Conservation of Mass Lab

Do gases have mass? Even at the high school level, we will run into students that don’t believe that gases have mass. This really simple quick lab packs a punch in concept development. Prove the Conservation of Mass Law, and prove that gases do indeed have mass!

Timing: One Class Period

Prior Knowledge:  Proper use of the electronic balance.
Materials:

  • clean 16 oz. water or drink bottle with cap (Note: soft drink bottles work best – see below)
  • water
  • electronic balance
  • small piece of Alka-Seltzer (about ¼ of a tablet)

Purpose:

In this quick lab, students will be able to prove, and actually see, the Law of Conservation of Mass. The mass of the reactants and the mass of the products will be the same! Students will learn the term “closed system” and “open system” which we will be using all year. As a final enrichment, students will remove some of the gas and determine the difference in mass.

Instructions:

1. Students will find and record the mass of the bottle, cap, water, and Alka-Seltzer tablet. Have students fill the bottle about ½ way. By putting the Alka-Seltzer (about ¼ tablet) in the upside-down cap, students can get the mass of all of the system at one time.

2. Have students turn the cap over – drop the tablet into the water, and secure the cap onto the bottle. This procedure reduces error as opposed to having students drop in the tablet and then secure the cap – preventing some of the gas from escaping.

3. Students should get pretty good results with the system massing the same before and after the reaction.

4. When students squeeze the bottle, have them be careful to not let any of the water spill out. Continue to squeeze the bottle until the water level moves into the neck of the bottle. Make sure that students understand that while continuing to squeeze the bottle and the water level up, they need to secure the cap. The bottle will remain squeezed in. They should be able to mass the bottle standing up.

Note: Soft drink bottles work the best for this experiment. Most water bottles do not have a very secure cap, and air will leak back into the bottle.

Discussion:

A few things we want students to get from this experiment are the Law of Conservation of Mass, gases do have mass, and closed system of reaction versus open system of reaction.

Students should be able to identify this as a chemical reaction. The introduction explains how this happens. First the Alka-Seltzer is dissolved and dissociates into ions. This is a physical change. Then, the acid and base react forming sodium citrate, water, and carbon dioxide. This is a chemical reaction.

The data will show that the Law of Conservation of Mass is proved as mass is conserved.

Analysis questions will go into the definitions of closed system and open system of reaction. Students should conclude that this reaction took place in a closed system.

Finally, students should be able to conclude that when they squeezed the bottle after the reaction, air was displaced by water – air was squeezed out. Therefore, the mass of the system will be less. Concluding that gases do have mass.

Post lab questions ask students to apply the Law of Conservation of Mass and the idea of closed and open system.

For the first question, we are looking for students to realize that this reaction is in an open system, and the mass of the wax moves into the surroundings as a gas. In order to prove the Law, we would have to come up with a closed system to contain all products.

The second question is looking for students to apply the law – the mass of the reactants will equal the mass of the products.

The third question is along the same vein, but has students looking at the law in a different way. This is a challenge, but hopefully with discussion, they will come up with a proportion to solve the problem. Even though we haven’t studied stoichiometry yet, they should be able to think through this problem.

This activity is included in our Introduction to Chemistry Unit:

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