Everything is atoms: from a Greek argument to a jiggling pollen grain
By the end of this lesson you will be able to
- Outline the disagreement between Democritus and Aristotle about matter, and explain why it could not be settled for two thousand years.
- Describe Dalton’s atomic model of 1803 and state the two pieces of evidence — constant composition and multiple proportions — that it explained.
- Explain Brownian motion and diffusion as evidence that matter is made of moving particles.
- Define and distinguish atom, molecule, element and compound, using particle diagrams and the formulas H₂O, CO₂, O₂ and NaCl.
- Interpret a chemical formula: what the symbols and small numbers count.
The puzzle
Take a piece of gold and cut it in half. Cut one half in half again, and again. Every piece is still gold. Could you keep going for ever, so that gold is smooth all the way down like a perfect jelly — or would you eventually reach a piece so small that cutting it once more would leave you with something that is not gold? A Greek thinker asked exactly this question around 400 BC, and nobody could answer it with evidence until the 1800s. Today we can photograph the answer.
A drop of purple potassium manganate(VII) solution is placed gently at the bottom of a glass of perfectly still water. Nobody touches the glass. Predict what the water looks like after ten minutes, after an hour, and after a day — and say what must be true about water for that to happen.
Compare your answer
The purple spreads upwards and outwards on its own — a few centimetres in ten minutes, most of the glass in an hour, an even pale purple everywhere after a day. Nothing stirred it, so something inside the water must be moving all the time and carrying the colour along. If water were a smooth, still jelly, the drop would sit where it was put. The spreading (diffusion) is only possible if water is made of tiny, separate particles in constant motion, with gaps between them that the purple particles can move into. This lesson is the story of how we found that out.
The science — structured notes
1.1 The oldest argument in chemistry
Around 400 BC the Greek thinker Democritus argued that if you kept cutting, you would reach pieces that could not be cut: he called them atomos, “uncuttable”. Between the atoms was empty space, and the differences between materials came from the shapes and arrangements of their atoms. His rival Aristotle (about 350 BC) disagreed. He taught that matter was continuous — no gaps, no smallest piece — and made of four “elements”, earth, water, air and fire, mixed in different proportions. Aristotle won. His ideas matched everyday experience (water pours smoothly; nothing looks grainy) and they were taught in universities for nearly two thousand years.
| Democritus, about 400 BC | Aristotle, about 350 BC | |
|---|---|---|
| What matter is | Tiny uncuttable particles (atoms) moving in empty space | Continuous — smooth all the way down, no gaps |
| Why materials differ | Different shapes, sizes and arrangements of atoms | Different proportions of earth, water, air and fire |
| Evidence offered | Reasoning only: wear on stone steps, the smell of bread | Reasoning only: matter looks and feels continuous |
| Fate | Ignored for 2 000 years; revived by Dalton | Taught until the 1600s; replaced by evidence-based chemistry |
1.2 Dalton, 1803: atoms that can be weighed
The English schoolteacher John Dalton revived the atom in 1803 (published in full in 1808) — not from philosophy but from the balance. Chemists had begun measuring the masses of substances that reacted together, and two patterns kept appearing that a smooth, continuous matter could not explain.
Carbon and oxygen make two gases. Gas A: 12 g carbon with 16 g oxygen. Gas B: 12 g carbon with 32 g oxygen. Explain why this supports Dalton and not Aristotle.
For the same 12 g of carbon, the oxygen masses are 16 g and 32 g — a ratio of 16 : 32 = 1 : 2.
If matter were continuous, any proportion could combine — 16 g, 19.3 g, 27.6 g — and there would be no reason for a whole-number ratio.
If each carbon atom joins a whole number of oxygen atoms, the only possibilities are one, two, three… oxygen atoms per carbon atom. Gas A is one oxygen atom per carbon (we now write it CO); gas B is two (CO₂). Atoms cannot be split, so the ratio must be whole-number. The balance had found the atom.
