Conservation of Mass

• During a chemical reaction no atoms are lost and no atoms are made
• There are the same number and types of atoms on each side of a reaction equation
• No mass is lost or gained-we say that mass is conserved (stays the same) in a chemical reaction
• Chemical reaction


  • 2Li + F2 → 2LiF
• The total mass of all the reactants will be the same as 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 of the equation

  2. Add up the relative formula masses on the right-hand side of the equation

  3. The total mass on the left-hand side of the equation is the same as the total mass on the right-hand side, so mass is conserved
• Conservation of mass


  The idea that you can use to work out the mass of a reactant or product in a reaction
• Calculate the mass of a reactant or product


  1. Know the masses of all the reactants and products except for one

  2. Work out the total mass of everything on one side of the equation

  3. Work out the total mass of everything on the other side of the equation, except for the thing you don't know the mass of

  4. The mass of the thing you don't know is the difference between these two totals
• If the mass of an unsealed reaction container seems to change during a reaction, there's usually a gas involved
• If the mass goes up

  1. One of the reactants is a gas found in air (e.g. oxygen)

  2. The gas atoms become part of the product, which is held inside the reaction container

  3. The total mass of the stuff inside the reaction container goes up
• If the mass goes down

  1. One of the products is a gas

  2. The gas can escape from the reaction container as it's formed

  3. The total mass of the stuff inside the reaction container goes down
• Thermal decomposition


  Reactions where substances are heated down