Everything around you is made of tiny moving pieces — even the air a horse breathes.
Welcome to chemistry. Physics asked how things move; chemistry asks what things are made of. The answer to almost every chemistry question starts with one enormous idea: all matter is made of unbelievably tiny particles that are always moving. You can't see them, but once you learn to picture them, ice cubes, morning fog, the smell of hay drifting across a stable, and steam rising off a mug of cocoa in the Alps all suddenly make sense.
🔺 The Chemist's Trick — see everything three ways
Good chemists look at every substance on three levels at once. Train this habit now and the whole subject gets easier:
What you SEE — the real thing: ice, water, steam (the "macro" view).
The PARTICLES — the invisible dots and how they're arranged (the "particle" view).
The SYMBOLS — the short-hand chemists write, like $\text{H}_2\text{O}$ (the "symbol" view).
This is exactly the Concrete → Pictorial → Abstract ladder from your maths — real thing, then a drawing, then the symbols. In chemistry we call the drawing a particle diagram. Whenever you're stuck, drop down a level and draw the particles.
The particle model — five ideas that explain almost everything:
All matter is made of tiny particles (far too small to see).
The particles are always moving — even in a solid rock.
There are spaces between the particles.
Particles attract each other — the closer they are, the stronger the pull.
Heating gives particles more energy, so they move faster and spread apart.
Same particles, three arrangements. Only the spacing and movement change — never the particles themselves.
The three states, in the particle view:
State
Arrangement
Movement
Shape / Volume
Solid
Packed, neat rows
Vibrate in place
Fixed shape, fixed volume
Liquid
Close but random
Slide past each other
Takes container's shape, fixed volume
Gas
Far apart
Fly around freely
Fills any container completely
A dancer analogy: A solid is dancers standing in tidy rows, swaying on the spot. A liquid is the same dancers in a packed crowd, shuffling and sliding past one another. A gas is those dancers sprinting around an empty field, hardly ever touching. Nobody left the party — they just spread out.
Changes of State
Heating or cooling doesn't make new particles — it changes how much they move, so the substance switches state. Each switch has a name:
Orange = heating (particles speed up, spread out). Blue = cooling (particles slow down, pull together). Jumping straight from solid to gas is called sublimation — that's how dry ice "smokes".
Maths connection — reading a graph: If you heat ice steadily and plot temperature against time, you get a heating curve. It rises, then goes flat while the ice melts, then rises again, then flat again while the water boils. Those flat stretches (plateaus) are moments where the slope is zero — all the heat is being spent breaking particles apart, not raising the temperature. You already know how to read slope from a graph (Lesson 17); here a zero slope tells you a state change is happening.
A heating curve for water. The two flat plateaus are melting (0 °C) and boiling (100 °C).
🔬 Try It at home — Predict · Observe · Explain
This is how real scientists learn: predict before you look, then observe, then explain the gap. Try both:
A · The travelling smell
Predict: Someone peels an orange (or opens a jar of coffee) at one end of the room. How long until you smell it across the room? Why should a smell travel at all?
Observe: Time it.
Explain: The smell is gas particles flying and spreading out — this is called diffusion. It's proof that gas particles really are moving and really do have space to move into.
B · The disappearing puddle
Predict: Wipe a wet cloth across a table. Will the water vanish faster in warm sun or cool shade? By how much?
Observe: Do both and time them.
Explain: Warmth gives the water particles more energy, so more of them escape as gas (evaporate) each second — no boiling required.
Worked Examples
Worked Example 1 — naming the state from the particles
A substance's particles are packed close together in no fixed pattern and can slide past one another, but they can't fly apart. Which state is it, and what happens to its shape if you pour it into a differently-shaped bucket?
Read the clues: "close together" rules out gas. "Slide past one another / no fixed pattern" rules out solid. → It's a liquid.
Shape: A liquid keeps the same volume but takes the shape of its container — so it spreads out to fit the new bucket, sitting flat on top.
Worked Example 2 — reading a heating curve
Mia heats a block of chocolate for a Bavarian cake. The temperature climbs, then holds steady at 34 °C for two minutes even though the stove is still on, then climbs again. What is happening during those two flat minutes?
The slope is zero — temperature isn't rising even though heat is going in.
Explanation: All the incoming heat is being used to melt the chocolate (pulling solid particles free into a liquid), not to raise the temperature. 34 °C is the melting point of that chocolate. Once it has all melted, the temperature can climb again.
