3.1 Conservation of mass
Why mass is conserved in chemical reactions, how this links to balanced equations, and why the mass seems to change in open systems when a gas is given off or taken in.
The law of conservation of mass
Key terms
- Law of conservation of mass: no atoms are lost or made during a chemical reaction, so the total mass of the products equals the total mass of the reactants.
- Closed system: a system that substances cannot enter or leave.
In a reaction, atoms are only rearranged. This is why a balanced symbol equation has the same number of each type of atom on both sides.
Example: 2Mg + O2 → 2MgO
48 g of magnesium reacts with 32 g of oxygen to make 48 + 32 = 80 g of magnesium oxide.
Closed systems
If nothing can get in or out, the mass does not change. For example, in a sealed flask:
copper sulfate(aq) + sodium hydroxide(aq) → copper hydroxide(s) + sodium sulfate(aq)
A blue precipitate (an insoluble solid) forms, but the reading on the balance stays the same.
Open systems: apparent changes in mass
Mass is still conserved, but in an open container the mass on the balance can seem to change:
- Mass decreases when a gas is given off and escapes into the air. For example, heating calcium carbonate (CaCO3 → CaO + CO2) or reacting marble chips with acid: the carbon dioxide escapes.
- Mass increases when a reactant is a gas from the air. When magnesium burns, oxygen atoms from the air join on to make magnesium oxide (2Mg + O2 → 2MgO), so the solid is heavier than the magnesium.
In particle terms: gas particles move quickly and spread out, so they escape from an open container (or join the product from the air) without being weighed.
Calculating a missing mass
Example: 10.0 g of calcium carbonate is heated and 5.6 g of calcium oxide is left.
Mass of carbon dioxide given off = 10.0 − 5.6 = 4.4 g
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