What a chemical reaction is
A chemical reaction is a rearrangement of atoms: bonds break, new bonds form, and you end up with different substances. The starting materials are the reactants (left of the arrow) and the new substances are the products (right of it). Burning wood, rusting iron and baking a cake are all reactions — matter hasn't vanished, it has been rearranged.
The rule that governs every equation is the law of conservation of mass: atoms are neither created nor destroyed. Whatever atoms go in must come out — same elements, same counts. That single law is why we balance equations at all: an unbalanced equation is literally claiming that atoms appeared out of nowhere.
- Reactants → products: atoms rearranged, never destroyed
- Law of conservation of mass: atoms in = atoms out
- Chemical change makes a new substance; physical change doesn't
- Signs of a reaction: color change, gas bubbles, precipitate, heat or light
The five reaction types
Most school reactions fall into five patterns. Synthesis (combination): A + B → AB — two things join. Decomposition: AB → A + B — one thing breaks apart. Single replacement: A + BC → AC + B — one element kicks another out. Double replacement: AB + CD → AD + CB — two compounds swap partners. Combustion: a fuel burns with oxygen, producing CO2 and H2O.
Identifying the type is pattern-matching, not deep chemistry. Count the pieces: two into one is synthesis, one into two is decomposition. Two compounds trading pieces is double replacement. If oxygen is a reactant and the products include CO2 and water, it's combustion. Learn each pattern's silhouette and categorizing takes seconds.
- Synthesis: A + B → AB (building up)
- Decomposition: AB → A + B (breaking down)
- Single replacement: A + BC → AC + B (one element swaps in)
- Double replacement: AB + CD → AD + CB (partners swap)
- Combustion: fuel + O2 → CO2 + H2O (burning)
CaCO3 → CaO + CO2: one compound becomes two — decomposition. Zn + CuSO4 → ZnSO4 + Cu: zinc replaces copper — single replacement. CH4 + 2O2 → CO2 + 2H2O: a fuel plus oxygen giving CO2 and water — combustion. Count the pieces, spot the pattern, done.
Balancing equations step by step
Balancing means adding coefficients — the big numbers in front of formulas — until the atom counts match on both sides. You never change subscripts, the little numbers inside formulas, because that would change the substance itself. H2O and H2O2 are different chemicals; balancing by touching subscripts is the classic error.
The method: write the equation, count each element's atoms on both sides, and adjust coefficients one element at a time, saving hydrogen and oxygen for last since they appear in several compounds. Finish with a full recount. For H2 + O2 → H2O: oxygen is unbalanced (2 in, 1 out), so put a 2 in front of H2O; then rebalance hydrogen with a 2 in front of H2. Result: 2H2 + O2 → 2H2O.
- Change coefficients (front numbers), never subscripts
- Count every element on both sides before and after
- Balance H and O last — they appear in multiple compounds
- Finish with a complete recount; one unbalanced atom fails everything
Carbon: 1 on each side, done. Hydrogen: 4 in (CH4) but only 2 in one H2O — put a 2 in front of H2O: now H = 4 = 4. Oxygen: right side now has 2 + 2 = 4, left has 2 — put a 2 in front of O2. Final: CH4 + 2O2 → CO2 + 2H2O. Recount every element: 1 C, 4 H, 4 O on both sides. Balanced.
Reading equations like a chemist
A balanced equation tells a story with numbers. The coefficients are ratios: 2H2 + O2 → 2H2O means two hydrogen molecules react with one oxygen molecule to make two water molecules — or, scaled up, 2 moles with 1 mole makes 2 moles. Those ratios are the foundation of stoichiometry, the next page in this hub.
Equations also carry state symbols: (s) solid, (l) liquid, (g) gas and (aq) aqueous, meaning dissolved in water. They look decorative but matter — a precipitate forms when two aqueous solutions produce a solid, and gas bubbling out is one of the signs a reaction actually happened. Read the equation as a sentence and the symbols become information, not decoration.
- Coefficients are mole ratios (the bridge to stoichiometry)
- State symbols: (s), (l), (g), and (aq) = dissolved in water
- Precipitate: a solid formed from two aqueous solutions
- Unbalanced equations can't be used for calculations
Key concepts to memorize
🎯 Study tips for this topic
- Write an atom-count table (element | left | right) under every equation you balance — invisible counting causes most errors.
- Remember the golden rule out loud: coefficients change, subscripts never do.
- Balance hydrogen and oxygen last, especially in combustion problems.
- Learn the five types as silhouettes (2→1, 1→2, swap one, swap two, burn) and classify by shape.
- Finish every balancing problem with a full recount — the last check catches the sneaky O or H you forgot.
