Question

What is the likely explanation to account for this sluggish reactivity of a marble chip, to a first approx. `CaCO_3`, with hydrochloric acid?

Answer 1

See here.

Explanation:

As you realize, the marble powder should have reacted faster and more completely. The answer is likely that the residual marble powder was insufficiently dry, and occluded a significant mass of water, raising its apparent mass.