Mass and Chemical Change.

You may recall that during one of your lab experiments in the chemistry unit, a solution of sodium carbonate and  solution of calcium chloride were mixed and allowed to react with each other.

You will probably carry out this exeriment at one point as it is a very simple but very important experiment.

Whatch thei YouTube video where they carry out a similar experiment using two different solutions:

When  these two substaces are mixed, a chemical change occurred.  We know that a chemical change occurred because the original substances disappare, a new substace was formed, and a temperature change occurred.


 This chemical reaction between Sodium  Carbonate (aq) & Calcium Chloride (aq) can be written as follows:

 Sodium Carbonate (aq) + Calcium Chloride (aq)  ==> Calcium Carbonate (s) + Sodium Chloride (aq)

 Note: the symbol (aq) means "aqueous" or "in a solution".

The chemicals in left side of the above equation are called the REACTANTS.

The chemicals in right side of the above equation are called the PRODUCTS.

We know this is a chemical change because:

  • A new substance was formed (a white solid precipitate -- called Sodium Carbonate)

  • Energy was being absorbed -- the bag felt cooler after the reaction

  • A colour change was produced from reactants to products

When we measured the masses of the chemicals before they were mixed and after they were allowed to react we found that there was no change.

Therefore we cn say that:

Mass of the Reactants (before the reaction) = Mass of the Products (after the reaction)

The above statement is generally known as the  LAW OF CONSERVATION OF MASS.

This law states that during a chemical reaction, in a closed system, no mass is either gained or lost and the total mass of reactants is the same as the total mass of products.

At the particle level, we can say that the total number of particles involved in the reaction remains constant.