I was told that an acid is only dangerous because it releases $\ce{H+}$ ions when in contact with water.

Therefore, powdered acid should be completely safe on skin provided that your skin is not covered with sweat.

Am I correct, or is my logic flawed?

  • 3
    $\begingroup$ What is ‘powdered acid’ - many strong acids are aqueous solutions. There are some strong organic acids that are non-aqueous, but these will cause damage with or without water. There are some weak acids (eg citric) found in foods which do come as powders, and are not harmful in small quantities with or without water $\endgroup$
    – NotEvans.
    Jul 16 at 17:28
  • 2
    $\begingroup$ @NotEvans. Powdered acid like Citric Acid powder. $\endgroup$
    – will-
    Jul 16 at 17:29
  • 9
    $\begingroup$ Oleum has no water, and you wouldn't like to touch it, I assure you. Being solid makes a difference because of hindered mixing. $\endgroup$
    – Mithoron
    Jul 16 at 18:08
  • 6
    $\begingroup$ Citric acid is found in food and has many household applications, it isn’t really a very strong acid at all. That said, contact with skin might cause some irritation, especially in large quantities. Even when ‘dry’ your skin still contains moisture, so the idea that keeping things dry isn’t really relevant $\endgroup$
    – NotEvans.
    Jul 16 at 18:09
  • 4
    $\begingroup$ If you need an inorganic example, take sodium hydrogen sulfate. $\endgroup$
    – Loong
    Jul 17 at 7:21

The state of a compound, solid, liquid, or gaseous is a property which is unrelated to the property / strength of a Brønsted or Lewis acid of said compound. Thus, you can not infer from its physical appearance if an acid is safe enough to be touched by skin; it may be irritant / corrosive even in absence of water (e.g., p-TsOH as a solid acid, triflic acid as a liquid acid). Similar, solid Brønsted bases like KOH or KOtBu are not magically safe to touch once obtained as a solid instead of in solution. The interaction of these acids and bases with the skin may be influenced by water. Your skin however includes many other chemicals which may interact and equally react as an acid, or a base, thus react with external acids and bases accordingly in absence of water, too.

In addition, there are plenty of chemicals (gaseous, or liquid, or solid) which neither are considered as an acid, nor as a base; still, for other reasons than those above these are not harmless enough to be touched over extended periods of time by human skin. Their origin, natural, biological, processed, or entirely artificial, is not a sufficient criterion to render them safe, or potentially dangerous, either. Equally note, skin is not uniform.

  • $\begingroup$ Or at least, there isn't a known relation of scientific interest. Even if there is stochastic dependence, I anticipate it to be rather weak. $\endgroup$
    – Galen
    Jul 16 at 19:38
  • 1
    $\begingroup$ There's also acids which are also poisonous, like hydrofluoric acid. $\endgroup$
    – nick012000
    Jul 17 at 10:32

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