Here's why:

* Chemical weathering involves a change in the chemical composition of the rock. In this case, the acid (usually carbonic acid formed from dissolved carbon dioxide in rainwater) reacts with the calcium carbonate in limestone, breaking it down into dissolved calcium ions and bicarbonate ions.

* Physical weathering, on the other hand, involves the breakdown of rock into smaller pieces without changing its chemical composition. Examples include frost wedging, abrasion, and root wedging.

So, because the limestone's chemical makeup is altered during the reaction with acid, it falls under chemical weathering.