Describe the chemical process that converts phosphate rock into a soluble phosphate salt suitable for use in NPK fertilisers.

Phosphate rock, which contains insoluble calcium phosphate, is reacted with phosphoric acid (H₃PO₄). The acid dissolves the calcium phosphate, forming soluble ammonium phosphate or mono‑phosphate salts, while calcium is precipitated as calcium phosphate. The resulting soluble phosphate salt can then be mixed with nitrogen and potassium salts to produce an NPK fertiliser.

A strong answer should directly address the approved learning objective to describe treatment of phosphate rock with phosphoric acid to produce soluble salts for fertilisers. (Chemistry only). This question belongs to Production and uses of NPK fertilisers within The Haber process and the use of NPK fertilisers, so the response should use that exact curriculum context rather than a generic statement. The answer is correct when it names the key idea, explains the link to chemistry only, and keeps the wording specific to AQA GCSE revision.