To demonstrate that matter is conserved during a chemical change or reaction, the best procedure would be:
Find the mass of an acid and a base. Combine them to produce water and a salt. Find the mass of the water and salt.
This procedure explicitly involves a chemical reaction and allows you to compare the total mass of the reactants (the acid and base) with the total mass of the products (the water and salt). According to the law of conservation of mass, the mass before and after the reaction should be the same, demonstrating that matter is conserved during the chemical change.