Science Fair Project: Where Does the Mass Go?

Does Mass Change in a Reaction?

1For the Westbrook Middle School science fair, seventh graders Amara Osei and Jonah Feldman asked a question: when a chemical reaction makes a gas, does the total mass change? They used fizzing antacid tablets, which contain baking soda and citric acid. When a tablet drops into water, these ingredients react and form new substances, including carbon dioxide gas, which makes the bubbles.

2Their teacher, Mr. Lindqvist, approved the plan and watched every trial. Each trial used 100 g of room-temperature water and one tablet of the same kind. In the open setup, the water was in a plastic cup. In the closed setup, the water was in a plastic bottle, and a balloon holding the tablet was stretched over the bottle's mouth. Lifting the balloon tipped the tablet into the water.

3For each trial, the students placed everything (container, water, tablet and, for the closed setup, the balloon) on a digital scale that reads to 0.1 g. They recorded the mass, started the reaction, waited until the fizzing stopped and recorded the mass again. They tested each setup three times. Their results are in Table 1.

4At the fair, a judge described a test her own class had done. They dampened a pad of fine steel wool, which is mostly iron, and left it in an open dish for a week. The steel wool turned reddish brown as rust formed, and the dish and steel wool together had gained mass.

5To explain their results, Amara and Jonah built a bead model of a different reaction: natural gas, or methane, burning in a stove. Each bead stood for one atom. They started with one methane molecule (CH₄) and two oxygen molecules (O₂). Then they pulled the beads apart and rebuilt them into one carbon dioxide molecule (CO₂) and two water molecules (H₂O), the products of the reaction. Table 2 in Section 2 shows their bead count.