: The Smelly Room
Worked Example: No Draught Needed 👃
Perfume is opened at the front of a still room; soon someone at the back smells it. What process carried it, and how?
- 1The perfume particles are always moving randomly among the air particles.
- 2They spread out from the crowded bottle to the empty back of the room — that's diffusion. "Still room" means it's the random motion doing it, not a draught. 👃
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