Here's why and what happens:
* Anhydrite is a salt: It's formed from the reaction of a strong base (calcium hydroxide) and a strong acid (sulfuric acid).
* HCl is a strong acid: It readily donates protons (H⁺).
* Reaction: When HCl is added to anhydrite, the acid reacts with the sulfate ion (SO₄²⁻) in anhydrite. This reaction produces calcium chloride (CaCl₂) and sulfuric acid (H₂SO₄).
* CaSO₄(s) + 2HCl(aq) → CaCl₂(aq) + H₂SO₄(aq)
Key points:
* The reaction is exothermic, meaning it releases heat.
* The reaction can be quite slow at room temperature, but speeds up significantly with heat.
* The reaction can be used to identify anhydrite in the field.
Important Note: While the reaction is relatively straightforward, it's important to note that the resulting sulfuric acid can further react with the anhydrite, creating more calcium chloride and water:
* H₂SO₄(aq) + CaSO₄(s) + H₂O(l) → Ca(HSO₄)₂(aq)
This secondary reaction can complicate the overall process.
