Chapter 1 Science Lecture 1

1. Chemical Reaction

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

Example:

2Mg + O₂ → 2MgO

Here, magnesium reacts with oxygen to form magnesium oxide.

2. Signs of a Chemical Reaction

A chemical reaction may be identified by:

• Change in state
• Change in colour
• Formation of a gas
• Change in temperature
• Formation of a precipitate (insoluble solid)

Example:

Zn + H₂SO₄ → ZnSO₄ + H₂

3. Chemical Equations

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

Word equation

Magnesium + Oxygen → Magnesium oxide

Chemical equation

Mg + O₂ → MgO

The substances on the left side are called reactants, while those on the right side are called products.

4. Balanced Chemical Equation

A chemical equation is balanced when the number of atoms of each element is equal on both sides of the equation.

❌ Unbalanced:

Mg + O₂ → MgO

✅ Balanced:

2Mg + O₂ → 2MgO

5. How to Balance a Chemical Equation

Example:

Fe + H₂O → Fe₃O₄ + H₂

Step 1

Balance iron:

3Fe + H₂O → Fe₃O₄ + H₂

Step 2

Balance oxygen:

3Fe + 4H₂O → Fe₃O₄ + H₂

Step 3

Balance hydrogen:

3Fe + 4H₂O → Fe₃O₄ + 4H₂

✅ Balanced equation:

3Fe + 4H₂O → Fe₃O₄ + 4H₂

6. State Symbols

State symbols can be written in a chemical equation:

Example:

Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g)

7. Important Examples

Magnesium burning in air

2Mg + O₂ → 2MgO

Magnesium burns with a bright white flame and forms magnesium oxide.

Zinc with dilute sulphuric acid

Zn + H₂SO₄ → ZnSO₄ + H₂

Hydrogen gas is released.

Iron with steam

3Fe + 4H₂O → Fe₃O₄ + 4H₂

Iron reacts with steam to form magnetic iron oxide and hydrogen.

⭐ Quick Revision

Chemical Reaction → Formation of one or more new substances.

Reactants → Substances that take part in a reaction.

Products → New substances formed.

Balanced Equation → Equal number of atoms of every element on both sides.

Main law → Law of Conservation of Mass.

State symbols → (s), (l), (g), (aq).

Most important skill → Learn how to balance chemical equations.