'''ケイ酸'''（&mdash;さん、珪酸、silicic acid）とは、化学式 [SiO<sub>x</sub>(OH)<sub>4&minus;2x</sub>]<sub>n</sub> で表されるケイ素、酸素、水素の化合物の総称である。確認されているものとしては、オルトケイ酸 (H<sub>4</sub>SiO<sub>4</sub>)、メタケイ酸 (H<sub>2</sub>SiO<sub>3</sub>)、メタ二ケイ酸 (H<sub>2</sub>Si<sub>2</sub>O<sub>5</sub>) などがある。単にケイ酸と呼ぶ場合、メタケイ酸のことを示すことが多い。

==化学的性質==
普通、これらは単独で存在することは少なく、混合体となっていることが多い。

アルカリの濃溶液に酸を加えると、白色無定形の膠状物体ができる。これをふつうケイ酸と呼び、二酸化ケイ素と水とが化合したものであるが、その水の量は一定していない。

このケイ酸が凝固したものを乾燥空気中で放置し、さらに加熱乾燥させると順次水を失い、500 ℃ で二酸化ケイ素のキセロゲルとなる（シリカゲル）。この間、二酸化ケイ素 1モルに対して一定量の水が化合したと思われるものは存在しないから、このゲル状のケイ酸を一定の化学式で表すことは困難であるが、これらの塩から考えて、オルトケイ酸・メタケイ酸が存在すると考えられている。

thumb|right|Annual mean sea surface '''silicic acid''' for the [[World Ocean.  Data from the World Ocean Atlas [http://www.nodc.noaa.gov/OC5/WOA01/ 2001].]]
'''Silicic acid''' is a general name for a family of chemical compounds of the element silicon, hydrogen, and oxygen, with the general formula [SiO<sub>x</sub>(OH)<sub>4-2x</sub>]<sub>n</sub>.  Some simple silicic acids have been identified in very dilute aqueous solution, such as ''metasilicic acid'' (H<sub>2</sub>SiO<sub>3</sub>), ''orthosilicic acid'' (H<sub>4</sub>SiO<sub>4</sub>, pK<sub>a1</sub>=9.84, pK<sub>a2</sub>=13.2 at 25° C), ''disilicic acid'' (H<sub>2</sub>Si<sub>2</sub>O<sub>5</sub>), and ''pyrosilicic acid'' (H<sub>6</sub>Si<sub>2</sub>O<sub>7</sub>); however in the solid state these probably condense to form polymeric silicic acids of complex structure.

Silicic acids may be formed by acidification of silicate salts (such as sodium silicate) in aqueous solution.  When heated they lose water to form silica gel, an active form of silicon dioxide.

In the oceans, silicon exists primarily as orthosilicic acid (H<sub>4</sub>SiO<sub>4</sub>), and its biogeochemical cycle is regulated by the group of algae known as the diatoms.  These algae polymerise the silicic acid to so-called biogenic silica, used to construct their cell walls (called ''frustules'').

Continuing research of the correlation of aluminium and Alzheimer's disease has in the last few years included the use of silicic acid in beverages<ref name="pmid16988476"></ref><ref name="pmid17697731"></ref><ref name="pmid18096288"></ref>, due to its abilities to both reduce aluminium uptake in the digestive system as well as cause renal excretion of aluminium.

Orthosilicic acid is the form predominantly absorbed by humans and is found in numerous tissues including bone, tendons, aorta, liver and kidney. Compelling data suggest that silica is essential for health although no RDI has been established. However, deficiency induces deformities in skull and peripheral bones, poorly formed joints, reduced contents of cartilage, collagen, and disruption of mineral balance in the femur and vertebrae.
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