Chemical Reactions and Equations: When Matter Rearranges
Understand what a chemical reaction is, how to balance equations using the law of conservation of mass, and the five main reaction types with everyday examples.
By the PadhoDost Team · 📖 7 min read · Updated 4 August 2026
Part of Class 10 (CBSE) prep🧠 Cooking is chemistry
When your mother makes chapatis, flour and water (reactants) turn into soft dough and then cooked rotis (products). Nothing vanishes - the ingredients simply rearrange into something new. A chemical reaction is exactly this: existing substances rearrange their atoms to form new substances, with the total amount of matter always staying the same.
What counts as a chemical reaction?
A chemical reaction produces one or more NEW substances with new properties. You can usually spot one by watching for tell-tale signs: a change in colour (a shiny iron gate turning reddish-brown), evolution of a gas (bubbles when antacid meets acid in your stomach), formation of a precipitate (a solid appearing in a clear liquid), a change in temperature (slaked lime warming up), or a change in state or smell.
Balancing chemical equations
We write a reaction as an equation: reactants on the left, an arrow, products on the right. Because of the law of conservation of mass, atoms are never created or destroyed - so the number of atoms of each element must be equal on both sides. An equation with equal atoms on both sides is called a balanced equation. We balance it by adjusting the coefficients (the big numbers in front), never by changing the small formula subscripts.
📝 Balance: Fe + H2O gives Fe3O4 + H2
Skeletal equation: Fe + H2O -> Fe3O4 + H2 (atoms not yet balanced).
Balance Fe: there are 3 Fe atoms on the right, so write 3Fe on the left.
Balance O: the right has 4 oxygen atoms (in Fe3O4), so write 4H2O on the left.
Now balance H: the left now has 4 x 2 = 8 hydrogen atoms, so write 4H2 on the right.
Balanced equation: 3Fe + 4H2O -> Fe3O4 + 4H2. Check: Fe 3 = 3, O 4 = 4, H 8 = 8.
The five main types of reactions
| Type | What happens | Example |
|---|---|---|
| Combination | Two or more reactants join to form a single product | CaO + H2O -> Ca(OH)2 |
| Decomposition | A single reactant splits into two or more products | 2FeSO4 -> Fe2O3 + SO2 + SO3 |
| Displacement | A more reactive element displaces a less reactive one | Fe + CuSO4 -> FeSO4 + Cu |
| Double displacement | Two compounds exchange ions, often forming a precipitate | Na2SO4 + BaCl2 -> BaSO4 + 2NaCl |
| Oxidation-Reduction (redox) | One substance gains oxygen while another loses it | CuO + H2 -> Cu + H2O |
In a redox reaction, gaining oxygen (or losing hydrogen) is called oxidation, while losing oxygen (or gaining hydrogen) is called reduction - and the two always happen together. In CuO + H2 -> Cu + H2O, copper oxide loses oxygen (it is reduced to copper) while hydrogen gains oxygen (it is oxidised to water). Reactions that release heat are exothermic (like burning natural gas), and those that absorb heat are endothermic.
Quick revision
- ✓A chemical reaction forms new substances; signs include colour change, gas, precipitate, and temperature change.
- ✓Balance equations by conserving atoms (law of conservation of mass) using coefficients, not subscripts.
- ✓Five key types: combination, decomposition, displacement, double displacement, and redox.
- ✓Oxidation = gain of oxygen / loss of hydrogen; reduction = loss of oxygen / gain of hydrogen.
- ✓Corrosion (rusting) and rancidity are everyday oxidation reactions.
⚡ Quick check
In sunlight, silver chloride breaks down as 2AgCl -> 2Ag + Cl2. Which type of reaction is this?
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