Class 10 Science Chemical Reactions and Equations Notes

1. Chemical Reactions: Meaning and Characteristics

A chemical reaction is a process in which one or more substances undergo a chemical change to form new substances with different properties.

Examples from everyday life

• Milk turning sour.
• Iron articles rusting in moist air.
• Fermentation of grapes.
• Cooking food.
• Digestion of food.
• Respiration in living organisms.

How can we identify a chemical reaction?

The following observations may indicate that a chemical reaction has taken place:

• Change in state: A substance changes from one physical state to another.
• Change in colour: A visible colour change occurs.
• Evolution of gas: A gas is released during the reaction.
• Change in temperature: Heat may be released or absorbed.

2. Chemical Equations

A chemical equation represents a chemical reaction using chemical symbols and formulae.

Word equation and chemical equation

Important terms

• Reactants: Substances that take part in a chemical reaction. They are written on the left-hand side (LHS).
• Products: New substances formed during a reaction. They are written on the right-hand side (RHS).
• Arrow (→): Indicates the direction of the reaction.
• Plus sign (+): Separates two or more reactants or products.
• Skeletal equation: An equation written using correct chemical formulae but not yet balanced.
• Balanced equation: An equation in which the number of atoms of each element is equal on both sides.

Symbols used for physical states

Reaction conditions such as heat, sunlight, pressure and catalysts may be written above or below the reaction arrow.

3. Balanced Chemical Equations

The law of conservation of mass states that mass can neither be created nor destroyed in a chemical reaction. Therefore, the total number of atoms of each element must remain the same before and after the reaction.

Steps for balancing equations: Hit-and-Trial Method

1. Write the correct chemical formulae of the reactants and products.
2. Count the atoms of each element on both sides.
3. Choose an element and balance its atoms by changing coefficients.
4. Continue balancing the remaining elements.
5. Check the number of atoms of every element on both sides.
6. Use the smallest whole-number coefficients.
7. Add physical-state symbols and reaction conditions if required.

Worked example

Balance the following skeletal equation:

Step 1: Balance oxygen. There are four oxygen atoms in Fe₃O₄, so place 4 before H₂O.

Step 2: Balance hydrogen. There are eight hydrogen atoms on the left, so place 4 before H₂.

Step 3: Balance iron. There are three iron atoms on the right, so place 3 before Fe.

Final balanced equation with state symbols:

4. Combination Reactions and Exothermic Reactions

Combination reaction

A combination reaction is a reaction in which two or more substances combine to form a single product.

General form:

Important examples

1. Formation of slaked lime:

Calcium oxide (quicklime) reacts with water to form calcium hydroxide (slaked lime), releasing heat.

2. Burning of coal:

3. Formation of water:

4. Burning of natural gas:

Exothermic reaction

An exothermic reaction releases heat or energy during the reaction.

Examples include burning fuels, the reaction of quicklime with water and respiration.

Respiration as an exothermic process

During respiration, glucose reacts with oxygen and releases energy.

Whitewashing reaction

Calcium hydroxide reacts slowly with carbon dioxide in the air to form calcium carbonate, which gives walls a shiny finish.

Calcium carbonate is also the main chemical component of marble.

5. Decomposition Reactions

A decomposition reaction is a reaction in which a single reactant breaks down into two or more simpler substances.

General form:

Decomposition reactions are the opposite of combination reactions.

A. Thermal decomposition

When a substance decomposes due to heat, the reaction is called thermal decomposition.

1. Calcium carbonate:

Calcium carbonate (limestone) decomposes to form calcium oxide (quicklime) and carbon dioxide.

2. Lead nitrate:

Brown fumes of nitrogen dioxide are released on heating lead nitrate.

3. Ferrous sulphate:

Green ferrous sulphate crystals change on heating, producing ferric oxide and sulphur oxides.

B. Electrolytic decomposition

When electricity causes a compound to decompose, the reaction is called electrolytic decomposition.

During electrolysis of water, hydrogen and oxygen gases are produced. The volume of hydrogen collected is twice the volume of oxygen under the same conditions.

C. Photochemical decomposition

When a compound decomposes in the presence of light, the reaction is called photochemical decomposition.

White silver chloride turns grey in sunlight due to the formation of silver.

These reactions are used in traditional black-and-white photography.

