Final answer:
The activity that demonstrates chemical weathering is the dissolving of limestone by acid rain, which involves a chemical reaction with acidic water that breaks down the rock.
Step-by-step explanation:
The activity that demonstrates chemical weathering is C dissolving of limestone by acid rain. Chemical weathering involves the breakdown of rocks through chemical reactions, leading to the alteration of the minerals within a rock. The dissolution of limestone involves a chemical reaction with acidic water, which is typically caused by the absorption of carbon dioxide or sulfur and nitrogen oxides creating carbonic, sulfuric, and nitric acids in the rainwater. When acid rain falls, it can react with the minerals in limestone, resulting in the rock slowly dissolving over time. This is in contrast to physical or mechanical weathering, which involves the breakdown of rocks without a change in their chemical composition, such as in option A - grinding of talc into powder, B - freezing of water in the cracks of a sandstone sidewalk, and D - abrasion of a stream bed by tumbling rocks.