Passage VI

Demonstration

A science teacher poured 400 mL of water into a beaker and heated the beaker with a Bunsen burner. After a few minutes, bubbles began to form in the water and float to the surface. Four students were asked to explain how the bubbles were formed and also to describe the composition of the bubbles.

Student 1

The chemical formula of water is H₂O. As H₂O is heated, the chemical bonds between the hydrogen (H) and oxygen (O) atoms break, leaving individual H and O atoms. The H atoms dissolve in the remaining water, while the O atoms combine to form oxygen molecules (O₂). The breakdown of water and the formation of O₂ can be summarized by the following balanced chemical equations:

H₂O2H + O

O + OO₂

Because O₂ is a gas, it forms bubbles in the water, and the bubbles contain O₂ only.

Student 2

Student 1 is correct about how the chemical bonds in H₂O break down and how O₂ molecules are formed. However, the H atoms do not dissolve in the remaining water; rather, they combine to form hydrogen molecules (H₂), as shown in the following equation:

H + HH₂

Because both O₂ and H₂ are gases, bubbles are formed in the water, and the bubbles contain a mixture of O₂ and H₂ only.

Student 3

The chemical bonds in water do not break when water is heated. Instead, water molecules form new chemical bonds—called hydrogen bonds—with other water molecules. As more and more hydrogen bonds form, a thin circular sheet of hydrogen-bonded water molecules is formed. When this sheet comes in contact with the surface of the water, air pressure pushes down on the center of the sheet, causing it to bow in the middle and close up on itself, forming a spherical bubble (see Figure 1). The bubbles contain air (a mixture of gases) only.

Student 4

As the water is heated, chemical bonds are neither broken nor formed. Instead, the water molecules absorb the heat energy, and this energy is converted into kinetic energy. Once the kinetic energy reaches a certain threshold, the water begins to change from the liquid phase to the gas phase, forming bubbles that contain water vapor only.