Chemistry
What matter is made of and how it changes: amounts, gases, acids, the energy and speed of reactions, electrochemistry, and the reactions every industry on the ladder runs on.
Home: Chemicals, Level 3 · Mill, level 2 of 3 in its track. Serves 12 steps across 5 subjects.
Chemistry turns recipes into numbers. 100 kg of limestone burns to 56 kg of quicklime; 0.8 ml of household bleach makes 20 litres of clear water safe; ammonia made from the air now grows the food for almost half the world's people (Erisman et al., 2008).
This page gives the results in the order each needs the last: atoms and amounts, gases and solutions, the energy and speed of reactions, electrochemistry, then a table of the reactions the ladder runs on and what chemistry has added since 2000. Every element is in the periodic table; the physics of heat and atoms is on physics.
At a glance: what builds on what
Each part uses the ones above it.
| Part | Results | What depends on it |
|---|---|---|
| 1 · Atoms and amounts | The mole, conservation of mass, the periodic law | Every recipe scaled by weight: soap, lime, smelting, fertiliser |
| 2 · Gases and solutions | The gas law, dilution, pH, buffers | Balloons, pressure vessels, disinfection, food safety, soil |
| 3 · Energy and rates | Heats of reaction, free energy, equilibrium, the Arrhenius rate | Fuels, kilns, the Haber process, storage and spoilage |
| 4 · Electrochemistry | Faraday's laws of electrolysis, cell voltage | Batteries, electroplating, aluminium, chlorine, corrosion |
| 5 · The reactions in use | Lime, iron, soap, glass, chlor-alkali, acids, ammonia, cement | The materials and chemical steps of the ladder |
| 6 · Since 2000 | The completed periodic table, cheap lithium-ion cells, metal–organic frameworks | What is new, and where it is going |
Part 1Atoms and amounts
The mole, conservation of mass and the periodic law
Chemistry counts atoms by weighing them. A reaction's equation gives its proportions in moles; the molar masses turn those into kilograms.
The amount of a substance n, in moles, is its mass m divided by its molar mass M, which in grams per mole equals its formula mass: water 18.02, salt 58.44, quicklime 56.08. One mole is exactly Nₐ particles.
Use: Scales any recipe from its equation: how much lye saponifies a given fat, how much acid dissolves an ore, how much nitrogen a fertiliser carries per kilogram.
First: Avogadro's hypothesis, 1811; Cannizzaro used it to fix atomic weights, 1858; the mole has been defined by an exact Nₐ since 2019 (BIPM SI Brochure).
Atoms are neither made nor destroyed in a reaction, so the masses on each side balance. The coefficients of a balanced equation are mole ratios; multiplied by the molar masses they become ratios by weight.
Use: Yields and raw materials: 100 kg of pure limestone gives 56 kg of quicklime (lime); a tonne of iron needs at least 1.43 tonnes of pure haematite, Fe₂O₃ (smelting).
First: Antoine Lavoisier, Traité élémentaire de chimie, 1789 (English translation, Gutenberg).
An element's place in the periodic table is its atomic number Z, the charge on its nucleus. Moseley found that each element's characteristic X-ray frequency rises as (Z − 1)², which fixed the order of the table and showed exactly which elements were still missing. The table is complete to Z = 118.
Use: The table predicts behaviour by column: alkali metals react violently with water, halogens disinfect and poison, noble gases do almost nothing (the periodic table). X-ray fluorescence still identifies the elements in an ore by these frequencies.
First: Mendeleev's periodic table, 1869; Henry Moseley, 1913 (Philosophical Magazine); the last four elements named in 2016 (IUPAC).
Part 2Gases and solutions
The gas law
In a gas the molecules are far apart, so every gas obeys nearly the same law whatever it is made of.
Pressure times volume is proportional to the amount of gas and its absolute temperature. A mole of any gas fills 22.4 litres at 0 °C and 1 atmosphere, 24.5 litres at 25 °C.
Use: A sealed vessel heated from 20 °C to 300 °C almost doubles its pressure, which is why closed tins and bottles must never go in a fire. Air at 100 °C is about a fifth lighter than at 20 °C, so a hot-air balloon lifts about 0.25 kg per cubic metre (balloons).
First: Boyle's law, 1662 (MacTutor); Charles and Gay-Lussac, 1787–1802; combined into one equation by Clapeyron, 1834 (MacTutor).
