Models and Conflicting Viewpoints

Passage III — The candle and the jar (conflicting viewpoints)

A teacher stood a short candle in the middle of a shallow dish of water and lit it. She then lowered an upside-down glass jar over the candle so that the rim of the jar was below the surface of the water. After several seconds, the flame went out, and water from the dish rose into the jar, eventually filling about one-tenth of the jar’s volume. Two students explained the observation.

Student 1

The flame uses up the oxygen (O₂) in the air inside the jar. Removing the O₂ leaves less gas in the jar, so the gas pressure inside the jar drops below the air pressure outside, and the outside air pushes water up into the jar. Because air is about 21% O₂ by volume, the water would fill about one-fifth of the jar if all of the O₂ were used up. The water fills less than that because the flame goes out before all of the O₂ is gone. The water rises steadily while the candle burns.

Student 2

Burning the candle uses O₂, but it also produces carbon dioxide (CO₂) and water vapor, which nearly replace the O₂ that is used. Burning by itself therefore changes the amount of gas in the jar very little. Instead, the flame heats the air in the jar. As the jar is lowered over the flame, the heated air expands, and some of it escapes as bubbles from under the rim. After the flame goes out, the remaining gas cools and contracts, the gas pressure inside the jar drops below the air pressure outside, and the outside air pushes water up into the jar. Most of the rise happens after the flame goes out.

Passage IV — How desert pavements form (conflicting viewpoints)

In many deserts, the ground is covered by a single layer of closely packed pebbles and stones called a desert pavement. Directly beneath the stones there is usually a layer of fine silt and clay, often 10 to 50 cm thick. Three scientists explain how desert pavements form.

Scientist 1

Desert pavements form by deflation, the removal of fine particles by wind. The original ground surface was a mixture of stones and fine particles. Over thousands of years, wind blew the fine particles away, while the stones, too heavy to be moved, were left behind and became concentrated at the surface as the ground was lowered. Once the stones were packed closely enough to shelter the fine particles between them, the wind could no longer remove particles, and the pavement stopped changing. Below the pavement, the ground is still the original mixture of stones and fine particles that the wind never reached.

Scientist 2

The stones have been at the surface since the pavement began to form. Wind carries dust into the desert from dry lake beds and other sources. The dust settles among the stones, and rain washes it down into the spaces beneath them. As dust collects beneath the stones, it lifts them, so the stones ride upward on top of a thickening layer of dust and are never buried. The fine layer under a pavement is therefore made of windblown dust that arrived after the stones, and the pavement slowly grows upward.

Scientist 3

The fine layer was part of the original soil, and the stones were once scattered through it. The soil contains clay that swells when it is wet and shrinks when it dries. Each time the clay swells, it pushes the stones in it upward by a tiny amount; when the clay shrinks, fine particles fall into the gap left beneath each stone, so the stone cannot settle back. After many thousands of cycles of wetting and drying, the stones are moved to the surface, leaving a layer below that has no stones.