Question

select the correct answer.
which of these actions is an example of chemical weathering?
a. freezing water widens the cracks in rocks.
b. plant roots force their way into cracks in rocks and break them.
c. oxygen in air reacts with iron content in rocks, causing them to change color.
d. strong winds peel soft rock away, leaving harder rock behind.

Explanation:

• Option A: Freezing water widening rock cracks is physical weathering (freeze - thaw action), no chemical change.
• Option B: Plant roots breaking rocks is biological weathering, due to mechanical force from roots, no chemical reaction.
• Option C: Oxygen reacting with iron in rocks (like rusting, a chemical reaction: $4Fe + 3O_2 = 2Fe_2O_3$) causes a chemical change, so it's chemical weathering.
• Option D: Wind eroding soft rock is physical weathering (wind abrasion), no chemical change.

Answer:

C. Oxygen in air reacts with iron content in rocks, causing them to change color