Chemical Reactions and Equations — Detailed Notes

Chapter 1 | NCERT Class 10 Science

1. Introduction

Reaction signs — Signs of a chemical reaction

Daily changes such as milk turning sour, iron rusting, fermentation of grapes, cooking, digestion, and respiration involve chemical reactions: the identity of the starting substances changes.

How do we know a reaction has occurred? From activities (Mg burning; Zn + acid; lead nitrate + KI), any of these observations indicates a chemical reaction:

change in state
change in colour
evolution of a gas
change in temperature

Better Method / Strategy
In “give reason / observe” questions, name the observable change first, then write the equation.

1.1 Chemical Equations

Word equations

Magnesium burns in oxygen to form magnesium oxide:

$Magnesium + Oxygen \to Magnesium oxide$

Reactants (LHS): substances that change
Products (RHS): new substances formed
Arrow shows direction of the reaction

Formula (skeletal) equations

$Mg + O_{2} \to MgO$

If atom counts on LHS and RHS differ, the equation is unbalanced (skeletal). Mass would appear not conserved.

Balancing — law of conservation of mass

Mass (and number of atoms of each element) is the same before and after a reaction. Balance by placing whole-number coefficients outside formulae; never change subscripts.

Already balanced example:

$Zn + H_{2} SO_{4} \to ZnSO_{4} + H_{2}$

Worked balance — iron + steam (NCERT):

$Fe + H_{2} O \to Fe_{3} O_{4} + H_{2} (skeletal)$

Box each formula; do not alter insides.
Start with the formula having most atoms ( $Fe_{3} O_{4}$ ): balance O → $4 H_{2} O$ .
Balance H → $4 H_{2}$ on RHS.
Balance Fe → $3 Fe$ on LHS.

$3 Fe (s) + 4 H_{2} O (g) \to Fe_{3} O_{4} (s) + 4 H_{2} (g)$

This trial method is called the hit-and-trial method.

Physical states and conditions

Symbol	Meaning
$(s)$	solid
$(l)$	liquid
$(g)$	gas
$(a q)$	aqueous solution

Example with conditions:

$CO (g) + 2 H_{2} (g) 340 atm CH_{3} OH (l)$

Photosynthesis (conditions above/below arrow): chlorophyll, sunlight.

Better Method / Strategy
Include states only when the question asks or when state clarifies the chemistry (e.g. steam as $H_{2} O (g)$ ).

1.2 Types of Chemical Reactions

Reaction types — Types of chemical reactions

Chemical reactions involve breaking and making of bonds; atoms are not created or destroyed.

1.2.1 Combination reaction

Two or more reactants form one product.

$CaO (s) + H_{2} O (l) \to Ca (OH)_{2} (a q) + Heat$

Other examples: $C + O_{2} \to CO_{2}$ ; $2 H_{2} + O_{2} \to 2 H_{2} O$ .

Exothermic reactions release heat with the products. Examples: burning of natural gas; respiration; composting of vegetable matter.

$CH_{4} (g) + 2 O_{2} (g) \to CO_{2} (g) + 2 H_{2} O (g)$

$C_{6} H_{12} O_{6} (a q) + 6 O_{2} (a q) \to 6 CO_{2} (a q) + 6 H_{2} O (l) + energy$

Whitewashing link: slaked lime reacts with $CO_{2}$ to form shiny $CaCO_{3}$ (same formula as marble):

$Ca (OH)_{2} (a q) + CO_{2} (g) \to CaCO_{3} (s) + H_{2} O (l)$

1.2.2 Decomposition reaction

One reactant breaks into two or more products — opposite of combination. Energy (heat, light, or electricity) is usually needed → often endothermic.

Thermal decomposition:

$2 FeSO_{4} (s) Heat Fe_{2} O_{3} (s) + SO_{2} (g) + SO_{3} (g)$

$CaCO_{3} (s) Heat CaO (s) + CO_{2} (g)$

$2 Pb (NO_{3})_{2} (s) Heat 2 PbO (s) + 4 NO_{2} (g) + O_{2} (g)$

Electrolysis of water (dilute $H_{2} SO_{4}$ ): $H_{2}$ and $O_{2}$ ; volume of $H_{2}$ is double that of $O_{2}$ .

Photochemical decomposition (photography):

$2 AgCl (s) Sunlight 2 Ag (s) + Cl_{2} (g)$

$2 AgBr (s) Sunlight 2 Ag (s) + Br_{2} (g)$

Mixing $Ba (OH)_{2}$ with $NH_{4} Cl$ is a useful endothermic classroom example (mixture cools).

1.2.3 Displacement reaction

A more reactive element displaces a less reactive element from its compound.

$Fe (s) + CuSO_{4} (a q) \to FeSO_{4} (a q) + Cu (s)$

Blue $CuSO_{4}$ fades; iron nail becomes brownish (copper deposit).

Also: $Zn + CuSO_{4}$ ; $Pb + CuCl_{2}$ (Zn, Pb more reactive than Cu).

1.2.4 Double displacement reaction

Exchange of ions between reactants. If an insoluble salt forms → precipitation reaction.

$Na_{2} SO_{4} (a q) + BaCl_{2} (a q) \to BaSO_{4} (s) + 2 NaCl (a q)$

White $BaSO_{4}$ precipitate from $Ba^{2 +}$ and $SO_{4}^{2 -}$ .

Lead nitrate + KI → yellow PbI₂ (also double displacement / precipitation).

1.2.5 Oxidation and reduction (redox)

Heating copper powder: black $CuO$ forms (oxygen added → oxidised).

$2 Cu + O_{2} Heat 2 CuO$

Passing $H_{2}$ over heated $CuO$ :

$CuO + H_{2} Heat Cu + H_{2} O$

$CuO$ loses O → reduced
$H_{2}$ gains O → oxidised

NCERT Class 10 definition (O/H focus):

Oxidation: gain of oxygen or loss of hydrogen
Reduction: loss of oxygen or gain of hydrogen

Other redox: $ZnO + C \to Zn + CO$ ; $MnO_{2} + 4 HCl \to MnCl_{2} + 2 H_{2} O + Cl_{2}$ .

1.3 Effects of Oxidation in Everyday Life

Corrosion

Metal attacked by moisture, acids, etc.

Iron → reddish-brown rust
Silver → black coating
Copper → green coating

Damages structures; costly to replace (details continued in Chapter 3).

Rancidity

Oxidation of fats and oils → unpleasant smell/taste. Prevention: antioxidants; airtight containers; flushing chip bags with nitrogen.

Exam-Oriented Analysis

Aspect	Detail
Weight	Core chemistry chapter; frequent short questions
Question types	Balance; type of reaction; exo/endo; redox identify; corrosion/rancidity reasons
Common mistakes	Changing formulae while balancing; calling all ion-exchange “displacement”; confusing oxidised/reduced
Scoring tips	Box formulae; count atoms twice; name type + write one equation

Question-type table

Style	Typical marks	What examiners look for
Balance equation	1–2	Correct coefficients; unchanged formulae
Identify type	1–2	Correct category + equation
Oxidised/reduced	2	Clear gain/loss of O or H for each species
Exothermic/endothermic	2	Definition + example
Corrosion / rancidity	1–2	Link to oxidation + prevention

Better Method / Strategy
For “identify type”: ask (1) one product or many? (2) element replacing element or ions swapping? (3) O/H transferred?