Conservation of Mass (OCR GCSE Chemistry A (Gateway))

The Law of Conservation of Mass

The Law of Conservation of Mass

• The Law of Conservation of Mass states that no matter is lost or gained during a chemical reaction.

• Mass is always conserved, therefore the total mass of the reactants is equal to the total mass of the products, which is why all chemical equations must be balanced

• The sum of the relative atomic/molecular masses of the reactants will be the same as the sum of the relative atomic/ molecular masses of the products, taking into account the stoichiometry (balancing)

• Changes of state do not affect the conservation of mass, but can appear to do so if gaseous products are allowed to escape

• If the reaction flask is closed and no other substance can enter or leave the system, then the total mass of the reaction flask will remain constant

  • This can be demonstrated using many reactions including precipitation reactions, in which two solutions react to form an insoluble solid called a precipitate

  • For example, the reaction between calcium chloride and sodium sulfate produces a precipitate of calcium sulfate.

  • If carried out in a closed system then the mass before and after the reaction will be the same

  • The balanced equation is:

CaCl₂ (aq) + Na₂SO₄ (aq) ⟶ CaSO₄ (s) + 2NaCl (aq)

Diagram showing the conservation of mass in a precipitation reaction

Explaining Changes in Mass

• Some chemical reactions may appear to involve a change in mass due to the presence of a gaseous reactant or product

• If the reaction flask is open (a non-enclosed system) and a gaseous product is allowed to escape, then the total mass of the reaction flask will decrease as product mass is lost when the gas particle leaves the system

  • For example, the reaction between hydrochloric acid and calcium carbonate produces carbon dioxide gas:

2HCl (aq) + CaCO₃ (s) ⟶ CaCl₂ (aq) + H₂O (l) + CO₂ (g)

• Another example of mass loss occurs when metal carbonates thermally decompose. Carbon dioxide is produced and the gas escapes into the atmosphere, leaving the metal oxide as the only solid product and an apparent decrease in mass as the CO₂ cannot be weighed

• If the mass of a reaction container is found to increase then it is probably due to one of the reactants being a gas found in the air and all of the products are either solids or liquids

  • For example, when a metal reacts with oxygen the mass of the oxide produced is greater than the mass of the metal you started with

  • An illustration of this is the reaction of magnesium with oxygen which produces magnesium oxide

2Mg (s) + O₂ (g) → 2MgO (s)

• If you leave some iron wool to rust in a flask the mass will increase as iron(III) oxide has a larger mass than iron

4Fe (s) + 3O₂ (g) → 2Fe₂O₃ (s)