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It is known that acid should be added to water and not the opposite because it results in an exothermic reaction.

Our stomach contains HCl, so why don't we explode when we drink water?

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    $\begingroup$ It is surely better to dilute acid by adding water and not viceversa. But it is not that all acids are H2SO4, as far "explosion" is of concern. Plus see answer $\endgroup$ – Alchimista Mar 25 '18 at 10:18
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    $\begingroup$ The point of the acid to water rule is to spread the acid around: a given mass of a given type of acid will make a certain amount of heat, and by having all the water already there, you give plenty of heat capacity to spread this heat around. But if there's not too much acid in the first place, then it doesn't matter. $\endgroup$ – Ian Mar 25 '18 at 18:32
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    $\begingroup$ @RamKeswani That sounds like safety advice for doing experiments. Safety advice is often much more cautious than is technically necessary. $\endgroup$ – zwol Mar 26 '18 at 20:19
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    $\begingroup$ @RamKeswani I'm sorry, it is a shame how lousy textbooks (and teachers!) are especially in developing countries. You should never accept anything written in a book or said by a teacher if it is not satisfactorily, logically explained why and under what circumstances it is the case. $\endgroup$ – Karl Mar 26 '18 at 21:38
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    $\begingroup$ The "other answer" would be...evolution. A species with a high enough concentration of acid in its body to explode when water is added would not survive. As mankind is not yet extinct[citation needed], we can assume that evolution has dealt with this and obviously found a way to mitigate this issue. Unfortunately, this answer belongs into Biology, not Chemistry. $\endgroup$ – Klaws Mar 27 '18 at 8:16
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The hydrochloric acid in the stomach is already quite dilute; its pH is in fact no less than 1.5 so that at the extreme maximum there is only 0.03 molar hydrochloric acid. And even that small amount is, of course, stabilized by being dissociated into solvated ions. There is just not enough stuff to react violently.

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  • $\begingroup$ Is there a further stabilising effect from a buffer solution? $\endgroup$ – J.G. Mar 26 '18 at 17:19
  • $\begingroup$ Not really. Once ypu have a set pH the effective concentration is also set, regardless of what else is in there. I am unaware that any buffering would be used because the acid needs unbuffered solvated protons (in moderatoon) to work. $\endgroup$ – Oscar Lanzi Mar 26 '18 at 18:13
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The rule that acid should be added to water and not the opposite only applies to the dilution of some concentrated acids that results in a strongly exothermic reaction, in particular to the dilution of concentrated sulfuric acid.

The dilution of concentrated sulfuric acid with a large amount of water at a temperature of 20 °C releases 95.33 kJ per mol $\ce{H2SO4}$. That means that, ignoring the heat capacity of the sulfuric acid itself, dilution of 1 mol of $\ce{H2SO4}$ releases enough heat to increase the temperature of 2 mol of water from 20 °C to the boiling point and evaporate this amount to steam at a pressure of 1 bar (Δ​H = 46.678 kJ/mol). Therefore, diluting concentrated sulfuric acid with small amounts of water (i.e. when adding water to the acid) can result in evaporation of the mixture of water and acid at the point where the water is dropping into the acid.

Gastric acid is not a concentrated acid. The free concentration of $\ce{HCl}$ is between 0.5 % and 1 % (which is about 0.17 mol/l–0.33 mol/l). Diluting an aqueous solution of $\ce{HCl}$ with a typical molality of 0.2 mol/kg to infinite dilution yields only 0.761 kJ/mol at 25 °C. This is not enough for a dramatic increase in temperature of the mixture.

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  • $\begingroup$ Minor point. My source gave a pH range suggesting a lower acid concentration in gastric juice, ad I answered accordingly. $\endgroup$ – Oscar Lanzi Mar 26 '18 at 18:14

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