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A condensation reaction joins molecular units while eliminating a small molecule such as water or an alcohol. Formation of an ester from a carboxylic acid and an alcohol is a familiar example.

The term classifies the overall transformation, not one universal mechanism. In polycondensation, repeated joining can build a polymer while small molecules are released. An aldol condensation combines carbon-carbon bond formation with a subsequent elimination, so its full outcome must be distinguished from the initial aldol reaction.

Orthosilicic acid is the monomeric silicic acid $\mathrm{Si}(\mathrm{OH}{)}_4$, with four hydroxyl groups attached to a silicon center in approximately tetrahedral molecular geometry. The formula can also be written ${\mathrm{H}}_4\mathrm{Si}{\mathrm{O}}_4$. These are two ways of counting the same atoms, not two different compounds. Dissolved orthosilicic acid is a molecular precursor from which organisms can construct silica.^[ref]

Its distinction from a silica network is visible in a condensation reaction: two hydroxyl-bearing sites can form a silicon–oxygen–silicon bridge while releasing water. Continuing such reactions produces connected species, so total dissolved silicon is not automatically the concentration of free monomer.

For a numerical example, one millimole of molecular orthosilicic acid contains one millimole of silicon atoms and four millimoles of oxygen atoms. In a transport experiment, however, counting atoms alone is insufficient: a silicon transporter must recognize the actual dissolved species. Purified diatom transporters tested near neutral pH transported silicic acid in a sodium-dependent assay, linking this molecular form to biological uptake.^[ref]

Silicic acid names a family of silicon oxoacids; the simple dissolved monomer is orthosilicic acid, $\mathrm{Si}(\mathrm{OH}{)}_4$. It contains one silicon atom bonded to four hydroxyl groups. Distinguishing a soluble monomer from a solid network of silica matters in both chemistry and biology: organisms that build silica structures must first acquire a dissolved source of silicon.

A simplified condensation reaction illustrates how monomers can begin to join:Two hydroxyl-bearing sites form a silicon–oxygen–silicon bridge and release water. Further joining creates more connected structures, so a nominal concentration of total silicon does not alone specify which molecular forms are present.

A 2016 experiment reconstructed diatom silicon transporters in artificial liposomes. The investigators trapped a zinc-based indicator inside and applied a sodium gradient; entry of silicic acid increased fluorescence. Controls separated transport from background signals. The near-neutral conditions supported transport of undissociated orthosilicic acid, illustrating how a soluble mineral precursor can be studied with a purified membrane system.^[ref]