In industrial phosphorus chemistry,\nwhat product is predominantly formed when orthophosphoric acid (H3PO4) is strongly heated to about 900 °C under dehydrating conditions?

Introduction / Context:
Phosphoric acids undergo stepwise dehydration on heating. Understanding which condensed phosphate forms at elevated temperature is essential in fertilizer manufacture and inorganic chemistry. This question focuses on the thermal transformation of orthophosphoric acid (H3PO4) at very high temperature, around 900 °C, where extensive dehydration occurs.

Given Data / Assumptions:

• Feed: orthophosphoric acid (H3PO4).
• Strong heating near 900 °C with no special catalysts mentioned.
• Dehydration (loss of water) is the dominant process under these conditions.

Concept / Approach:
Orthophosphoric acid first condenses to pyrophosphoric acid (H4P2O7) on moderate heating by eliminating water. With further heating and deeper dehydration, cyclic or polymeric metaphosphoric acids (general formula HPO3)n form. At still higher severities, phosphorus pentoxide (P2O5) may be present as a volatilized species, but the recognized condensed phase product of strong dehydration of H3PO4 is metaphosphoric acid.

Step-by-Step Solution:

Verification / Alternative check:
Textbook dehydration sequences list orthophosphoric → pyrophosphoric → metaphosphoric with increasing temperature. Industrial drying and calcination steps exploit these equilibria to target specific phosphate species.

Why Other Options Are Wrong:

• Pyrophosphoric acid: forms at lower temperatures; further heating continues dehydration.
• No change: contradicted by well-known dehydration behavior.
• None of these / phosphorus pentoxide only: while P2O5 can appear at very high severities, the recognized condensed product of heating H3PO4 is metaphosphoric acid.

Common Pitfalls:
Confusing the first step (pyro formation) with the ultimate high-temperature product; overlooking that “metaphosphoric acid” represents cyclic/polymeric species.

Final Answer:
Metaphosphoric acid