Chemical Reactions and Equations: Turning kitchen-table chemistry into precise, balanced equations
The opening chapter of Class 10 Science — how to recognise a chemical reaction, write and balance its equation, and sort every reaction you'll ever meet into a handful of core types.
🔥1.1 Spotting a Chemical Reaction & Writing It Down
Milk turning sour in summer, an iron nail rusting in humid air, grapes fermenting, food cooking, food digesting inside you, even the simple act of breathing — in every one of these, the identity of the starting substance genuinely changes into something new. Whenever that happens, a chemical reaction has taken place.
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Think of baking a cake.. Flour, eggs, and sugar go in — a cake comes out. You can't un-bake it back into raw eggs and flour. That one-way transformation into something genuinely new is the heart of every chemical reaction, unlike just melting butter, which is only a physical change you could reverse by cooling it back down.
ACTIVITY 1.1Clean a magnesium ribbon with sandpaper, then burn it using tongs, keeping it away from your eyes.
OBSERVEThe ribbon burns with a dazzling white flame and changes into a white powder.
IDENTIFYThat white powder is magnesium oxide — a brand new substance, formed by magnesium reacting with oxygen in the air.
🔍Four telltale signs of a chemical reaction:. Change in state, change in colour, evolution of a gas, or change in temperature. Activity 1.2 (lead nitrate + potassium iodide → a bright yellow precipitate) and Activity 1.3 (zinc + dilute acid → bubbling gas and a rise in temperature) both show these signs in action.
Activity 1.1: Burning magnesium ribbonMagnesium ribbon held with tongs is burnt over a flame; the white ash (magnesium oxide) is collected on a watch-glass below.
✍️Writing a chemical equation
Describing 'magnesium burns in oxygen to form magnesium oxide' in a full sentence every time gets tedious fast. A word-equation shortens this: the substances that go IN are reactants (written on the left, joined by '+'), and the new substance that comes OUT is the product (written on the right), with an arrow showing the direction of change.
Magnesium + Oxygen → Magnesium oxide
(Reactants) (Product)
Swap the words for chemical formulae and you get an even more compact chemical equation: Mg + O₂ → MgO. But count the oxygen atoms — there are 2 on the left and only 1 on the right! This mismatched version is called a skeletal chemical equation — a correct sketch of what reacts with what, but not yet balanced.
⚖️1.1.1 – 1.1.2 Balancing Equations Step by Step
Recall the law of conservation of mass from Class IX: mass can neither be created nor destroyed in a chemical reaction. That means the number of atoms of every element must be identical before and after the reaction — a skeletal equation that doesn't satisfy this needs to be balanced.
🚫Golden rule: you can NEVER change the little subscript numbers inside a chemical formula to balance an equation (H₂O₄ is not a real substance!). You can only add whole-number coefficients in front of formulas — like changing H₂O to 4H₂O.
STEP 1 — BOX EACH FORMULA[Fe] + [H₂O] → [Fe₃O₄] + [H₂]. Nothing inside a box may change.
STEP 3 — BALANCE THE MOST COMPLEX COMPOUND FIRSTFe₃O₄ has the most atoms. Its oxygen (4 on RHS, 1 on LHS) needs 4 H₂O on the left: Fe + 4H₂O → Fe₃O₄ + H₂.
STEP 4 — BALANCE HYDROGEN NEXT4H₂O now gives 8 H atoms on the LHS, so we need 4H₂ on the RHS: Fe + 4H₂O → Fe₃O₄ + 4H₂.
STEP 5 — BALANCE WHAT'S LEFT (IRON)Fe₃O₄ has 3 iron atoms, so we need 3Fe on the LHS: 3Fe + 4H₂O → Fe₃O₄ + 4H₂.
Reaction conditions (temperature, pressure, a catalyst, or sunlight) are often written above or below the arrow. Photosynthesis is a classic example: 6CO₂(aq) + 12H₂O(l) —Sunlight/Chlorophyll→ C₆H₁₂O₆(aq) + 6O₂(aq) + 6H₂O(l).
Now that you can write and balance equations, it's time to sort reactions into types — starting with two that are exact opposites of each other.
🧲Combination reaction:. Two or more reactants combine to form a single product. Example (Activity 1.4): quicklime reacts vigorously with water, releasing a large amount of heat — CaO(s) + H₂O(l) → Ca(OH)₂(aq) + Heat.
C(s) + O₂(g) → CO₂(g) — burning of coal
2H₂(g) + O₂(g) → 2H₂O(l) — formation of water
CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g) — burning of natural gas
C₆H₁₂O₆(aq) + 6O₂(aq) → 6CO₂(aq) + 6H₂O(l) + energy — respiration inside your own cells
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Whitewashed walls get their shiny finish from a follow-up combination reaction: Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l), forming a thin layer of calcium carbonate (the same compound marble is made of!) over two to three days.
🌡️All the reactions above release heat as they form products — these are exothermic reactions. Respiration, burning fuels, and decomposing compost are everyday exothermic combination reactions.
💥Decomposition reactions — the reverse process
If combination glues reactants together, decomposition pulls a single reactant apart into two or more simpler products. Because breaking bonds needs energy input, decomposition reactions are typically endothermic — they absorb energy rather than release it.
