Changed: How Does Oxygen Weather a Rock?

Problem

How does oxygen weather a rock?

Materials

• cup
• tap water
• rubber gloves
• lemon-size steel wool pad without soap (found at stores that carry painting supplies)
• saucer
• clear plastic drinking glass

Procedure

CAUTION: Steel wool can splinter. Wear rubber gloves when handling steel wool.

1. Half-fill the cup with water.
2. Put on the rubber gloves.
3. Dip the steel wool pad into the cup of water. Hold the steel wool above the cup and allow the excess water to drain into the cup.
4. Place the moistened steel wool on the saucer.
5. Invert the glass and stand it in the saucer so that it covers all of the steel wool.
6. Each day for 5 days, put on the rubber gloves, pick up the steel wool, and rub the wool between your fingers.



7. Place the saucer where it will not be disturbed for 5 days.

Results

Each day, more of the steel wool turns reddish brown and crumbles when touched.

Why?

Oxygen in the air combines with the iron in the steel wool pad to form iron oxide, commonly called rust. The rust weakens the structure of the steel wool, causing it to fall apart when touched. Since humidity speeds up the rusting process, the glass in this experiment is used to hold moist air around the steel wool.

The chemical process in which oxygen combines with other substances is called oxidation. When oxygen combines with materials in rocks, the compounds formed, such as rust, weaken the structure of the rocks, making them more likely to weather.

Rocks that contain iron often have yellow, orange, or reddish brown colors. Moist air combines with the iron at the surface of these rocks to form iron oxide, and the rocks eventually crumble away as did the steel wool. This breaking down of rock by a change in its chemical composition is called chemical weathering.

Let's Explore

1. Would the iron rust in the same time with no moisture? Repeat the experiment, but this time do not moisten the steel wool.
2. How would acid rain (rain with a higher than normal amount of acid) affect the rusting of iron? Acid rain is caused when rain reacts with acid gases from automobile exhausts and factories. Repeat the original experiment, adding cup (63 ml) of white vinegar to the water. Make observations as often as possible every day for 3 days or until no further changes occur. Science Fair Hint: Keep a written record of observations and take photographs of each experiment to show the results as part of a project display.

Show Time!

1. Water can chemically weather rock by dissolving minerals out of the rock. Demonstrate the dissolving effect of rain on rocks by placing 6 or more sugar cubes, which represent rocks, in a bowl. Place the bowl in a shallow baking pan. Ask an adult to use the point of a pencil to punch three small holes in the bottom of a paper cup, spacing the holes as far apart as possible. Hold your hand over the bottom of the cup while a helper fills it with water, which represents rain. Immediately place the cup about 12 inches (30 cm) above the sugar cubes in the bowl. Remove your hand and observe the effect of the water on the sugar cubes.



2. Rain chemically weathers all rocks, but usually the change is very slow. Much of the weathering of statues and buildings made of rocks is chemical weathering due to acid rain. Rocks, such as marble, that contain carbonates weather quickly by acid. The acid combines with the carbonate to produce a gas. The weathering of marble by acid rain can be demonstrated by placing marble chips (found at plant nurseries) inside a glass jar. Fill the jar with white vinegar, a mild acid. Observe and record the effect of the acid on the rocks as often as possible for 1 to 2 days, or until no further changes are seen.

Check it Out!

Water dissolves more substances than any other liquid. Use an earth science text to find out more about chemical weathering by water. What acid is formed when carbon dioxide in air dissolves in water? What kinds of rock can this acid dissolve? How are caverns created?

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Janice VanCleave's Rocks and Minerals
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