In a chemical reaction, mass is always conserved

• During a chemical reaction no atoms are destroyed and no atoms are created

• This means there are the same number and types of atoms on each side of a reaction equation

• Because of this, no mass is lost or gained, we say that mass is conserved during a reaction

• By adding up the relative formula masses of the sugbstances on each side of a balanced symbol equation, you can see that mass is conserved

  • The total M1 of all the reactants equals the total M1 of the products

If the mass seems to change, there’s usually a gas involved

• In some experiments you might observe a change of mass of an unsealed reaction vessel during a reaction, there’s two explanations for this

  • If the mass inceases it’s probably because one of the reactants is a gas that’s found in air and all the products are solids, liquids or aqueous

    • Before the reaction, the gas is floating around in the air, It’s there but it’s not contaijed in the reaction vessel so you can’t account for its mass

    • When the gas reacts to form part of the product, it becomes contained inside the reaction vessel so the total mass of the stuff inside the reaction vessel increases

  • If the mass decreases, it’s probably because one of the products is a gas and all the reactants are solids, liquids or aqueous

    • Before the reaction, all the reactants are contained in the reaction vessel