Nitrogen forms several oxides. In one, 14 g of nitrogen combines with 16 g of oxygen; in another, 14 g of nitrogen combines with 32 g of oxygen; in a third, 28 g of nitrogen combines with 16 g of oxygen.
(a) Work out the mass of oxygen that combines with 14 g of nitrogen in the third oxide.
(b) State the ratio of oxygen masses per 14 g of nitrogen across the three oxides, and explain what it shows.
Reveal answer
(a) 28 g of nitrogen with 16 g of oxygen means 14 g of nitrogen with 8 g of oxygen.
(b) Per 14 g of nitrogen: 8 g, 16 g and 32 g of oxygen — a ratio of 1 : 2 : 4. Small whole numbers again: the nitrogen atoms are combining with one, two or four oxygen atoms for every two nitrogen atoms (N₂O, NO, NO₂ in modern symbols). A continuous substance would have no reason to prefer these steps.
1.3 Seeing the evidence: Brownian motion and diffusion
Dalton’s atoms explained the balance, but they were still invisible, and many chemists treated them as a useful fiction — a bookkeeping trick, not real objects. The evidence that atoms are real and moving came from a botanist who was not looking for it.
In 1827 Robert Brown put pollen grains in water under his microscope and saw tiny particles from the pollen jiggling constantly — a jerky, random dance that never stopped, in water that was perfectly still and had no living thing in it. Dust from a stone did the same. For nearly eighty years nobody could explain it. In 1905 Albert Einstein showed mathematically that the dance is exactly what you would expect if the grains were being hit, thousands of millions of times a second, by water molecules far too small to see, and that the hits do not quite cancel. He predicted how far a grain should wander in a given time and how that depended on temperature and on the size of the water molecules. In 1908 the French physicist Jean Perrin measured it and found Einstein’s numbers were right — and from them worked out the size of a molecule. The doubters gave up: the leading sceptic, Wilhelm Ostwald, publicly accepted atoms in 1909.
| Everyday observation | What is happening at the particle level |
|---|---|
| You smell food cooking from another room | Gas particles from the food move randomly through the air, spreading from where they are crowded to where they are scarce — diffusion |
| A purple crystal colours a whole beaker of still water | Dissolved particles are knocked about by moving water particles and drift into the gaps between them |
| Smoke in a sunbeam wanders rather than falling straight | Air particles hit each smoke particle unevenly — Brownian motion you can see with a lens |
| Diffusion is faster in warm water | Particles move faster at higher temperature, so they spread sooner |
| Diffusion is slow in glycerol or honey | Long, tangled molecules get in the way, so each step of the random walk is shorter |
A smoke cell is a tiny glass box of smoke lit from the side and viewed under a microscope. The smoke particles are seen as bright specks that jerk about randomly. Explain what the jerking shows, why the specks do not simply fall to the bottom, and why the motion is more violent if the smoke is heated.
Reveal answer
Each speck is being struck from all sides by air particles that are far too small to see. In any instant more hit one side than the other, so the speck is nudged in a random direction, and the next instant a different side wins — the jerky path. The specks are so small that these nudges are comparable to their weight, so gravity cannot pull them steadily down; they stay suspended. Heating makes the air particles move faster, so each hit is harder and the specks are thrown further each time — the same reason diffusion is faster in warm water.
“Brownian motion is the movement of the water molecules.”
Reveal answer
False. Brownian motion is the movement of the visible particle — the pollen fragment or the smoke speck — caused by the water or air molecules hitting it. The molecules themselves are far too small to see; we infer their motion from what they do to something bigger. That is why it counts as evidence: the model (moving invisible particles) predicts the dance, and the dance is observed.
