During a chemical reaction, no atoms are oestroyed and no atoms are created. This means there are the same number and types of atoms on each side of the equation. Becausc of this, no mass is lost or gained so we say that mass is conserved during a reaction. By adding up all of the relative formula masses of the substances on each side of the equation you can see that mass is conserved.
In some experiments, you might observe a change in mass of an usealed reaction vessel during a reaction. There are usually two explanations for this:
- If the mass increases, it's probably because one of the reactants is a gas that is 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.
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