Chemical Reactions & Equations Topic Hub

Chemical Reactions & Equations — Class 10 Notes

CBSE 2027 — dates, rules and official papers →
Chemistry · Ch 1Class 10 · CBSE 2026–27High weightage

Chemical Reactions and Equations

What the board actually asks: balance a skeletal equation and add the physical-state symbols (s)/(l)/(g)/(aq); identify the reaction type (combination, decomposition, displacement, double displacement/precipitation, or oxidation-reduction); classify a decomposition by its energy source (heat, light or electricity); say which substance is oxidised and which is reduced; and explain corrosion, the everyday effect of oxidation on metals. The balanced equation IS the answer in most questions here.

NCERT VERBATIM sourced, word-for-wordAUTHORED LazyTopper teaching scaffolding
The Big Idea AUTHORED

New substances form - and we record them as a balanced equation.

A chemical reaction makes one or more new substances; you notice it as a change in state, colour or temperature, or a gas being given off. We write the change as a chemical equation and then balance it, because atoms are only rearranged, never created or destroyed. Almost every reaction in this chapter sorts into one of five types, and many are also redox reactions - oxygen or hydrogen changing hands. Most marks sit in writing the correct balanced equation with the right classification.

Formulae
Combination
(two or more reactants a single product)
Tip: Two or more reactants, ONE product.
Decomposition
(one reactant two or more products)
Tip: Needs heat, light or electricity.
Displacement
(A more reactive than B)
Tip: The more reactive element takes the place.
Double displacement
(ions exchanged)
Tip: Ions exchange; look for a precipitate.
Oxidation / Reduction
Oxidation: gain O or lose H · Reduction: lose O or gain H
Tip: Oxidised = gains oxygen or loses hydrogen.
Conservation of mass
atoms of each element (reactants) = atoms of each element (products)
Tip: Count atoms of each element on both sides.
Everyday oxidation
Corrosion (rusting of iron) - prevented by painting, oiling, greasing or galvanising
Tip: Corrosion colours: iron gives reddish-brown rust; silver gets a black coating; copper a green coating.

How to Balance a Chemical Equation

Steps
(1) Write the skeletal equation with the correct formulae. (2) Count the atoms of each element on both sides. (3) Balance one element at a time by hit and trial, changing only the numbers in front of formulae, never the formulae themselves. (4) Check that each element has the same number of atoms on both sides. (5) Add the state symbols and conditions.
State Symbols and Conditions
Write the physical state after each formula: (s) solid, (l) liquid, (g) gas, (aq) aqueous solution. Write conditions such as heat, catalyst, sunlight or electric current over the arrow. General word equations take no state symbols. Examiners usually reward a balanced equation and correct state symbols and conditions separately.
Exothermic and Endothermic Reactions
In an exothermic reaction heat is released: burning of natural gas, respiration, decomposition of vegetable matter into compost, and quicklime reacting with water. In an endothermic reaction energy is absorbed.
Thermal, Electrolytic and Photolytic Decomposition
By heat:
By electricity:
By light:
Precipitation Reaction
A reaction that forms an insoluble product, called a precipitate, is a precipitation reaction. For example, sodium sulphate solution and barium chloride solution form white barium sulphate: .
Colour Changes in Reactions (Observations)
Iron nail in copper sulphate: the blue solution turns pale green and the nail becomes brownish. Silver chloride: white, turns grey in sunlight. Copper powder heated in air turns black (copper(II) oxide). Lead nitrate on heating gives brown fumes of nitrogen dioxide. Crystals of ferrous sulphate are green.