Energy source
Called
Example
Heat
Thermal decomposition
CaCO₃(s) —Heat→ CaO(s) + CO₂(g) (limestone → quicklime, used in cement)
Electricity
Electrolytic decomposition
2H₂O(l) —Electricity→ 2H₂(g) + O₂(g) (electrolysis of water; H₂ collected is exactly double the O₂ by volume)
Light
Photolytic decomposition
2AgCl(s) —Sunlight→ 2Ag(s) + Cl₂(g) (white AgCl turns grey — the basis of black-and-white photography)
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Other thermal decompositions worth knowing: heated ferrous sulphate crystals (green) lose water and break down into ferric oxide, sulphur dioxide, and sulphur trioxide (2FeSO₄ —Heat→ Fe₂O₃ + SO₂ + SO₃); heated lead nitrate releases brown fumes of nitrogen dioxide (2Pb(NO₃)₂ —Heat→ 2PbO + 4NO₂ + O₂).
EndothermicProducts sit higher: energy is absorbed.
Drop clean iron nails into a blue copper sulphate solution (Activity 1.9), wait 20 minutes, and something dramatic happens: the nails turn brownish, and the blue colour of the solution visibly fades.
Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)
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Think of it as 'king of the hill.'. A more reactive element (iron) physically kicks a less reactive element (copper) out of its compound and takes its place — exactly like a stronger competitor displacing a weaker one from the top spot. This is a displacement reaction.
Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s)
Pb(s) + CuCl₂(aq) → PbCl₂(aq) + Cu(s)
Zinc and lead are both more reactive than copper, so they displace it from its compounds.
🌨️Double displacement — swapping partners
Mix colourless sodium sulphate solution with colourless barium chloride solution (Activity 1.10) and a white solid immediately appears, insoluble in water. This solid is called a precipitate.
Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq)
🔁Double displacement reaction:. Two compounds in solution swap ions with each other — here, SO₄²⁻ pairs up with Ba²⁺ to form insoluble barium sulphate, while Na⁺ and Cl⁻ remain dissolved as sodium chloride. Because a solid forms out of solution, this is also called a precipitation reaction.
💡Every double displacement reaction involves an exchange of ions — but not every one produces a visible precipitate. What made this one special is that BaSO₄ specifically happens to be insoluble in water.
Heat copper powder in a china dish (Activity 1.11) and its shiny surface turns black, coated with copper(II) oxide. Now pass hydrogen gas over that same heated black coating — it turns back to brown copper metal!
2Cu(s) + O₂(g) —Heat→ 2CuO(s)
CuO(s) + H₂(g) —Heat→ Cu(s) + H₂O(l)
⚗️Oxidation vs Reduction:. If a substance GAINS oxygen (or LOSES hydrogen), it is oxidised. If a substance LOSES oxygen (or GAINS hydrogen), it is reduced. In the second reaction above, CuO loses oxygen (reduced) while H₂ gains oxygen (oxidised) — both happening simultaneously. Reactions where this dual gain-and-loss occurs are called oxidation-reduction reactions, or redox reactions for short.
ZnO + C → Zn + CO — carbon is oxidised to CO; zinc oxide is reduced to zinc
MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂ — HCl is oxidised to Cl₂; MnO₂ is reduced to MnCl₂
🏗️Corrosion & rancidity — oxidation in daily life
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Corrosion. Shiny iron left exposed slowly coats itself in reddish-brown rust; silver tarnishes black, copper develops a green coating. When a metal is attacked by substances around it (moisture, acids), it's said to corrode — and corrosion of iron alone costs enormous sums in damage to vehicles, bridges, and railings every year.
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Rancidity. Fats and oils left out for a long time slowly oxidise, developing an unpleasant smell and taste — this is rancidity. Food manufacturers fight it with antioxidants, air-tight packaging, and by flushing chip packets with unreactive nitrogen gas to keep oxygen away from the oils inside.
📝Chapter Summary
A complete chemical equation shows reactants, products, and (optionally) their physical states.
A chemical equation must be balanced: equal atoms of each element on both sides, per the law of conservation of mass.
Combination: two or more substances → one new substance.
Decomposition (opposite of combination): one substance → two or more simpler substances; usually needs heat, light, or electricity (endothermic).
Displacement: a more reactive element pushes out a less reactive one from its compound.
Double displacement: two compounds exchange ions; if an insoluble solid forms, it's also a precipitation reaction.
Oxidation = gain of oxygen / loss of hydrogen. Reduction = loss of oxygen / gain of hydrogen. Both happen together in a redox reaction.
TOOL 1 Chemical Equation Balancer
Enter (or pick a preset) skeletal chemical equation; the tool walks through the hit-and-trial balancing method step by step — boxing formulas, counting atoms, and adding coefficients one element at a time — mirroring the textbook's Fe + H₂O → Fe₃O₄ + H₂ walkthrough.
Enter (or pick a preset) balanced chemical equation; the tool identifies whether it's a combination, decomposition, displacement, or double displacement reaction, flags whether it's also a redox reaction, and explains its reasoning.
Fe + CuSO₄ → FeSO₄ + Cu
Reaction typeDisplacementA free element replaces another element inside a compound.
Redox flagRedoxRedox: Fe is oxidised to Fe²⁺ while Cu²⁺ is reduced to Cu.
Reactants: Fe, CuSO₄ | Products: FeSO₄, Cu
§3 Flashcards
Click a card to flip it. Use Prev/Next to move through the deck.
Card 1 / 12
Q
What is a balanced chemical equation?
A
One where the number of atoms of every element is equal on both the reactant (LHS) and product (RHS) sides.
§4 Quiz
Modelled on the CBSE Class 10 Section A paper — Multiple Choice and Assertion–Reason questions, each worth 1 mark. 20 questions per attempt, drawn from a pool of 40.
