Passage IV

A teacher placed 50 mL of Liquid A at 20°C in a buret (see Figure 1). A buret is a graduated tube with a stopcock. Liquid flows out of the buret when the stopcock is opened.

The stopcock was opened and the flow time, the time it took for 20 mL of the liquid to flow out of the buret, was measured and found to be 7 sec. The procedure was repeated with Liquid B, which had a flow time of 14 sec.

The teacher asked 3 students to try to explain why the liquids had different flow times.

Student 1

Liquid B drained more slowly than Liquid A because it is denser than Liquid A. As a liquid flows, some of its molecules bump into each other. This causes the molecules to lose speed in the direction of flow, slowing the overall flow of the liquid. Since molecules of a denser liquid are closer together than are molecules of a less dense liquid, collisions occur more often in the denser liquid. Thus, if 2 liquids are at the same temperature, the less dense liquid will always flow more easily.

Student 2

Liquid B drained more slowly than Liquid A because it has a greater molecular mass (the mass of each mole­cule) than Liquid A. Consider 2 objects of different mass. More force is required to move the object with the greater mass. Thus, if 2 liquids are at the same temperature, the liquid with the smaller molecular mass will always flow more easily.

Student 3

Liquid B drained more slowly than Liquid A because it has a larger molecular volume (the volume occupied by each molecule) than Liquid A. Large molecules readily bump into each other when a liquid flows, forming molecu­lar "logjams" that slow down the overall flow. Thus, if 2 liquids are at the same temperature, the liquid with the smaller molecular volume will always flow more easily.

Table 1 gives the density, molecular mass (in atomic mass units, amu), and molecular volume (in nm³; 1 billion nm = 1 m) for several liquids at 20°C.