Endothermic reaction

An endothermic reaction absorbs energy from its surroundings. Decomposition reactions often require heat, light or electricity to break down the reactant.

6. Displacement Reactions

A displacement reaction occurs when a more reactive element displaces a less reactive element from its compound.

Reaction between iron and copper sulphate

When an iron nail is placed in copper sulphate solution:

• The blue colour of copper sulphate solution fades as iron sulphate forms.
• A brown coating of copper appears on the iron nail.
• Iron displaces copper from copper sulphate because iron is more reactive than copper.

Other examples

7. Double Displacement and Precipitation Reactions

A double displacement reaction occurs when two compounds exchange ions to form two new compounds.

Reaction between sodium sulphate and barium chloride

A white insoluble solid called barium sulphate is formed.

Precipitation reaction

A precipitation reaction is a reaction in which an insoluble solid, called a precipitate, forms from a solution.

Another important example

Lead iodide forms a yellow precipitate.

8. Oxidation, Reduction and Redox Reactions

Oxidation and reduction: definitions

Example 1: Oxidation of copper

Copper gains oxygen and forms black copper(II) oxide. Therefore, copper is oxidised.

Example 2: Reduction of copper oxide

• Copper oxide loses oxygen and is reduced to copper.
• Hydrogen gains oxygen and is oxidised to water.

When oxidation and reduction occur simultaneously in the same reaction, the reaction is called a redox reaction.

Example 3

Carbon is oxidised to carbon monoxide, while zinc oxide is reduced to zinc.

9. Effects of Oxidation in Everyday Life

A. Corrosion

Corrosion is the gradual damage of a metal due to reactions with substances in its surroundings, such as moisture and acids.

• Iron: develops a reddish-brown coating called rust.
• Silver: develops a black coating.
• Copper: develops a green coating.

Corrosion damages vehicles, bridges, iron railings, ships and other metal structures.

Why is paint applied to iron articles?

Paint prevents iron from coming into direct contact with air and moisture, thereby reducing rusting.

B. Rancidity

Rancidity occurs when fats and oils in food are oxidised, causing changes in smell and taste.

Methods to prevent rancidity:

• Add antioxidants to foods containing fats and oils.
• Store food in airtight containers.
• Keep food away from excessive heat and exposure to air.
• Manufacturers may flush packets of chips with nitrogen to reduce oxidation.

10. CBSE Board Exam Preparation: Important Areas

Practise these question types while revising the chapter. They cover the key concepts and skills presented in the NCERT chapter.

How to identify reaction types quickly

Important NCERT-style practice prompts

1. Why should magnesium ribbon be cleaned before burning in air?
2. What is a balanced chemical equation? Why must chemical equations be balanced?
3. Write and balance equations for combination and decomposition reactions.
4. Differentiate between displacement and double displacement reactions with examples.
5. Why is respiration considered an exothermic reaction?
6. Why are decomposition reactions called the opposite of combination reactions?
7. Explain oxidation and reduction in terms of gain or loss of oxygen.
8. Why do we paint iron articles?
9. Why are food packets containing fats and oils flushed with nitrogen?
10. Explain corrosion and rancidity with examples.

⚡ Quick Revision: Chapter at a Glance

• Chemical reaction: Formation of new substance(s).
• Signs of reaction: Change in state, colour or temperature, or evolution of gas.
• Reactants: Left-hand side; products: right-hand side.
• Balanced equation: Equal numbers of atoms of each element on both sides.
• Balancing rule: Change coefficients, never subscripts in chemical formulae.
• Combination: Two or more substances form one product.
• Decomposition: One substance breaks into simpler substances.
• Exothermic: Releases energy.
• Endothermic: Absorbs energy.
• Displacement: A more reactive element replaces a less reactive one.
• Double displacement: Two compounds exchange ions.
• Precipitation: An insoluble solid is formed.
• Oxidation: Gain of oxygen or loss of hydrogen.
• Reduction: Loss of oxygen or gain of hydrogen.
• Redox: Oxidation and reduction occur together.
• Corrosion: Gradual damage to a metal due to its surroundings.
• Rancidity: Oxidation of fats and oils changes food's smell and taste.

For better board-exam performance, revise definitions, practise balancing equations, learn the key observations and solve sample papers under timed conditions.