To demonstrate that matter is conserved during a chemical change or reaction, the best procedure from the options provided 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 involves a chemical reaction that produces new substances (water and salt) and allows for the measurement of the total mass before and after the reaction, illustrating the principle of conservation of mass in a chemical change. The other options either involve physical changes or processes where some products (like smoke) may escape the system, making it difficult to accurately measure and demonstrate conservation of mass.