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Formaldehyde has the formula $\ce{CH2O}$, and the ratio of atoms in a simple carb is $\ce{1C:2H:1O}$. This fits the formula of formaldehyde. When I researched this, I found some sources saying that formaldehyde is the simplest carb, but other sources saying it is glycolaldehyde ($\ce{C2H4O2}$). Is formaldehyde a carbohydrate?

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According to IUPAC definition:

‘carbohydrate’ includes monosaccharides, oligosaccharides and polysaccharides as well as substances derived from monosaccharides by reduction of the carbonyl group (alditols), by oxidation of one or more terminal groups to carboxylic acids, or by replacement of one or more hydroxy group(s) by a hydrogen atom, an amino group, a thiol group or similar heteroatomic groups. It also includes derivatives of these compounds [...]


But according to Wikibooks

the term is generally understood in the biochemistry sense, which excludes compounds with only one or two carbons. Natural saccharides are generally built of simple carbohydrates called monosaccharides with general formula $\ce{(CH2O)_n}$ where $n$ is three or more [...]

Because formaldehyde and glycolaldehyde (not a true sugar) has one and two carbon respectively in its structures we can exclude both as a carbohydrate.

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The term carbohydrate literally means a "hydrate of carbon". With respect to this definition, anything with the general formula of $\ce{C_x(H2O)_x}$ could be considered a "hydrate of carbon." Examples would include molecules like formaldehyde, acetic acid and, yes, glucose.

However, when most everyone uses the term "carbohydrate", they are talking about the biomolecules (like glucose, fructose, etc). In this much more common definition, a carbohydrate should be defined as a polyhydroxy aldehyde or ketone. Or in other words, the molecule must have either the ketone or aldehyde functional group AND multiple (more than 1) hydroxyl groups. So now molecules like formaldehyde and acetic acid would NOT be considered carbohydrates, but molecules like glucose, fructose, allose, mannose, glyceraldyde, dihydroxyacetone, etc would be considered carbohydrate.

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