Conservation of Mass | Chemistry for EmSAT Achieve PDF Download

According to the Law of Conservation of Mass, matter is neither created nor destroyed in a chemical reaction. The total mass of the reactants is always equal to the total mass of the products, which is why it is necessary to balance chemical equations.

The sum of the relative atomic or molecular masses of the reactants is equal to the sum of the relative atomic or molecular masses of the products.

In a precipitation reaction, two solutions combine to form an insoluble solid called a precipitate.

If a reaction takes place in a closed system where no substances can enter or leave, the total mass of the system will remain constant.

For instance, when calcium chloride reacts with sodium sulfate, a precipitate of calcium sulfate is formed. If this reaction occurs in a closed system, the mass before and after the reaction will be the same.

• The balanced equation for the reaction 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, the total mass of the reaction flask will change as the mass of the product is lost when the gas leaves the system.
• For instance, when hydrochloric acid reacts with calcium carbonate, carbon dioxide gas is produced:
  2HCl (aq) + CaCO₃ (s) ⟶ CaCl₂ (aq) + H₂O (l) + CO₂ (g)
• Unless the reaction flask is closed, there will be a loss of mass from the system.
• If the mass of a reaction flask is observed to increase, it may be due to one of the reactants being a gas that is present in the air, while all of the products are either solids or liquids.