Conservation of Mass (WJEC GCSE Chemistry: Combined Science)

Re-arranging atoms

• When chemicals react together, there is a re-arrangement of atoms as the reactants form one or more products
• This re-arrangement typically causes a change in properties, e.g.:

The reaction of sodium and chlorine

The properties of sodium chloride are quite different from sodium and chlorine

• Looking at the atoms involved in any chemical reaction, the same number of atoms are present at the start and end of the reaction
  • This requires the use of a balanced chemical equation 
• For example, the reaction of sodium with chlorine:

The atoms in the reaction of sodium with chlorine

There are the same number of sodium and chlorine atoms on both sides of the chemical equation

• • In the balanced chemical equation, there are:
    • 2 atoms of sodium and 2 atoms of chlorine on the reactants side
    • 2 atoms of sodium and 2 atoms of chlorine on the products side
  • This shows that there are the same number of each atom throughout the chemical reaction
  • The only thing that has changed is how the atoms are arranged

The Law of Conservation of Mass

• The Law of Conservation of Mass follows the principles shown in how atoms re-arrange during a chemical reaction
• It 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

Demonstrating conservation of mass

• A precipitation reaction is one where 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
• The balanced chemical equation is:

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

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

Diagram showing the conservation of mass

The mass at the start and end of the reaction is the same

• If the reaction flask is open and a gaseous product is allowed to escape, then the total mass of the reaction flask will decrease
  • This is because the gas is lost from the system to the surroundings 
• 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