1.4 Atoms, molecules, elements, compounds
Four words that every chemistry question depends on. Two of them describe particles; two describe substances.
| Word | Definition | Particle or substance? | Examples |
|---|---|---|---|
| Atom | The smallest particle of an element that still is that element; cannot be split by chemical reactions | Particle | One atom of gold (Au), one atom of oxygen (O) |
| Molecule | Two or more atoms bonded together into one particle — the atoms may be the same element or different | Particle | O₂ (two oxygen atoms), H₂O (one oxygen, two hydrogen) |
| Element | A substance made of only one kind of atom; cannot be broken down into simpler substances | Substance | Gold, oxygen gas, carbon (diamond or graphite), iron |
| Compound | A substance made of two or more elements chemically bonded in a fixed ratio; its properties are different from the elements in it | Substance | Water (H₂O), carbon dioxide (CO₂), sodium chloride (NaCl) |
The particle diagrams below are the way examiners ask this: each circle is an atom, each colour an element, and touching circles are bonded into a molecule.
Classify each as an element, a compound or a mixture, and say whether its smallest particle is an atom or a molecule: (i) chlorine gas, Cl₂; (ii) carbon dioxide, CO₂; (iii) argon, Ar; (iv) sea water; (v) sodium chloride, NaCl.
Reveal answer
(i) Element, made of Cl₂ molecules (two atoms of the same element). (ii) Compound, made of CO₂ molecules (one carbon, two oxygen). (iii) Element, made of single Ar atoms — the noble gases do not form molecules. (iv) Mixture: water molecules with dissolved salts and gases in no fixed ratio; the sea near Walvis Bay and the Dead Sea have different saltiness, which a compound never could. (v) Compound; its particles are ions (charged atoms) in a giant lattice rather than molecules — for now, “a compound of sodium and chlorine in a 1 : 1 ratio” is the correct description, and Year 3 explains the ions.
1.5 Reading a formula
A chemical formula is a count of atoms. The capital letter (with a small letter if needed) is the element’s symbol; the small number after it, written low, says how many atoms of that element are in one particle. No number means one.
| Formula | Name | Atoms in one particle | Element or compound |
|---|---|---|---|
| O₂ | oxygen | 2 oxygen | Element (a molecule of one element) |
| H₂O | water | 2 hydrogen, 1 oxygen — 3 atoms | Compound |
| CO₂ | carbon dioxide | 1 carbon, 2 oxygen — 3 atoms | Compound |
| NaCl | sodium chloride | 1 sodium, 1 chlorine (ratio 1 : 1) | Compound |
| CH₄ | methane | 1 carbon, 4 hydrogen — 5 atoms | Compound |
| NH₃ | ammonia | 1 nitrogen, 3 hydrogen — 4 atoms | Compound |
How many atoms of each element are in 3H₂O, and how many atoms in total?
The large 3 in front means three molecules. Each molecule has 2 H and 1 O.
Hydrogen: 3 × 2 = 6 atoms. Oxygen: 3 × 1 = 3 atoms. Total: 9 atoms.
Two traps: the small number applies only to the symbol just before it (in CO₂ the 2 counts oxygen, not carbon), and a large number in front multiplies everything in the formula.
A student writes: “CO₂ is a mixture of carbon and oxygen. It has 1 carbon atom and 2 oxygen molecules, so it is made of 3 molecules.” Identify three errors.
Reveal answer
(1) CO₂ is a compound, not a mixture — the atoms are chemically bonded in a fixed 1 : 2 ratio and cannot be separated by filtering or evaporating. (2) The 2 counts oxygen atoms, not molecules; O₂ molecules are what oxygen gas is made of, but inside CO₂ the two oxygen atoms are each bonded to the carbon. (3) CO₂ is one molecule containing three atoms, not three molecules.
1.6 How small is small?
An atom is about 0.1 nanometres across — a ten-millionth of a millimetre. Ten million atoms side by side span the width of a full stop. A single drop of water holds about 1 500 000 000 000 000 000 000 molecules (1.5 × 10²¹). If you could count one molecule per second, you would need fifty million million years. That is why chemists weigh substances rather than count particles — and why Dalton’s balance was the right instrument all along.
| Object | Approximate size | How many atoms across |
|---|---|---|
| Hydrogen atom | 0.1 nm | 1 |
| Water molecule | 0.3 nm | about 3 |
| Virus | 100 nm | about 1 000 |
| Bacterium | 1 μm (1 000 nm) | about 10 000 |
| Human hair (width) | 80 μm | about 800 000 |
| Full stop on this page | 0.5 mm | about 5 000 000 |
Aristotle believed matter was continuous. Suppose he had been right. Describe two everyday observations that would then be impossible, and explain why.
