The best procedure to demonstrate that matter is conserved during a chemical change or reaction is:
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 effectively shows the conservation of mass in a chemical reaction, as it involves a chemical change where reactants (acid and base) yield products (water and salt). By measuring the mass before and after the reaction, you can demonstrate that the total mass remains constant, illustrating the principle of conservation of matter.