Dilution, acids and buffers
Diluting a solution leaves the amount of what is dissolved unchanged, so concentration times volume is the same before and after: c₁V₁ = c₂V₂. Household bleach at 5 % holds about 50,000 mg/L of chlorine, so dosing 20 litres of clear water at WHO's 2 mg/L takes 2 × 20 ÷ 50,000 litres, 0.8 ml (water guide). Always pour acid into water, never water into acid: the heat released can boil the first drops and spray acid.
Acidity is counted in powers of ten of the hydrogen-ion concentration, in moles per litre. pH 7 is neutral at 25 °C; each unit lower is ten times more acid, each unit higher ten times more alkaline.
Use: pH 4.6 is the line below which Clostridium botulinum cannot grow (preservation); chlorine disinfects well only below about pH 8; most crops grow best between pH 6 and 7.5, raised with lime (soils).
First: Arrhenius described acids as sources of hydrogen ions, 1887 (Zeitschrift für physikalische Chemie); Sørensen defined the pH scale at the Carlsberg Laboratory, 1909.
A weak acid HA mixed with its salt A⁻ resists changes of pH. The pH sits at the acid's pKₐ when the two are equal, and moves only one unit for a tenfold change in their ratio.
Use: Blood is held at pH 7.35–7.45 by carbonic acid and bicarbonate; outside that range is a medical emergency. Pickles, cheese and silage stay acid by the same chemistry, and a soil's buffering is why liming takes months to show.
First: Henderson, 1908 (American Journal of Physiology); Hasselbalch's logarithmic form, 1916.
Part 3Energy and rates
Heat of reaction and free energy
Whether a reaction gives out heat, and whether it runs at all, are different questions. The first is its enthalpy; the second its free energy, which includes the disorder it creates.
The heat a reaction gives out or takes in depends only on where it starts and ends, not the route, so it can be added up from tables of heats of formation. Negative ΔH gives out heat.
Use: Fuel values, as heats of combustion: methane 55.5 MJ/kg, hydrogen 142 MJ/kg, coal 24–30 MJ/kg, dry wood about 19 MJ/kg. Slaking quicklime gives out 65 kJ per mole, enough to boil its water and char wood: store it dry (lime).
First: Germain Hess, 1840–1841 (Annalen der Physik).
A reaction runs of its own accord when ΔG is negative. Heat taken in (positive ΔH) can be paid for by a gain in disorder (positive ΔS) once the temperature is high enough. ΔG° also fixes the equilibrium constant K.
Use: Burning lime takes ΔH = +178 kJ/mol but gains ΔS = +161 J/(mol K), so it runs above 178,000 ÷ 161 ≈ 1,110 K, about 840 °C: why kilns are fired to about 900 °C. Plotted for every metal oxide, the same sum tells which ores carbon can smelt, and how hot the furnace must be.
First: J. Willard Gibbs, On the Equilibrium of Heterogeneous Substances, 1876–1878 (MacTutor).
Equilibrium and the speed of reactions
Many reactions go both ways and settle at an equilibrium; all of them go faster when hotter.
At equilibrium the products and reactants, each raised to the power of its coefficient, stand in a fixed ratio K at a given temperature. Disturb it, by adding a reactant, raising the pressure or changing the temperature, and it shifts to undo part of the disturbance.
Use: The Haber–Bosch process: high pressure (150–300 bar) favours ammonia because four molecules of gas become two; low temperature favours it too but is slow, so plants compromise at 400–500 °C over an iron catalyst. The fertiliser made this way supports almost half the world's population (synthetic nitrogen).
First: Guldberg and Waage's law of mass action, 1864 (MacTutor); Le Chatelier's principle, 1884; Haber's ammonia synthesis, 1909, scaled up by Bosch, 1913 (Erisman et al., 2008).
The rate constant k rises steeply with absolute temperature T: only molecules colliding with more than the activation energy Eₐ react. For a typical Eₐ near room temperature, each 10 °C rise roughly doubles the rate.
Use: Food, medicines and seed keep several times longer cold than warm; cooking at 3,000 m, where water boils at about 90 °C, takes markedly longer; a catalyst works by lowering Eₐ.
First: Svante Arrhenius, 1889 (Zeitschrift für physikalische Chemie).
Part 4Electrochemistry
Electrolysis and cells
A battery runs a reaction to push electrons round a circuit; electrolysis pushes electrons in to run a reaction backwards. The same constants govern both.