Warm-Up
Problem 1
For each of these in a stable in Bavaria, name the state (solid / liquid / gas): (a) a metal horseshoe, (b) water in the trough, (c) the steam rising off a horse after a gallop, (d) a sugar cube, (e) the smell of fresh hay.
name each state →
Problem 2
Draw a particle diagram (just circles) for a solid, a liquid, and a gas. Under each, write one sentence about how the particles are moving.
draw three boxes of circles →
Problem 3
Name the change of state for each: (a) a puddle drying up on a beach in Mexico, (b) ice cream going soft, (c) mist forming on a cold window, (d) a pond freezing over in winter.
melting / freezing / evaporating / condensing →
Problem 4
True or false, and fix any false ones: (a) "Particles in a solid stop moving completely." (b) "A gas can be squashed into a smaller space more easily than a liquid." (c) "When ice melts, the particles get bigger." (d) "Gas particles have lots of space between them."
true/false + corrections →
Core Problems
Problem 5
Use the particle model to explain, in your own words: why can you smell dinner cooking from another room, but you can't smell a solid metal spoon? Mention movement and spacing of particles.
diffusion + spacing →
Problem 6
Mia heats a beaker of ice and records the temperature every minute:
Time (min)
0
1
2
3
4
5
6
Temp (°C)
−10
−5
0
0
0
5
10
(a) During which minutes is the ice melting? How do you know from the numbers? (b) What is the slope (°C per minute) between minute 0 and minute 2? (c) What is the slope during the flat part, and what does a zero slope tell you?
spot the plateau → slope = rise ÷ run → interpret →
Problem 7
A sealed syringe holds 20 mL of air. You push the plunger and squash it to 8 mL without letting any air out. (a) Did you destroy any air particles? (b) Use the particle model to explain why a gas can be squashed but the same-sized syringe full of water can barely be squashed at all.
think about the spaces between particles →
Problem 8
Show these three views of steam (the "chemist's trick"): (a) the macro view — describe what you actually see coming off a kettle; (b) the particle view — draw the particles and how they move; (c) the symbol view — steam is water, written $\text{H}_2\text{O}$. Which two views change when the steam cools and condenses back into water, and which one stays the same?
three levels → what changes, what doesn't →
Problem 9 Challenge
Two identical wet paintings (Mia's watercolours) are left to dry: one flat on a table, one hung upright so the same amount of water is spread over a larger area exposed to the air. Predict which dries faster and explain using the particle model. Then design a fair test to check your prediction — state the one variable you change and three things you keep the same.
predict → explain with particles → design a fair test →
Problem 10 Open
Perfume evaporates and you smell it across a room; a puddle evaporates but you don't "smell" the water spreading. Both are evaporation. Invent an explanation for why one has a strong smell and the other barely any — and describe an experiment you could do to explore what makes a substance smellier. There's no single right answer; make your reasoning clear.
Solid: circles in neat packed rows — "vibrate in place". Liquid: circles close but jumbled — "slide past each other". Gas: few circles far apart — "fly around freely, filling the space".
(a) False — solid particles still vibrate on the spot, they just don't move around. (b) True — gases have big spaces between particles, so they compress easily. (c) False — the particles don't change size; they just move apart and slide freely. (d) True.
Problem 5
Cooking smells are gas particles that break free and fly through the air (diffusion), spreading into the spaces between the air particles until they reach your nose. A metal spoon is a solid — its particles are locked in place and vibrate but can't fly off, so nothing reaches your nose.
Problem 6
(a) Minutes 2–4 — the temperature stays at 0 °C (flat) even though heating continues; that heat is melting the ice. (b) Slope = rise ÷ run = $(0-(-10)) \div (2-0) = 10 \div 2 = 5$ °C/min. (c) Slope = 0 °C/min during the plateau; a zero slope means the heat is changing the state (melting), not raising the temperature.
Problem 7
(a) No particles are destroyed — there are just the same particles in a smaller space. (b) A gas has large gaps between particles, so squashing pushes them closer into that empty space. In water (liquid) the particles are already touching with almost no gaps, so there's nowhere to push them — it barely compresses.
Problem 8
(a) Macro: a white cloud/wisp of steam rising and curling. (b) Particle: widely spaced particles moving fast in all directions. (c) Symbol: $\text{H}_2\text{O}$. On condensing, the macro view (cloud → droplets) and the particle view (spread out → packed close, slower) both change; the symbol $\text{H}_2\text{O}$ stays the same — it's still water, just a different state.
Problem 9
The upright painting (larger surface area exposed to air) dries faster: more water particles are at the surface where they can escape as gas, so more evaporate per second. Fair test — change only the surface area (or orientation); keep the same amount of water, same paper, same paint, same room temperature and airflow.
Problem 10
Open answer. Look for: an idea that "smelliness" depends on the kind of particle (some substances release particles our nose detects strongly, water doesn't), plus a sensible experiment — e.g. leave equal drops of water, vinegar, and perfume out and rank how far/fast each smell travels, keeping amount, temperature, and room the same.
Next up → s10: Mixtures & How to Separate Them
Now you can picture particles, the next question is: what happens when different particles get mixed together — salt in water, colours in a marker, sand in a rock pool? And how do chemists pull them back apart? You'll do real kitchen chemistry, and you'll use ratios and percentages to describe exactly how "strong" a mixture is.