Shortcuts & memory tricks →

Definitions the board rewards NCERT VERBATIM

State these in NCERT’s exact words

Balanced chemical equation
NCERT verbatim — memorise this exact wording· Ch 1,
“A chemical equation is balanced so that the numbers of atoms of each type involved in a chemical reaction are the same on the reactant and product sides of the equation.”
What it means: The same number of atoms of every element on the left and the right of the arrow - it must obey the law of conservation of mass.
Combination reaction
NCERT verbatim — memorise this exact wording· Ch 1,
“Such a reaction in which a single product is formed from two or more reactants is known as a combination reaction.”
What it means: Two or more reactants join to give a single product, e.g. calcium oxide + water gives slaked lime.
Decomposition reaction
NCERT verbatim — memorise this exact wording· Ch 1,
“a single reactant breaks down to give simpler products. This is a decomposition reaction.”
What it means: The opposite of combination - one reactant breaks down into two or more simpler products, usually needing heat, light or electricity.
Displacement reaction
NCERT verbatim — memorise this exact wording· Ch 1,
“When an element displaces another element from its compound, a displacement reaction occurs.”
What it means: A more reactive element pushes a less reactive one out of its compound - iron displaces copper from copper sulphate solution.
Double displacement reaction
NCERT verbatim — memorise this exact wording· Ch 1,
“Such reactions in which there is an exchange of ions between the reactants are called double displacement reactions.”
What it means: The two reactants swap their ions/partners; an insoluble product often drops out as a precipitate.
Oxidation and reduction
NCERT verbatim — memorise this exact wording· Ch 1,
“if a substance gains oxygen or loses hydrogen during a reaction, it is oxidised. If a substance loses oxygen or gains hydrogen during a reaction, it is reduced.”
What it means: Oxidation adds oxygen (or removes hydrogen); reduction removes oxygen (or adds hydrogen). When one substance is oxidised and the other reduced in the same reaction it is a redox reaction.
Redox reaction (oxidation-reduction)
NCERT verbatim — memorise this exact wording· Ch 1,
“one reactant gets oxidised while the other gets reduced during a reaction. Such reactions are called oxidation-reduction reactions or redox reactions.”
What it means: A reaction in which one substance is oxidised and, at the same time, another substance is reduced.
Exothermic reaction
NCERT verbatim — memorise this exact wording· Ch 1,
“Reactions in which heat is released along with the formation of products are called exothermic chemical reactions.”
What it means: A reaction that gives out heat to the surroundings - respiration, combustion, and quicklime added to water are examples.
Endothermic reaction
NCERT verbatim — memorise this exact wording· Ch 1,
“Reactions in which energy is absorbed are known as endothermic reactions.”
What it means: A reaction that takes in energy from the surroundings - most decomposition reactions are endothermic.
Corrosion
NCERT verbatim — memorise this exact wording· Ch 1,
“When a metal is attacked by substances around it such as moisture, acids, etc., it is said to corrode and this process is called corrosion.”
What it means: The slow eating-away of a metal by air, moisture or acids - the rusting of iron is the everyday example.
Key terms — NCERT verbatim (state word-for-word)
Skeletal chemical equation
Such a chemical equation is a skeletal chemical equation for a reaction. (Ch 1, )
Precipitation reaction
Any reaction that produces a precipitate can be called a precipitation reaction. (Ch 1, )
Must-know concepts AUTHORED FRAMING

