In Chemistry, mass is never lost or gained during a chemical reaction. This is known as the law of conservation of mass.

To show this in equations, we use balanced symbol equations, where the number of atoms is the same on both sides.

What is conservation of mass?

• In a chemical reaction, atoms are rearranged, not destroyed
• This means the total mass of reactants = total mass of products
• No atoms are lost or gained – just rearranged into new substances

💡 If a reaction seems to lose mass, it’s often because a gas escapes

💡 If a reaction seems to gain mass, a gas from the air (like oxygen) may have combined with the substance

Example: magnesium + oxygen → magnesium oxide

Word equation:
Magnesium + Oxygen → Magnesium oxide

Symbol equation (unbalanced):
Mg + O₂ → MgO

Balanced:
2Mg + O₂ → 2MgO

Now there are 2 Mg atoms and 2 O atoms on both sides.

Balancing equations step-by-step

1. Write the correct formulas for all substances
2. Count the atoms of each element on both sides
3. Add big numbers in front of formulas to balance the atoms
4. Never change small numbers in formulas – only the big numbers in front

Example: H₂ + O₂ → H₂O

Unbalanced:

• 2 H on the left, 2 H on the right
• 2 O on the left, only 1 O on the right

Balanced:
2H₂ + O₂ → 2H₂O

Now there are 4 H atoms and 2 O atoms on each side.

Another example: Fe + O₂ → Fe₂O₃

Unbalanced:

• Fe = 1 on left, 2 on right
• O = 2 on left, 3 on right

Balanced:
4Fe + 3O₂ → 2Fe₂O₃

Now Fe and O are both balanced.