Conservation of mass

• In a Chemical Reaction, Mass is Always Conserved
• There are the same number and types of atoms on each side of a reaction equation
• During a chemical reaction no atoms are destroyed and no atoms are created
• By adding up the relative formula masses of the substances on each side of a balanced symbol equation, you can see that mass is conserved
• The total mass of the reactants equals the total mass of the products
• Show that mass is conserved in this reaction: 2Li + F₂→2LiF


  1. Add up the relative formula masses on the left-hand side: 2x M(Li)+2x M(F)=52

  2. Add up the relative formula masses on the right-hand side: 2x M(LiF)=52

  3. The total mass on the left hand side equals the total mass on the right hand side, so mass is conserved
• If the Mass Seems to Change, There's Usually a Gas Involved
• If the mass increases


  It's probably because one of the reactants is a gas that's found in air (e.g. oxygen) and all the products are solids, liquids or aqueous
• If the mass decreases


  It's probably because one of the products is a gas and all the reactants are solids, liquids or aqueous
• Never, ever forget that, in a reaction, the total mass of reactants is the same as the total mass of products