Chapter 1 · Introduction

Chapter 1 — Chemical Reactions and Equations

How to recognise a chemical change, write it as a balanced equation, sort reactions into types, and understand the oxidation that rusts iron and spoils food.

Every day you watch chemistry happen without naming it. The LPG flame under the tawa, the curd setting overnight, the antacid fizzing in a glass of water, the brown flakes on an old bicycle, the potato chips that taste stale once the packet has been open for a week — in each, substances are turning into new substances. Cooks, farmers, doctors and engineers all need to know which changes will happen, how much of each substance takes part, and how to speed a change up or stop it.

This chapter gives you the language and the first big ideas for all of that. You will learn to recognise a chemical change from what you can see, to write it compactly as a chemical equation, and to balance that equation so that it respects the law of conservation of mass. Then you will sort reactions into a handful of families — combination, decomposition, displacement, double displacement — and meet oxidation and reduction, the pair of processes behind burning, respiration, rusting and rancidity.

Signs of a change→Word equation→Skeletal equation→Balanced equation→Types of reaction→Oxidation & reduction→Corrosion & rancidity
🎯 By the end of this chapter you can…
  • recognise a chemical change from evolution of a gas, change in colour, change in temperature, formation of a precipitate or change in state;
  • write word equations and skeletal equations, and balance them by the hit-and-trial method, adding state symbols and reaction conditions;
  • explain why equations must be balanced, using the law of conservation of mass;
  • classify reactions as combination, decomposition (thermal, electrolytic, photolytic), displacement or double displacement, and as exothermic or endothermic;
  • identify oxidation, reduction, oxidising agents and reducing agents in terms of gain or loss of oxygen or hydrogen;
  • explain corrosion and rancidity and describe how they are prevented.
Teaser — What Tells You a Reaction Is Happening?

Pick a classic school experiment and watch it run. The thermometer and the checklist on the right record each sign of a chemical change as it appears. Which experiments show more than one sign?

temperature (illustrative)—
signs observed—
new substance formed—

Sections in this chapter

1.1

Chemical Reactions and Balanced Chemical Equations

Signs of a chemical change, word and skeletal equations, conservation of mass, balancing, state symbols and conditions.

$\mathrm{3Fe + 4H_{2}O \rightarrow Fe_{3}O_{4} + 4H_{2}}$
Open section →1.2

Types of Chemical Reactions

Combination, decomposition, displacement and double displacement; exothermic and endothermic reactions.

$\mathrm{CaO + H_{2}O \rightarrow Ca(OH)_{2}}$
Open section →1.3

Oxidation and Reduction in Everyday Life

Gain and loss of oxygen or hydrogen, redox reactions, corrosion and rancidity.

$\mathrm{CuO + H_{2} \xrightarrow{\text{heat}} Cu + H_{2}O}$
Open section →
✅ Before you start
You need the Class 9 ideas of atoms, molecules and chemical formulas: element symbols, valency, and how to write formulas such as $\mathrm{MgO}$, $\mathrm{Al_{2}O_{3}}$ and $\mathrm{Ca(OH)_{2}}$. If you are unsure, try the readiness check and the formula builder in the welcome page first.
← Welcome to Class 10 Chemistry1.1 Chemical Reactions and Balanced Chemical Equations →