Reveal answer
Diffusion would be impossible: with no gaps between particles and no particle motion, a drop of colour placed in still water would stay exactly where it was put, and the smell of cooking would never leave the pot. Brownian motion would be impossible: a pollen fragment in still, continuous water would sit motionless, because there would be nothing hitting it. Also: gases could not be compressed (there would be no empty space to squeeze out), and compounds could form in any proportion rather than whole-number ratios. Every one of these observations was seen, so the continuous model fails.
Try it — how fast does colour spread through still water?
Drop a crystal of purple potassium manganate(VII) into a beaker of still water and time how long the colour takes to reach a line 2 cm above it, at water temperatures from 10 °C to 70 °C. Then change the crystal size and the liquid. You are watching diffusion — the visible result of invisible particles in motion — and measuring how temperature changes it.
Set the water temperature with the slider (this stands for a water bath checked with a thermometer). Keep the crystal small and the liquid water for your first run.
Run, wait for the purple front to touch the 2 cm line, and record the time. Take three trials at each temperature — 10, 20, 30, 40, 50, 60 and 70 °C — and calculate the mean to the nearest second.
Plot mean time (s) against temperature (°C). Describe the shape, quoting values, and test whether doubling the temperature halves the time.
Switch the liquid to the glycerol mixture at 20 °C and compare; then try the large crystal. Which control variable matters more, and why?
Suppose your mean times in water with a small crystal were: 10 °C → 322 s, 20 → 240, 30 → 185, 40 → 146, 50 → 118, 60 → 98, 70 → 82 s.
(a) Describe the relationship, quoting values, and state whether time is inversely proportional to temperature.
(b) Explain the result using the particle model.
(c) In the glycerol mixture at 20 °C the time was about 1 440 s. Explain why, at the particle level.
Reveal answer
(a) The time falls as the temperature rises, from 322 s at 10 °C to 82 s at 70 °C; the fall is steepest at the start (82 s between 10 and 20 °C) and smallest at the end (16 s between 60 and 70 °C), so the graph is a curve that flattens. It is not inversely proportional: doubling the temperature from 20 to 40 °C does not halve the time (240 s to 146 s, about 61 %), and from 30 to 60 °C gives 185 to 98 s, about 53 %.
(b) The dissolved purple particles spread because they and the water particles are in constant random motion. At a higher temperature the particles move faster, so each is knocked further each second, and the water particles also get in the way less because warm water is less viscous. Both effects make the purple front travel the 2 cm sooner.
(c) Glycerol molecules are large and long and attract each other strongly, so the liquid is very viscous. A dye particle trying to move is blocked far more often and travels much less far between collisions, so the random spread covers the same 2 cm about six times more slowly. The particle motion has not stopped — it is obstructed.
Common mistakes to avoid
- “Dalton saw atoms” — he inferred them from mass ratios; the first image of atoms was made in 1981.
- “Brownian motion is the water molecules moving” — it is the visible particle being knocked about by molecules you cannot see.
- “Molecule” and “compound” used as the same word — O₂ is a molecule but an element; NaCl is a compound but not a molecule.
- “The 2 in CO₂ means two carbon dioxides” — it counts oxygen atoms in one molecule; a number in front counts molecules.
- “Particles stop moving when the water is still” — still water means no flow; the particles never stop above absolute zero.
- “A compound is the elements mixed together” — bonded, in a fixed ratio, with new properties; a mixture keeps the old ones.