The mass m of a substance freed at an electrode is the charge Q passed, divided by the Faraday constant F (the charge on a mole of electrons), times the molar mass M over the number of electrons z each atom needs.
Use: Aluminium (z = 3) needs 2,980 ampere-hours per kilogram; at the voltage of a real cell that is 13–15 kWh per kg, which is why smelters stand beside cheap power (electrochemistry). Splitting water for 1 kg of hydrogen takes at least 33 kWh of electricity, about 50–55 kWh in practice. Electroplating thickness follows from the charge passed.
First: Michael Faraday, 1834 (Philosophical Transactions).
A cell's voltage E is its reaction's free energy per charge moved. It is set by the chemistry (E°) and drops slightly as the reactants are used up (the reaction quotient Q rises).
Use: Fixes each cell's voltage: Daniell 1.1 V, lead-acid 2.1 V, lithium iron phosphate 3.2 V, other lithium-ion about 3.6–3.7 V; cells in series add (batteries). Explains corrosion and its cure: zinc bolted to a steel hull in seawater corrodes in the steel's place.
First: Walther Nernst, 1889 (Zeitschrift für physikalische Chemie).
Part 5The reactions in use
The reactions the ladder runs on
Each line is the chemistry under a step; the step gives the method, quantities and hazards.
| Process | Reaction | Where |
|---|---|---|
| Burning lime | CaCO₃ → CaO + CO₂, at about 900 °C | Lime |
| Slaking, and lime mortar setting | CaO + H₂O → Ca(OH)₂ + heat; then Ca(OH)₂ + CO₂ → CaCO₃ + H₂O over months | Lime guide |
| Smelting iron | Fe₂O₃ + 3 CO → 2 Fe + 3 CO₂; the CO comes from charcoal or coke burnt short of air | Smelting, blast furnace |
| Soap | Fat (a triglyceride) + 3 NaOH → glycerol + 3 soap molecules (sodium salts of fatty acids) | Soap |
| Fermentation | C₆H₁₂O₆ → 2 C₂H₅OH + 2 CO₂, by yeast without oxygen | Fermentation and distilling |
| Glass | SiO₂ + Na₂CO₃ → Na₂SiO₃ + CO₂, with lime added so the glass does not dissolve in water | Glass |
| Chlor-alkali | 2 NaCl + 2 H₂O → Cl₂ + H₂ + 2 NaOH, by electrolysis of brine | Chlorine, alkalis |
| Sulfuric acid | S → SO₂ → SO₃ (over a vanadium oxide catalyst) → H₂SO₄ | Acids |
| Lead-acid cell, discharging | Pb + PbO₂ + 2 H₂SO₄ → 2 PbSO₄ + 2 H₂O, 2.1 V per cell | Batteries |
| Portland cement | Limestone and clay fired at about 1,450 °C to clinker (Ca₃SiO₅, Ca₂SiO₄), which sets by reacting with water | Cement and concrete |
| Ammonia | N₂ + 3 H₂ ⇌ 2 NH₃, at 150–300 bar and 400–500 °C over iron | Synthetic nitrogen |
Part 6Since 2000
What chemistry has added since 2000
The laws above are settled. These are the advances of the last quarter-century that change what can be made, each from its primary source.
| Advance | Year | What it changed | Source |
|---|---|---|---|
| The periodic table completed | 2016 | Elements 113, 115, 117 and 118 confirmed and named, filling the seventh row | IUPAC |
| Cheap lithium-ion cells | 1991– | The real price of lithium-ion cells per unit of energy fell by about 97 % in the three decades after their introduction in 1991, making electric vehicles and grid storage practical (Nobel Prize 2019) | Ziegler and Trancik, 2021 |
| Metal–organic frameworks | 2017–2025 | Crystals full of designed pores that soak up chosen gases; one pulled drinking water out of desert air using sunlight alone (Nobel Prize 2025) | Kim et al., 2017, Royal Swedish Academy |
| Protein structures predicted, and genes edited | 2012–2024 | Chemistry's prizes of 2020 and 2024 went to CRISPR gene editing and to computing a protein's shape from its sequence | Biology as engineering |
Where to go next
This page covers what you are most likely to meet. It is not a field guide, and on identification that difference can kill you. These are the works that go further — chosen because they can be held offline.