How is pyrophosphoric acid formed from phosphoric acid molecules?

Pyrophosphoric acid (also known as diphosphoric acid, ${\text{H}}_4{\text{P}}_2{\text{O}}_7$) is formed when two molecules of phosphoric acid interact under suitable conditions, typically involving high temperatures. This process results in the condensation of the molecules, which involves the loss of one molecule of water.1

This reaction is a form of polymerization that creates a $\text{P–O–P}$ linkage between the two phosphoric acid units. Pyrophosphoric acid serves as the simplest member of a homologous series of long-chain molecules known as polyphosphoric acids.21