How we found out — the man who measured the invisible
By 1900 chemists used atoms every day, but a powerful group of scientists — led by the German chemist Wilhelm Ostwald and the Austrian physicist Ernst Mach — refused to believe they existed. Nobody had seen one; the mass ratios could be a pattern with some other cause; “atom” was a convenient story. Mach’s challenge to anyone who mentioned atoms was: “Have you seen one?” It was a fair question. A model that explains the evidence is not proved by it; a different model might explain the same evidence.
What settled it was a prediction. Einstein’s 1905 paper did not just say “molecules hit the pollen”; it said how far a grain of a given size should wander, on average, in a given time, at a given temperature — and the number depended on how many molecules there are in a gram of water. Jean Perrin spent three years in Paris following thousands of tiny resin spheres under a microscope, timing them with a stopwatch and plotting their positions every thirty seconds. The wanderings matched Einstein’s formula, and from them Perrin calculated the number of molecules in a known mass — a figure that agreed, to within a few per cent, with numbers found in four completely different ways (gas behaviour, the blue of the sky, radioactivity, the charge on an electron). Five independent routes to the same number cannot be a coincidence. Ostwald wrote in 1909 that he was now convinced; Perrin received the Nobel Prize in 1926 “for his work on the discontinuous structure of matter”. The atom went from useful fiction to measured fact because a model made a numerical prediction and the measurement matched.
Mach asked “Have you seen one?” Explain why Perrin’s experiment answered him even though Perrin never saw an atom either.
Reveal answer
Perrin saw the effect of atoms on something he could see, and — more importantly — the size of that effect matched a number predicted by the atomic model. If atoms did not exist, there was no reason for the pollen grains’ wandering to obey Einstein’s formula, and no reason for the number of molecules per gram calculated from it to agree with the number calculated from gases, from sky colour and from radioactivity. A model that makes a testable numerical prediction, and passes, and agrees with independent methods, is accepted whether or not the thing itself is visible. Seeing is one kind of evidence; a successful prediction is a stronger one.
Where it is used now
- Botswana’s diamonds are atoms in a pattern. Every diamond from Jwaneng or Orapa is carbon atoms bonded in a rigid three-dimensional lattice; graphite in a pencil is the same carbon atoms in flat sheets. Same element, different arrangement — Democritus’s idea, proved right.
- Salt from Sua Pan. Botswana Ash at Sowa extracts sodium chloride and sodium carbonate from brine by evaporation — a physical separation of a mixture. The NaCl itself cannot be split by evaporating: it is a compound.
- Water treatment. Chlorine added to Gaborone’s water supply diffuses through the whole volume of a reservoir without stirring; engineers calculate the contact time from diffusion and flow.
- Medicine. Oxygen reaches your blood by diffusing across the thin walls of the lungs’ air sacs; a drug patch works by diffusion through the skin; dialysis cleans blood by diffusion across a membrane.
- Imaging atoms. In 1981 the scanning tunnelling microscope produced the first images of individual atoms; in 1989 researchers at IBM moved 35 xenon atoms to spell the company’s name. The picture Mach demanded finally exists.
Science now — and the scientist you need to be
What is happening in 2026
The atom is now a building block that engineers place by hand. Electron microscopes resolve individual atoms in a crystal and can watch them shift during a chemical reaction; scanning-probe microscopes push atoms one at a time to build circuits a few atoms wide. Artificial intelligence has changed what happens before any experiment: machine-learning models trained on the known crystals have proposed millions of new stable compounds — atoms in arrangements nobody had tried — and automated laboratories in several countries now synthesise and test the most promising ones without a person touching a beaker. The batteries, catalysts and solar materials of the 2030s are being chosen this way. Quantum computers built from individual atoms held in laser beams passed a thousand qubits in 2024 and are being used to simulate other atoms — using atoms to predict atoms. And a century after Perrin, Brownian motion is a working tool: biologists track single proteins jiggling inside living cells to measure how crowded the cell is. Democritus’s guess has become an industry.