…and where they’re tested

Writing and balancing an equation
Write the reactants on the left and products on the right of an arrow, using chemical formulae. Balance by the hit-and-trial method - start with the compound that has the most atoms and change only the coefficients, never the formula itself. Finally add state symbols (s), (l), (g), (aq) to make the equation informative.
3-mark: balance a skeletal equation and write it with correct state symbols.
Combination reactions
Two or more reactants combine to form a single product, for example calcium oxide + water gives slaked lime, or magnesium burns in air to give magnesium oxide. Combination reactions are usually exothermic - heat is given out.
NCERT Fig 1.1Burning of a magnesium ribbon in air and collection of magnesium oxide in a watch-glass.
A combination (and redox) reaction: magnesium burns with a dazzling white flame; . Labels: tong, magnesium ribbon, burner, watch-glass, magnesium oxide.
identify a combination reaction, write the equation, and state whether it is exothermic.
Decomposition reactions
A single reactant breaks down into two or more simpler products. The energy needed is supplied as heat (thermal, e.g. limestone to quicklime), light (photolytic, e.g. silver chloride in sunlight), or electricity (electrolytic, e.g. electrolysis of water). Decomposition reactions are usually endothermic.
NCERT Fig 1.6Electrolysis of water.
Electrolytic decomposition: passing electricity through acidified water gives hydrogen and oxygen in a 2:1 volume ratio. Labels: plastic mug, oxygen, hydrogen, test tube, water, graphite rod, rubber stopper, anode, cathode, switch, 6 V battery.
classify a decomposition by its energy source (heat/light/electricity) and write one equation for each.
Displacement reactions
A more reactive metal displaces a less reactive metal from its salt solution, for example iron displaces copper from copper sulphate solution. The iron nail turns brownish and the blue colour of the copper sulphate solution fades.
reactivity comparison plus the colour changes observed with the iron nail and copper sulphate.
Double displacement and precipitation
The reactants exchange ions; when an insoluble salt forms it drops out as a precipitate, for example sodium sulphate + barium chloride gives a white precipitate of barium sulphate. Any reaction that produces a precipitate is a precipitation reaction.
NCERT Fig 1.9Formation of barium sulphate and sodium chloride.
A double displacement (precipitation) reaction: mixing sodium sulphate and barium chloride solutions gives a white precipitate of barium sulphate. Labels: test tube containing solution of sodium sulphate, test tube containing solution of barium chloride.
name the precipitate and write the balanced equation with state symbols.
Oxidation-reduction (redox)
In a redox reaction one substance is oxidised (gains oxygen or loses hydrogen) while the other is reduced (loses oxygen or gains hydrogen) at the same time. For example, when hydrogen is passed over heated copper(II) oxide, CuO is reduced to copper and hydrogen is oxidised to water.
state which substance is oxidised and which is reduced, with the reason.
Effects of oxidation in everyday life
Corrosion is the slow attack of a metal by moisture and acids in its surroundings - the rusting of iron is the commonest example, and it damages railings, bridges, ships and car bodies.
2-mark: define corrosion with an example; name two ways to prevent the rusting of iron.
Worked examples — real solved problems NCERT VERBATIM

Solved exactly as the source does them

Every problem is a real sourced item, quoted verbatim with its page; the step-by-step decoding and mark-logic are LazyTopper-authored coaching.

Ch 1, Example Balancing worked example (Steps I-VII, Eq. 1.10),
“[authored prompt] Balance the skeletal equation Fe + H2O -> Fe3O4 + H2 by the hit-and-trial method and write it with physical states.”
Count the atoms.LHS: Fe 1, H 2, O 1. RHS: Fe 3, H 2, O 4 - the skeletal equation is unbalanced.
½ mark
Start with the biggest formula. has 4 oxygen atoms, so balance oxygen by putting 4 before .
Concept: start with the most atoms
½ mark
Balance hydrogen.There are now 8 H atoms on the left, so put 4 before on the right.
½ mark
Balance iron.There are 3 Fe atoms on the right, so put 3 before Fe on the left.
½ mark
Add physical states.Water is used here as steam, so the complete balanced equation is:
1 mark
Mark-scheme logic (authored): CBSE 3-mark: the correct coefficients reached stage by stage (2 - about half a mark per balanced element/step) plus a fully balanced equation carrying correct state symbols (1). Never change a formula - only the coefficients.
Ch 1, Example Activity 1.4, Eq. 1.13,
“[authored prompt] Quicklime (calcium oxide) is added to water and the beaker becomes warm. Write the balanced equation and classify the reaction.”
Ch 1, Example Eqs. 1.20 and 1.22; Activity 1.7 (Fig. 1.6),
“[authored prompt] Give one decomposition reaction each that is brought about by heat, by light, and by electricity, naming the type in each case.”
Ch 1, Example Activity 1.9, Eq. 1.24,
“[authored prompt] An iron nail is left in copper sulphate solution for some time. Write the balanced equation, name the reaction type, and state the two colour changes observed.”
Ch 1, Example Activity 1.10, Eq. 1.27,
“[authored prompt] Sodium sulphate solution is mixed with barium chloride solution. State what is observed, write the balanced equation with state symbols, and classify the reaction.”
Ch 1, Example Eq. 1.29,
“[authored prompt] In the reaction CuO + H2 -> Cu + H2O, identify the substance that is oxidised and the one that is reduced, giving a reason.”
Problem-solving strategies AUTHORED
Balance first.Draw a box around each formula and change only the coefficients, never the subscripts. Start with the compound that has the most atoms, then work through the other elements.
Which type is it?Count reactants and products - one product means combination; one reactant means decomposition; a single element swapped out means displacement; ions exchanged between two compounds means double displacement.
Redox?Track the oxygen (or hydrogen): whichever substance gains oxygen or loses hydrogen is oxidised; the other is reduced.
Complete equation asked?Add the state symbols (s), (l), (g), (aq) and write the condition - Heat, Sunlight or Electricity - above the arrow when the question wants a complete equation.
Common mistakes that cost marks

