Passage V

When vinegar (a solution of acetic acid, CH₃COOH, in H₂O) is mixed with baking soda (sodium bicarbonate, NaHCO₃), the following reaction occurs:

CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂

A teacher constructed the apparatus shown in Figure 1 for a demonstration. First, 30 mL of vinegar was poured into a bottle. Then, 5 g of baking soda was placed in a bal­loon. The balloon was affixed to the top of the bottle with wire to form an airtight seal.

The apparatus was placed on a balance, and the mass reading was 68.7 g. The balloon was then lifted up so that all the baking soda fell into the bottle. As bubbles formed in the bottle, the balloon inflated and the mass reading on the balance steadily decreased. When the reaction was fin­ished, the mass reading was 68.1 g (see Figure 2).

The teacher asked 3 students to explain what occurred in the demonstration. The students agreed that the balloon inflated because bubbles of CO₂ formed and then exited the reaction mixture. They disagreed about why the mass read­ing on the balance decreased.

Student 1

No gas exited the apparatus because it was airtight. Gases, however, have no mass, so as CO₂ was produced, the mass of the apparatus decreased.

Student 2

Gases have mass, so the mass of the apparatus should not have changed. A balloon, however, will deflate over time because there are tiny pores in its surface. Due to the rapid pressure increase in the apparatus, a measurable mass of CO₂ exited through the pores in the quickly expanding balloon, which caused the mass of the apparatus to decrease.

Student 3

Gases have mass, and although there are tiny pores in a balloon's surface, there was not enough time for a mea­surable mass of gases to have exited the balloon. The air exerted an upward buoyant force on the apparatus that was equal to the weight of the volume of air that was displaced by the apparatus. As the balloon inflated, the volume of the apparatus increased, causing it to displace a greater volume of air. Thus, the buoyant force increased, which caused the mass reading on the balance to decrease. The actual mass of the apparatus did not measurably change.