Which agent produces blue discoloration in copper sulfate crystals during the endpoint test for decalcification?

Anhydrous copper sulfate is the agent that produces blue discoloration in the presence of moisture during the endpoint test for decalcification. This is due to the property of anhydrous copper sulfate to absorb water. When it comes into contact with decalcified tissue, which contains moisture, it reacts and forms hydrated copper sulfate, a blue compound. The blue color signifies that decalcification is complete and that there is enough moisture present for the reaction to occur.

The other substances mentioned do not exhibit this characteristic when mixed with copper sulfate. Water alone does not provide a unique reaction since it is already the source of moisture. Vegetable oil does not interact with copper sulfate in the same way, as oils are hydrophobic and do not contribute to the hydration of the copper sulfate crystals. Sodium chloride does not induce a color change in copper sulfate, as it does not influence the hydration state of the compound or participate in any chemical reaction that would produce a blue color.

Thus, the reaction mechanism and the properties of anhydrous copper sulfate are key to understanding why this agent is responsible for the blue discoloration observed during the test.