Where students lose easy marks

Changing a formula to balance an equation - e.g. writing instead of putting a 4 before . Only the coefficients may be changed, never the subscripts inside a formula.
Leaving out the state symbols (s)/(l)/(g)/(aq) when the question asks for a complete or a balanced-with-states equation.
Confusing displacement (a single element is displaced) with double displacement (ions are exchanged between two compounds).
Reversing oxidation and reduction. Remember it by the oxygen: gain of oxygen is oxidation, loss of oxygen is reduction.
Calling a precipitation reaction a combination reaction. A white insoluble product means a double displacement (precipitation) reaction, not a combination.
Formulae
Combination
(two or more reactants a single product)
Tip: Two or more reactants, ONE product.
Decomposition
(one reactant two or more products)
Tip: Needs heat, light or electricity.
Displacement
(A more reactive than B)
Tip: The more reactive element takes the place.
Double displacement
(ions exchanged)
Tip: Ions exchange; look for a precipitate.
Oxidation / Reduction
Oxidation: gain O or lose H · Reduction: lose O or gain H
Tip: Oxidised = gains oxygen or loses hydrogen.
Conservation of mass
atoms of each element (reactants) = atoms of each element (products)
Tip: Count atoms of each element on both sides.
Everyday oxidation
Corrosion (rusting of iron) - prevented by painting, oiling, greasing or galvanising
Tip: Corrosion colours: iron gives reddish-brown rust; silver gets a black coating; copper a green coating.

How to Balance a Chemical Equation

Steps
(1) Write the skeletal equation with the correct formulae. (2) Count the atoms of each element on both sides. (3) Balance one element at a time by hit and trial, changing only the numbers in front of formulae, never the formulae themselves. (4) Check that each element has the same number of atoms on both sides. (5) Add the state symbols and conditions.
State Symbols and Conditions
Write the physical state after each formula: (s) solid, (l) liquid, (g) gas, (aq) aqueous solution. Write conditions such as heat, catalyst, sunlight or electric current over the arrow. General word equations take no state symbols. Examiners usually reward a balanced equation and correct state symbols and conditions separately.
Exothermic and Endothermic Reactions
In an exothermic reaction heat is released: burning of natural gas, respiration, decomposition of vegetable matter into compost, and quicklime reacting with water. In an endothermic reaction energy is absorbed.
Thermal, Electrolytic and Photolytic Decomposition
By heat:
By electricity:
By light:
Precipitation Reaction
A reaction that forms an insoluble product, called a precipitate, is a precipitation reaction. For example, sodium sulphate solution and barium chloride solution form white barium sulphate: .
Colour Changes in Reactions (Observations)
Iron nail in copper sulphate: the blue solution turns pale green and the nail becomes brownish. Silver chloride: white, turns grey in sunlight. Copper powder heated in air turns black (copper(II) oxide). Lead nitrate on heating gives brown fumes of nitrogen dioxide. Crystals of ferrous sulphate are green.
Shortcuts & Tricks
Order of marks: skeletal equation → balanced → state symbols/conditions. Examiners usually reward a balanced equation and correct state symbols separately.
Safe in written answers
After balancing, count each element on both sides once more.
MCQ / check only: don't write this as working
A new insoluble solid formed from two solutions ⇒ precipitation (double displacement).
MCQ / check only: don't write this as working
Decomposition by heat (thermal); by light (silver salts); by electricity (electrolysis).
Safe in written answers
In one reaction one substance is oxidised while another is reduced (CuO is reduced, H₂ is oxidised). Use the Class 10 definition: oxygen/hydrogen.
Safe in written answers
If the test tube warms up it is exothermic.
MCQ / check only: don't write this as working
Memory Tricks
Jodo, Todo, Hatao, Badlo.
The four types. Jodo = combination · Todo = decomposition · Hatao = displacement · Badlo = double displacement (the NCERT order 1.2.1–1.2.4).
Oxidation = Oxygen Aaye ya Hydrogen Jaye. Reduction = Oxygen Jaye ya Hydrogen Aaye.
Class 10 redox definition. Oxidation: gain O or lose H. Reduction: lose O or gain H. (No electron-transfer rule.)
Chuna-patthar tapish se, chaandi ka namak roshni se, paani bijli se.
Which energy for which example. CaCO₃ → heat · AgCl → light · H₂O → electricity.
Exo = Energy bahar, Endo = Energy andar.
Heat out / heat in. exothermic releases; endothermic absorbs.
Neela gaya, Hara aaya; keel ka rang bhura.
CuSO₄ + Fe. blue copper sulphate → green iron sulphate; the iron nail turns brownish.
Barium + sulphate = safed gaadha.
BaSO₄ precipitate is white. Ba + SO₄ → white precipitate.
Mindmap
      • Reactants Products (arrow)
      • Skeletal vs balanced equation
      • Balance: change coefficients only
      • State symbols (s)(l)(g)(aq)
      • Combination (A+B AB)
      • Decomposition (AB A+B)
      • Displacement (A+BC AC+B)
      • Double displacement (ion swap)
      • Precipitation (insoluble salt)
      • Heat - thermal ()
      • Light - photolytic ()
      • Electricity - electrolytic (water)
      • Oxidation = gain O / lose H
      • Reduction = lose O / gain H
      • (redox)
      • Exothermic - heat given out (respiration)
      • Endothermic - energy absorbed (most decompositions)
      • Corrosion - rusting of iron
      • Prevented by paint, oil, grease, galvanising