The mindshift
What you need to be able to do
Three examinable skills from this lesson: define the four words precisely and sort substances into them from a formula or a particle diagram; explain diffusion and Brownian motion at the particle level with the words random, collide and faster at higher temperature; and explain why whole-number ratios are evidence for atoms. Examiners report every session that “explain” answers stop at the description — the mark is for the particle mechanism.
See an example
Cold tap water (about 18 °C): the colour had spread about 2 cm from the drop after ten minutes, in a wispy plume. Warm water (about 45 °C): the whole glass was tinted in six minutes. The AI’s first answer was “warm water dissolves the dye faster”; asked about particles, it added that water molecules move faster at higher temperature and knock the dye molecules further each second, so they diffuse more quickly — the answer that earns the mark.
Key terms
| Atom | The smallest particle of an element that is still that element; the building block of all matter. |
| Molecule | Two or more atoms bonded together into one particle (O₂, H₂O, CO₂). |
| Element | A substance made of one kind of atom only; cannot be broken down chemically. |
| Compound | A substance made of two or more elements bonded in a fixed ratio, with its own properties. |
| Mixture | Two or more substances together in no fixed ratio, keeping their own properties; separable physically. |
| Formula | A count of the atoms in one particle of a substance, e.g. H₂O = 2 hydrogen, 1 oxygen. |
| Constant composition | A pure compound always has the same elements in the same mass ratio. |
| Multiple proportions | When two elements form more than one compound, the combining masses are in small whole-number ratios. |
| Brownian motion | The random, jerky movement of small visible particles caused by collisions with invisible moving molecules. |
| Diffusion | The net movement of particles from where they are more concentrated to where they are less, because of their random motion. |
| Model | A scientific idea that explains evidence and makes testable predictions; kept while it works, changed when it fails. |
Think like a scientist
Criterion A A perfume bottle is opened at the front of a classroom with the windows closed. Students in the front row smell it after about 15 s; students at the back (8 m away) after about 90 s. The teacher repeats it on a cold morning with the heater off and the back row waits about 2 minutes. The perfume liquid is a mixture of an alcohol (C₂H₆O) and scent molecules dissolved in it.
(a) Explain, using the particle model, how the scent reaches the back of the room without any wind. [3]
(b) Explain why the back row waits longer on the cold morning. [2]
(c) State how many atoms of each element are in one molecule of C₂H₆O, and classify the alcohol as an element, a compound or a mixture, giving a reason. [3]
(d) A student says the perfume “must be a compound because it always smells the same”. Evaluate this claim. [2]
(e) Outline one piece of evidence, other than smell spreading, that matter is made of moving particles, and explain why it counts as evidence rather than as a description. [3]
Model answer
(a) Scent molecules evaporate from the liquid into the air as a gas. Gas particles — the scent molecules and the air molecules around them — are in constant, rapid, random motion, colliding with each other thousands of millions of times a second. Because there are far more scent molecules near the bottle than at the back, more of them move away from the bottle than towards it, so there is a net spread from where they are concentrated to where they are not: diffusion. No wind is needed; the random motion alone carries them, slowly, because each molecule changes direction at every collision.
(b) At a lower temperature the air and scent molecules move more slowly, so each travels a shorter distance per second between collisions and the net spread across 8 m takes longer. (Fewer molecules evaporate per second from the cold liquid too, so the concentration at the bottle is lower and the gradient driving the spread is smaller.)
(c) 2 carbon atoms, 6 hydrogen atoms, 1 oxygen atom — 9 atoms in the molecule. It is a compound: three different elements chemically bonded in a fixed ratio (2 : 6 : 1), giving a substance whose properties differ from carbon, hydrogen and oxygen.
(d) The claim confuses a consistent product with a compound. Perfume is a mixture — scent molecules dissolved in alcohol — and it smells the same because the maker mixes the same ingredients in the same proportions, not because the ratio is fixed by bonding. A test: the alcohol can be evaporated off, leaving the scent behind, which is a physical separation; a compound could not be separated that way. Smell is not the test for a compound — a fixed composition that survives physical separation attempts is.
