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🧪 Conservation of Mass and Balanced Equations

Spec 5.3.1.1 📗 Foundation
📖 In-Depth Theory

The Law of Conservation of Mass

The LAW OF CONSERVATION OF MASS states:
In a chemical reaction, the TOTAL MASS OF REACTANTS equals the TOTAL MASS OF PRODUCTS.
Mass is always CONSERVED — it is neither created nor destroyed.
Why? Because atoms are simply REARRANGED during a chemical reaction:
No atoms are gained or lost.
The same atoms that were in the reactants end up in the products, just in different arrangements.
Since mass depends only on the number and type of atoms present, and these don't change, total mass stays the same.
Example:
Magnesium + oxygen → magnesium oxide
2Mg + O₂ → 2MgO
If 24 g of Mg reacts with 16 g of O₂ → 40 g of MgO is produced.
24 + 16 = 40 ✓ Mass is conserved.

Balancing Chemical Equations

A BALANCED equation has the same number of each type of atom on both sides — reflecting conservation of mass.
RULES:
1. Write the correct formulae for reactants and products (do NOT change formulae).
2. Count atoms on each side.
3. Add LARGE NUMBERS (coefficients) in front of formulae to balance — never change subscripts within a formula.
4. Check all atoms balance.
Example — balancing H₂ + O₂ → H₂O:
Left: 2H, 2O. Right: 2H, 1O. Oxygen unbalanced.
Add coefficient 2 in front of H₂O: H₂ + O₂ → 2H₂O
Left: 2H, 2O. Right: 4H, 2O. Hydrogen now unbalanced.
Add coefficient 2 in front of H₂: 2H₂ + O₂ → 2H₂O
Left: 4H, 2O. Right: 4H, 2O. ✓ Balanced.
Example — iron + oxygen → iron oxide:
Fe + O₂ → Fe₂O₃
4Fe + 3O₂ → 2Fe₂O₃ ✓
Left: 4Fe, 6O. Right: 4Fe, 6O. ✓

Word Equations and Symbol Equations

WORD EQUATIONS name the reactants and products:
Magnesium + hydrochloric acid → magnesium chloride + hydrogen
SYMBOL EQUATIONS use chemical formulae — more precise, more useful:
Mg + 2HCl → MgCl₂ + H₂
Symbol equations should include STATE SYMBOLS:
Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)
(s) = solid, (l) = liquid, (g) = gas, (aq) = aqueous (dissolved in water)
IMPORTANT: The coefficients in a balanced equation tell us the RATIO in which substances react and are produced — not the number of grams, but the number of formula units (atoms, molecules or formula units for ionic compounds).
2H₂ + O₂ → 2H₂O means:
2 molecules of H₂ react with 1 molecule of O₂ to give 2 molecules of H₂O.
OR: 2 moles H₂ + 1 mole O₂ → 2 moles H₂O.
⚠️ Common Mistake

Never change the SUBSCRIPT numbers in a formula when balancing — only add or change the large numbers (coefficients) IN FRONT of formulas. Changing H₂O to H₂O₂ would create a different compound (hydrogen peroxide, not water). Always balance by adjusting coefficients only.

📌 Key Note

Conservation of mass: atoms are rearranged, never created or destroyed → total mass reactants = total mass products. Balance equations by adjusting coefficients only — never change chemical formulae. Check all atoms balance on both sides.

🎯 Matching Activity — Balance these Equations

Match each unbalanced equation to the correct balanced version. — drag the symbols on the right to match the component names on the left.

Na + Cl₂ → NaCl
Drop here
H₂ + O₂ → H₂O
Drop here
Fe + O₂ → Fe₂O₃
Drop here
Ca + H₂O → Ca(OH)₂ + H₂
Drop here
4Fe + 3O₂ → 2Fe₂O₃ (4 Fe and 6 O each side)
Ca + 2H₂O → Ca(OH)₂ + H₂ (need 2 water molecules)
2H₂ + O₂ → 2H₂O (need 2 H₂ and 2 H₂O to balance all atoms)
2Na + Cl₂ → 2NaCl (need 2 Na to balance the 2 Cl)
🎯 Test Yourself
Question 1 of 3
1. Why must chemical equations be balanced?
2. 24 g of magnesium reacts completely with oxygen to form magnesium oxide. 40 g of magnesium oxide is produced. How much oxygen reacted?
3. Which of these is a correctly balanced equation?
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