Mass the Gas

In this demo, baking soda and vinegar are mixed in a plastic storage bag. The reaction container is massed before and after the reaction for students to see the Law of Conservation of Mass in action.

NaHCO3 (s) + CH3COOH (aq) →CO2 (g) + H2O (l) + CH3COONa (aq)

– chemical change
– evolution of a gas
– Law of Conservation of Mass
– pressure

– baking soda
– small paper or plastic cups
– Ziploc bag, quart or gallon size
– vinegar
– 10 mL graduated cylinder
– scale/balance

1. Add ~1 g of baking soda (NaHCO3) to Cup 1.
2. Add ~10 mL of vinegar (CH3COOH) to Cup 2.
3. Place both cups into the plastic bag and seal.
4. Carefully mass the whole reaction container and record.
5. Add Cup 2 to Cup 1

Bags and cups can all be rinsed and reused. The bag needs to be sealed well, as any gas that escapes causes mass to be lost. This also makes a quick lab for students to do.

Safety goggles should be worn at all times.


This lesson plan‘s topic is the Introduction of Chemical Reactions. This lesson introduces students to the symbols associated with reactions, using this PowerPoint, and has a short inquiry style lab shows students the Law of Conservation of Mass. Mass the Gas is the short inquiry style lab in this lesson.