(e) Brownian motion: a smoke particle in a smoke cell (or a pollen fragment in water) is seen under a microscope to jerk about randomly in air or water that is completely still. It is evidence rather than a description because the particle model predicts it — invisible molecules moving randomly must hit a small visible particle unevenly and knock it about — and a continuous model of matter predicts the particle should stay still. An observation that one model predicts and the other forbids is what decides between them. (Also acceptable: whole-number mass ratios in compounds, which only make sense if atoms combine in whole numbers.)
Probe deeper — exam-style questions
Attempt every question before opening the answer. Marks in brackets show what a full answer is worth.
Define an element and a compound, and give one example of each.
Reveal answer
An element is a substance made of only one kind of atom, which cannot be broken down into simpler substances by chemical reactions, e.g. oxygen or gold [1]; a compound is a substance made of two or more elements chemically bonded in a fixed ratio, with properties different from its elements, e.g. water or sodium chloride [1].
State the number of atoms of each element in 2CO₂, and the total number of atoms.
Reveal answer
Two molecules, each with 1 carbon and 2 oxygen [1]; carbon 2, oxygen 4 [1]; total 6 atoms [1].
A student writes: “Democritus discovered atoms in 400 BC. Dalton later proved them by looking through a microscope, and Brownian motion is the movement of atoms that we can see.” Identify three errors.
Reveal answer
Democritus proposed atoms by reasoning, with no evidence — a discovery needs evidence [1]; Dalton inferred atoms from mass ratios measured on a balance, not from a microscope — no microscope of his time could show them [1]; Brownian motion is the movement of small visible particles caused by collisions with molecules too small to see — the atoms themselves are not seen [1].
Hydrogen and oxygen form water (1 g H : 8 g O) and hydrogen peroxide (1 g H : 16 g O). Explain how these masses support Dalton’s atomic model.
Reveal answer
For a fixed 1 g of hydrogen, the oxygen masses are 8 g and 16 g, a ratio of 1 : 2 [1]; a small whole-number ratio is what you expect if each hydrogen atom combines with one or two whole oxygen atoms, since atoms cannot be split [1]; a continuous substance could combine in any proportion, so the whole-number ratio is evidence for atoms (law of multiple proportions) [1].
True or false, with a reason: “Diffusion stops when a liquid is perfectly still.”
Reveal answer
False [1]: “still” means no overall flow, but the particles of a liquid are in constant random motion at any temperature above absolute zero, so dissolved particles continue to spread by collisions until they are evenly distributed [1].
A bottle of ammonia is opened at one end of a long glass tube and a bottle of hydrochloric acid at the other. A white ring of ammonium chloride forms inside the tube after a minute, closer to the acid end. Explain the ring, and why it is nearer the acid end (ammonia molecules are lighter than hydrogen chloride molecules).
Reveal answer
Both gases diffuse along the tube by random particle motion; where the two kinds of molecule meet they react to form solid ammonium chloride, seen as a white ring [1]; the ring is nearer the acid end because the ammonia molecules travelled further in the same time [1]; lighter molecules move faster at the same temperature, so ammonia diffuses faster than hydrogen chloride [1].
Summary
- Democritus proposed atoms and Aristotle denied them; without evidence, the argument could not be settled for two thousand years.
- Dalton (1803) inferred atoms from constant composition and multiple proportions: whole-number mass ratios mean whole atoms combining.
- Brownian motion (Brown 1827, explained by Einstein 1905, measured by Perrin 1908) and diffusion show that matter is made of particles in constant random motion, faster when hotter.
- Atom and molecule are particles; element and compound are substances. A compound has bonded elements in a fixed ratio and new properties; a mixture does not.
- A formula counts atoms: H₂O is 2 H and 1 O; a number in front multiplies the whole molecule.
Next time → Thomson’s plum pudding: a glowing tube, the discovery of the electron, and the first atom with parts inside it.