Chemical Reactions and Equations · Class 10 Science: quick revision, 45 bites

Chemical vs Physical Change

The one-question test

Takeaway. New substance formed? Then it's a chemical reaction.

Signs a Reaction Happened

Five clues at the crime scene

Takeaway. Any of the four signs = bonds being rearranged.

Burning Magnesium Ribbon

The dazzling classroom classic

Takeaway. 2Mg + O₂ → 2MgO — dazzling proof of a chemical change.

Reactants, Products and the Arrow

The grammar of chemistry

Word equation: zinc + sulphuric acid → zinc sulphate + hydrogen. Formula equation: Zn + H₂SO₄ → ZnSO₄ + H₂.

Takeaway. Left eats, right results — the arrow is the verb.

Conservation of Mass

Chemistry's accounting law

This law is WHY equations must balance: every atom on the left must appear on the right.

Takeaway. Atoms in = atoms out; balancing is bookkeeping for this law.

Coefficients Yes, Subscripts Never

The rule that saves your marks

And a coefficient multiplies the whole formula: 2Ca(OH)₂ = 2 Ca, 4 O, 4 H.

Takeaway. Fix the FRONT, never the FORMULA.

Hit-and-Trial Balancing

The method that always works

Takeaway. Count → adjust coefficients → recount — repeat till equal.

Worked: Iron and Steam

The exam's favourite equation

Takeaway. Hardest compound first, lone element last, verify always.

State Symbols

Four brackets, full information

↑ marks a gas escaping; ↓ marks a precipitate settling.

Takeaway. s, l, g, aq — the four addresses of matter in equations.

Conditions: Heat, Light, Catalyst

Instructions on the arrow

Takeaway. The arrow carries the recipe's oven settings.

Worked: Burning Methane

One more rep

Takeaway. Balance H with water's coefficient, then close with O₂.

Coefficients Multiply Everything

The classic count slip

Takeaway. Multiply through brackets before you compare sides.

Everyday Chemical Reactions

Reactions running in you right now

Takeaway. Life and kitchen alike are stacks of chemical equations.

Board Favourites

How this topic is asked

Takeaway. Conservation law + three classic balancings = the marks.

Memory Pack

Whole topic in three lines

Takeaway. Three lines, the whole equations toolkit.

Join, Split, Kick, Swap

All of chemistry in four moves

Takeaway. Four patterns classify nearly every reaction you'll meet.

Combination Reactions

Two become one

Takeaway. Many in, ONE out = combination.

Quicklime + Water → White-Wash

The hissing bucket

Takeaway. Two linked reactions paint and polish every white-washed wall.

Decomposition Reactions

One becomes many

Takeaway. One in, many out — powered by heat, light or current.

Heating Ferrous Sulphate

Green crystals, sulphur smell

Takeaway. Green → brown + sulphur smell = FeSO₄ decomposing.

Silver Chloride in Sunlight

Chemistry that took photographs

Takeaway. Light splits silver halides — the camera's original chemistry.

Electrolysis of Water

Splitting water with a battery

Takeaway. Electric current splits water 2 volumes H₂ : 1 volume O₂.

Displacement Reactions

The stronger one takes the seat

Reactivity decides everything: iron displaces copper, zinc displaces both — but copper can displace neither.

Takeaway. More reactive kicks out less reactive — never the reverse.

Iron Nail in Copper Sulphate

The nail that turns brown

Takeaway. Blue fades + brown coat = displacement in action.

Double Displacement Reactions

Partner exchange

Takeaway. Ions swap partners; a precipitate or salt+water announces it.

Exothermic vs Endothermic

The heat ledger

Takeaway. Follow the heat: released = exo, absorbed = endo.

Respiration Is Exothermic

The quiet fire inside you

Digested glucose combines with breathed-in oxygen in your cells: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy. This slow, controlled 'burning' releases the energy that runs your body — respiration is an **exothermic** reaction.

Takeaway. Every breath fuels a slow exothermic burn.

Displacement ≠ Double Displacement

One kick vs a swap

Takeaway. Element attacking = kick; compounds exchanging = swap.

Board Favourites

The five reactions to know cold

Takeaway. These five equations answer most questions of this chapter.

Memory Pack

Whole topic in three lines

Takeaway. Three lines, the whole classification.

Oxidation and Reduction

Two definitions, one see-saw

Takeaway. Follow the oxygen: gained = oxidised, lost = reduced.

Redox Reactions

Never one without the other

CuO + H₂ → Cu + H₂O: CuO reduced (lost O), H₂ oxidised (gained O) — in the same breath.

Takeaway. Oxidation and reduction are two ends of one exchange.

The Copper Cycle

Brown → black → brown

Takeaway. One dish of copper shows both halves of redox.

Oxidising and Reducing Agents

Who is the agent of what?

Takeaway. Giver of O = oxidiser; taker of O = reducer.

ZnO + C → Zn + CO

The smelting preview

Takeaway. Metal extraction is applied redox.

Corrosion

Metals being eaten alive

Takeaway. Brown iron, black silver, green copper — corrosion's palette.

Rust Needs Air AND Water

Rust's strict recipe

The classic three-test-tube experiment: a nail in moist air RUSTS; a nail in dry air (with anhydrous CaCl₂ absorbing moisture) stays bright; a nail in boiled, oil-sealed water (air expelled and excluded) also stays bright. Conclusion: rusting requires **both oxygen and moisture together**.

Takeaway. Remove either air or water and iron cannot rust.

Preventing Corrosion

Fighting the brown enemy

Takeaway. Every method is one idea: keep O₂ and H₂O off the metal.

Rancidity

When the ghee smells wrong

Old fried snacks, stale butter and long-opened cooking oil all announce it by their 'off' odour.

Takeaway. Rancid = fat + air's oxygen, gone wrong.

Preventing Rancidity

Why the chips packet is a balloon

Takeaway. Block, replace, or chill the oxygen — food stays fresh.

Corrosion vs Rancidity

Two children of the same oxidation

Takeaway. Same enemy (oxidation), different victims, parallel defences.

Oxidised ⇒ Reducing Agent

The agent name-swap trap

Takeaway. Name agents by what they do to the OTHER substance.

How Galvanising Really Works

The sacrificial bodyguard

Zinc on galvanised iron is more reactive than the iron below — so ANY corrosion attacks the zinc first, even at a scratch exposing the iron. The coating is a bodyguard that takes the hit; only after the zinc is consumed does the iron start rusting.

Takeaway. Zinc doesn't just cover iron — it corrodes in its place.

Board Favourites

The reliable four

Takeaway. Four stock answers carry the redox topic's marks.

Memory Pack

Whole topic in three lines

Takeaway. Three lines, the whole redox story.