For Chemistry the subject-adaptive third tab holds the chapter's key balanced equations - the formal core you must be able to reproduce, grouped by reaction type. Every equation is a real NCERT Ch 1 reaction; the condition sits above/beside the arrow.

Combination - slaked lime
Quicklime + water slaked lime; strongly exothermic
Combination - burning of coal
Carbon burns in oxygen
Combination - formation of water
Hydrogen burns in oxygen
Exothermic - respiration
Glucose is broken down with oxygen in the cells; exothermic
Thermal decomposition - ferrous sulphate
Green crystals lose colour; smell of burning sulphur (Activity 1.5)
Thermal decomposition - limestone
Limestone quicklime; used in the cement industry
Thermal decomposition - lead nitrate
Brown fumes of nitrogen dioxide given off (Activity 1.6)
Photolytic decomposition - silver chloride
White silver chloride turns grey in sunlight; used in photography
Photolytic decomposition - silver bromide
Behaves like silver chloride; used in black-and-white photography
Electrolytic decomposition - water
Electrolysis of acidified water; : collected in a 2:1 volume ratio (Activity 1.7, Fig. 1.6 - the balanced equation is the standard form of this printed activity, not a printed equation)
Displacement - iron and copper sulphate
Iron is more reactive than copper; blue solution fades, nail turns brown
Displacement - zinc and copper sulphate
Zinc is more reactive than copper
Double displacement - precipitation of barium sulphate
White precipitate of barium sulphate (insoluble)
Redox - oxidation of copper
Copper gains oxygen (oxidised); black coating of copper oxide forms
Redox - reduction of copper oxide
CuO is reduced (loses oxygen); is oxidised (gains oxygen)

3 competency questions from the CBSE board pattern, with the full step-marked solution for each — the marks are shown per step, as the official scheme awards them.

1 markSection A

A single displacement reaction is represented below: PQ + R → PR + Q. Which of the following is true about the reactants and products?

  1. (A) R forms the cation in the product, and PR is more stable than PQ.
  2. (B) R forms the cation in the product, and PR is less stable than PQ.
  3. (C) R forms the anion in the product, and PR is more stable than PQ.
  4. (D) R forms the anion in the product, and PR is less stable than PQ.
Step-marked solution
  1. [0.5 mark] Single displacement: R displaces Q from PQ ⟹ R bonds with P. For R to displace Q, R must form a more stable compound (PR) than PQ.
  2. [0.5 mark] R in original was free element; after reaction in PR, P is the cation (metal) and R becomes the anion (gained electrons from displacement). So R = anion, PR more stable.
1 markSection A

Which of the following observations is correct for the experimental diagram in which copper metal is heated strongly in air?

