Given the task definition and input, reply with output. Given a scientific passage and an answer, generate a question for the given answer.

Passage: The way in which a reaction is written influences the value of the enthalpy change for the reaction. Many reactions are reversible, meaning that the product(s) of the reaction are capable of combining and reforming the reactant(s). If a reaction is written in the reverse direction, the sign of the changes. For example, we can write an equation for the reaction of calcium oxide with carbon dioxide to form calcium carbonate. Answer: calcium carbonate
The reaction of calcium oxide with carbon dioxide forms what?