Chemistry - Why don't we explode after drinking water?

Solution 1:

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.

Solution 2:

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.