Class 10 Science • Complete Chapter Notes
We observe many changes around us. Some changes are physical, such as a change in shape or state, while others produce new substances with different properties. A chemical change in which one or more new substances are formed is called a chemical reaction.
Chemical reactions are represented in chemistry by using chemical equations. These equations provide a concise way of describing the substances involved and the products formed.
A chemical reaction is often accompanied by observable changes. These observations provide evidence that new substances may have been formed.
A chemical equation represents a chemical reaction using the symbols and formulae of the substances involved.
In a word equation, the names of reactants and products are written in words.
The reaction can then be represented using chemical formulae.
The substances that take part in a chemical reaction are called reactants. The new substances formed are called products.
A chemical equation should be balanced so that the number of atoms of each element is the same on both sides of the equation. This follows the law of conservation of mass.
Atoms are not created or destroyed during a chemical reaction. Therefore, the total number of atoms of each element must remain conserved.
The balanced form is:
State symbols provide information about the physical state of the substances involved in a chemical reaction.
| Symbol | Meaning |
|---|---|
| (s) | Solid |
| (l) | Liquid |
| (g) | Gas |
| (aq) | Aqueous solution |
Conditions such as heat, light or electricity may also be indicated above or below the reaction arrow when required.
A combination reaction is a reaction in which two or more substances combine to form a single product.
Magnesium combines with oxygen to form magnesium oxide.
A decomposition reaction is one in which a single compound breaks down into two or more simpler substances.
A displacement reaction is a reaction in which a more reactive element displaces a less reactive element from its compound.
Zinc is more reactive than copper, so zinc displaces copper from copper sulphate solution.
In a double displacement reaction, ions are exchanged between two compounds to form two new compounds.
Barium sulphate is formed as an insoluble white precipitate.
A reaction in which an insoluble solid separates from a solution is called a precipitation reaction. The insoluble solid is called a precipitate.
A reaction that releases energy, usually in the form of heat, is called an exothermic reaction.
A reaction that requires absorption of energy from the surroundings is called an endothermic reaction.
Thermal decomposition reactions are examples where heat is supplied to bring about decomposition.
In the context used in this chapter, oxidation can involve the addition of oxygen or removal of hydrogen from a substance.
Reduction can involve the removal of oxygen or addition of hydrogen.
Here copper oxide loses oxygen and is reduced to copper, while hydrogen gains oxygen and is oxidised to water.
When oxidation and reduction occur simultaneously in a reaction, the reaction is called a redox reaction.
Corrosion is the gradual deterioration of a metal due to chemical reactions with substances present in the environment.
Iron reacts with oxygen and moisture over time and develops a reddish-brown coating commonly called rust.
Rancidity is the spoilage of food containing fats and oils due mainly to oxidation, producing unpleasant smell and taste.
| Reaction Type | Main Feature | General Form |
|---|---|---|
| Combination | Two or more reactants form one main product. | A + B → AB |
| Decomposition | One compound breaks into simpler substances. | AB → A + B |
| Displacement | A more reactive element replaces a less reactive one. | A + BC → AC + B |
| Double Displacement | Ions are exchanged between two compounds. | AB + CD → AD + CB |
For the current syllabus and textbook-aligned study, students should also refer to official CBSE and NCERT resources.
NCERT – Official Textbooks → CBSE – Class 10 Science Competency-Based Material → CBSE Academic – Curriculum 2026-27 →