Chapter 1 — Chemical Reactions and Equations

Chapter 1 of the Class 10 Science NCERT textbook, "Chemical Reactions and Equations", explains what chemical reactions are, how to write and balance chemical equations, the major types of chemical reactions (combination, decomposition, displacement, double displacement, and redox), and the everyday effects of oxidation such as corrosion and rancidity.

• Representing reactions with balanced equations — A chemical reaction rearranges atoms into new substances, so it is written as a symbolic equation. Balancing it — making each element's atom count equal on both sides — is required by the law of conservation of mass, since no atoms are created or destroyed.
• Classifying the five reaction types — Reactions are grouped by how reactants combine: combination builds one product, decomposition splits one substance apart, displacement swaps a more reactive element in, double displacement exchanges ions, and redox transfers oxygen or hydrogen between species.
• Energy changes and oxidation in daily life — Reactions either release heat (exothermic, like respiration) or absorb it (endothermic, like electrolysis). Oxidation of materials over time explains familiar problems such as the corrosion of metals and the rancidity of stored fats and oils.

1. 01A chemical reaction is indicated by changes in state, colour, temperature, or evolution of a gas
2. 02Chemical equations must be balanced to satisfy the law of conservation of mass — atoms of each element are equal on both sides
3. 03Combination reactions form one product from two or more reactants; decomposition reactions are the reverse — one substance breaks into two or more
4. 04Displacement reactions occur when a more reactive element displaces a less reactive one from its compound; double displacement reactions involve exchange of ions between two compounds
5. 05Exothermic reactions release heat (e.g. respiration, burning of natural gas); endothermic reactions absorb energy (e.g. electrolysis of water, decomposition by heat or light)
6. 06Oxidation is the gain of oxygen or loss of hydrogen; reduction is the loss of oxygen or gain of hydrogen — both occur simultaneously in redox reactions; corrosion and rancidity are everyday oxidation effects