Thermal dehydration of phosphoric acids — Heating orthophosphoric acid (H₃PO₄) to about 250°C chiefly produces which condensed phosphoric acid?

Introduction:Phosphoric acids undergo stepwise condensation upon heating, forming P–O–P linkages and releasing water. Recognizing which species forms at a given temperature is important in fertilizer and phosphate processing.

Given Data / Assumptions:

• Starting acid: orthophosphoric acid, H3PO4.
• Temperature around 250°C.
• Closed system without significant decomposition to other oxyacids.

Concept / Approach:Upon heating, two molecules of H3PO4 condense: 2 H3PO4 → H4P2O7 + H2O, giving pyrophosphoric (diphosphoric) acid. At still higher temperatures and with prolonged dehydration, polyphosphoric and finally metaphosphoric acids (general formula HPO3)n can form. But near 250°C, the predominant product is H4P2O7.

Step-by-Step Solution:Write the condensation: 2 H3PO4 → H4P2O7 + H2O.Match temperature regime: ≈200–300°C favors diphosphoric acid.Conclude that pyrophosphoric acid is produced.

Verification / Alternative check:Process texts on phosphate fertilizers describe controlled dehydration steps, with pyrophosphate formation preceding metaphosphate formation at higher temperature.

Why Other Options Are Wrong:

• Metaphosphoric acid typically forms at higher temperature/greater dehydration.
• “No change” contradicts known dehydration behavior.
• Phosphorous acid is a different oxyacid obtained by other routes, not thermal dehydration of H3PO4.

Common Pitfalls:Confusing orthophosphoric dehydration series; mixing up phosphoric (H3PO4) and phosphorous (H3PO3) acids.

Final Answer:Pyrophosphoric acid (H₄P₂O₇)