You are given a background paragraph that describes one or more causal or qualitative relationships such as a relationship in economics or a scientific law and a story that makes use of the concepts or the relationship described in the provided paragraph. You are also given a question about the story that requires an understanding of the relationship described in the background paragraph and the story. You need to come up with an answer to the given question; the answer will be a span from either the question or the story. In order to correctly answer the given question, you need to understand the relationship mentioned in the background paragraph and should be able to use it to understand that in the story. Your answer can not consist of any word that is not mentioned in any of these: the background paragraph, the story, or the question. You can directly copy and paste a span from the story or the question while answering the given question.

Background Paragraph: The "concentration" of a solid is essentially another way of measuring its density. A certain number of moles are present for a given volume of the solid, and this value does not depend on how much of the solid is present. As the reaction proceeds, the amount of solid reactant will decrease, but so will the volume it occupies. A similar argument can be made for liquids. As a result, the concentrations of solids and liquids are essentially constant over the course of a reaction.
Story: Cindy has a ball of iron. She measures its density then drops it in a vat of hydrochloric acid. Ashe then removes the irom from the acid noting that the acid ate away as a good potion of the iron. To her suprise however the density of the iron remained constant.
Question: Will a solids density decrease or stay constant when some of it reacts to a chemical?
constant.