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.
  • 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 + Na2SO4 ⟶ CaSO4 + 2NaCl

Law of Conservation of Mass, IGCSE & GCSE Chemistry revision notes

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 + CaCO3→ CaCl2+ H2O + CO2

  • 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.

Balancing Equations

  • Chemical equations use the chemical symbols of each reactant and product.
  • When balancing equations, there has to be the same number of atoms of each element on either side of the equation in accordance with the Law of Conservation of Mass.
  • The following nonmetals must be written as molecules: H2, N2, O2, F2, Cl2, Br2 and I2.
  • To balance an equation you work across the equation from left to right, checking one element after another.
  • If there is a group of atoms, for example a nitrate group (NO3) that has not changed from one side to the other, then count the whole group as one entity rather than counting the individual atoms.
  • Examples of chemical equations:
    • Acid-base neutralisation reaction:

NaOH + HCl → NaCl + H2O

    • Redox reaction:

2Fe2O3 + 3C → 4Fe + 3CO2

  • In each equation there are equal numbers of each atom on either side of the reaction arrow so the equations are balanced.

Example 1

Balancing Equation Example 1 Table, IGCSE & GCSE Chemistry revision notes

Example 2

Balancing Equation Example 2 Table, IGCSE & GCSE Chemistry revision notes

AQA GCSE Chemistry Notes

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Morgan Curtin Chemistry

Author: Morgan

Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.