Conservation of Mass (Edexcel GCSE Chemistry: Combined Science)

• 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
• A precipitation reaction is one in which two solutions react to form an insoluble solid called a precipitate
• 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
• 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

• If the reaction flask is open and a gaseous product is allowed to escape, then the total mass of the reaction flask will change as product mass is lost when the gas 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)

• Mass will be lost from the reaction flask unless it is closed
• If the mass of a reaction flask is found to increase then it may be due to one of the reactants being a gas found in the air and all of the products are either solids or liquids