Chemistry

Have you ever noticed how the inscriptions on some old tombstones are difficult to read? Weathering causes this problem. Weathering takes place through physical and chemical processes. Physical weathering, a process in which rocks are physically changed, results from abrasion, wetting and drying, freezing and thawing, expansion and contraction, crystallization of minerals, action of organisms, and the growth of plant roots. All of these processes help to break rocks into fragments. Once rocks have undergone physical weathering, they are especially vulnerable to chemical weathering, a process in which the original minerals in rocks change into other substances. A rock that is broken into fragments has more surface area, and therefore more regions that are exposed to chemicals, than a solid rock. In this experiment, you will find out how the surface area of a material affects the rate of chemical weathering.

Rocks and minerals can be worn down over a period by physical weathering. Several physical processes accelerate weathering of rocks and minerals by splitting them into fragments. For example, when water freezes in the cracks and crevices of a rock, it expands, pushing the rock segments apart. The roots of plants can have a similar effect. Since chemical reactions can only occur at the places where solid materials come in contact with liquids or gases, breaking a solid material apart increases the area where interaction can occur. To understand how this works, think about making a peanut butter sandwich from a loaf of bread. If the loaf of bread is not sliced, you can only spread peanut butter on outer surfaces. By cutting a slice of bread, you create a new surface area on which you can spread peanut butter. Each additional slice provides more surface area. In the same way, each "slice" of rock is exposed to chemical interactions.

• acid solution
• hard candies
• hammer
• clock
• 2 small clear cups
• piece of chalk
• hot plate
• six 100-ml beakers
• 2 pieces of steel wool
• science notebook

Part A

1. Fill each cup with dilute acid solution.
2. Place a whole hard candy in cup 1.
3. Crush a second hard candy with a hammer. Place the crushed candy in cup 2.
4. Allow both pieces of candy to remain in the acid solution for 10 minutes.
5. After 10 minutes, examine the amount of candy left in each cup. Note the color of each solution. Record your observations in your science notebook.

Part B

1. Pour water in beakers W1, W2, and W3. Pour 50 ml of dilute acid in beakers A1, A2, and A3.
2. Place a piece of chalk in beakers W1 and A1.
3. Place beakers W2 and A2 on a hot plate and adjust it to medium heat. Place a piece of chalk in each of these beakers.
4. Place a steel wool ball in beakers W3 and A3.
5. Observe all six beakers for 15 minutes. In your science notebook, record any changes in the chalk and steel wool.

1. In this experiment, what materials represent rocks?
2. What is the purpose of crushing a piece of candy in part A, step 3?
3. Based on the results of your experiment, how does surface area affect the rate of weathering?
4. Based on the results of your experiment, which of the following materials would weather the fastest-a jagged piece of rock or a smooth round pebble? Explain your answer.
5. How does temperature affect the rate of chemical weathering?

1. candy, chalk, and steel wool
2. Crushing breaks the material into fragments, which increases the surface area.
3. Increasing surface area increases the rate of weathering.
4. A jagged piece of rock will weather faster than a smooth rock because it will have more surface area.
5. As temperature increases, so does weathering.