  1. (A) Red-coloured copper is reduced to black-coloured copper(I) oxide
  2. (B) Red-coloured copper is oxidized to red-coloured copper(I) oxide
  3. (C) Red-coloured copper is reduced to black-coloured copper(II) oxide
  4. (D) Red-coloured copper is oxidized to black-coloured copper(II) oxide
Step-marked solution
  1. [0.5 mark] When copper is heated strongly in air, it combines with atmospheric oxygen — this is an oxidation reaction: 2 Cu + O2 → 2 CuO.
  2. [0.5 mark] The product CuO (copper(II) oxide) is BLACK in colour. Reddish-brown copper is converted to black CuO; the metal undergoes oxidation (gain of oxygen).
5 marksSection D

An iron nail is placed in each of three test tubes: (A) tap water, (B) boiled distilled water covered with a layer of oil, (C) anhydrous calcium chloride with dry air. After a week only the nail in tube A has rusted. (i) What is rusting, chemically? (ii) Explain the result in tube B. (iii) Explain the result in tube C. (iv) What TWO conditions does this experiment prove are necessary for rusting? (v) Suggest one method to protect the nail.

Step-marked solution
  1. [1 mark] Rusting is the slow oxidation of iron in moist air to form hydrated iron(III) oxide (rust), 4Fe + 3O2 + xH2O -> 2Fe2O3.xH2O.
  2. [1 mark] Tube B: boiling removes dissolved air (oxygen) and the oil layer keeps fresh air out, so no oxygen is available and the nail does not rust.
  3. [1 mark] Tube C: anhydrous calcium chloride absorbs moisture, so the air is dry; with no water the nail does not rust.
  4. [1 mark] The experiment proves that BOTH air (oxygen) and moisture (water) are necessary for rusting; only tube A had both.
  5. [1 mark] Protection: coat the nail — e.g. galvanising (zinc coat), painting, oiling or greasing — to keep out air and moisture.
Practise Chemical Reactions and Equations competency questions

Read NCERT Chapter 1 on LazyTopper (jesc101)

Official NCERT chapter PDF (jesc101) — ncert.nic.in

Sources: NCERT 2026-27 (tap to view)
Source ledger PROVENANCE

Source edition: NCERT Reprint 2026-27, Class 10 Science, Ch 1. One row per sourced item — every verbatim, example and NCERT figure traces to its page.

ItemTypeSource
Balanced chemical equationverbatimNCERT 2026-27 Ch1
Combination reactionverbatimNCERT 2026-27 Ch1
Decomposition reactionverbatimNCERT 2026-27 Ch1
Displacement reactionverbatimNCERT 2026-27 Ch1
Double displacement reactionverbatimNCERT 2026-27 Ch1
Oxidation and reductionverbatimNCERT 2026-27 Ch1
Redox reaction (oxidation-reduction)verbatimNCERT 2026-27 Ch1
Exothermic reactionverbatimNCERT 2026-27 Ch1
Endothermic reactionverbatimNCERT 2026-27 Ch1
CorrosionverbatimNCERT 2026-27 Ch1
Skeletal chemical equationverbatimNCERT 2026-27 Ch1
Precipitation reactionverbatimNCERT 2026-27 Ch1
Balancing a skeletal equationexampleNCERT 2026-27 Ch1 Steps I-VII, Eq. 1.10,
Combination + energy classificationexampleNCERT 2026-27 Ch1 Activity 1.4, Eq. 1.13,
Decomposition by energy sourceexampleNCERT 2026-27 Ch1 Eqs. 1.20/1.22, Activity 1.7, -9
Displacement + observationsexampleNCERT 2026-27 Ch1 Activity 1.9, Eq. 1.24,
Double displacement / precipitationexampleNCERT 2026-27 Ch1 Activity 1.10, Eq. 1.27,
Identify oxidised and reducedexampleNCERT 2026-27 Ch1 Eq. 1.29,
NCERT Fig 1.1figureNCERT 2026-27 Ch1 Fig 1.1
NCERT Fig 1.6figureNCERT 2026-27 Ch1 Fig 1.6
NCERT Fig 1.9figureNCERT 2026-27 Ch1 Fig 1.9

Chemical Reactions and Equations · spec-rendered note · NCERT Reprint 2026-27, Class 10 Science, Ch 1

Practise writing answers the way examiners mark them → Guide