Question

While diluting an acid, why is it recommended that acid should be added to water and not water to acid?

Answer 1

Hint : Dilution is the process of decreasing the concentration of a solute in a solution, usually simple by mixing with more solvent like adding more water to the solution. To dilute a solution means to add more solvent without the addition of more solute.

Complete Step By Step Answer:
When you mix strong acids and water, it makes a difference whether you add acid to water or water to acid. Always add acid to water and not water to acid.
If you add a little water to a lot of concentrated acid, the resulting solution is still concentrated. The hydration reaction uses all the water, in this acid is a limiting reactant. Generating a lot of heat. The solution violently boils, spitting concentrated acid out of its container. If you add a little concentrated acid to a lot of water, water is a limiting reactant and the resulting solution is more dilute. Here, all the acid reacts, but there is extra water to absorb heat, lessening the chance of boiling.
There are few factors that make it better to add acid to water. Diluting acid with water is an exothermic reaction, so it is easier to boil and splash water because water has a high heat capacity and can absorb a lot of heat.
Also, some strong acids have a specific gravity or density than water. So, if you put water on top of acid, the heat that’s generated is at the acid surface. In this situation, the liquid readily boils and spots. On the other hand, if you pour acid on top of or into water, the water rises over the acid before mixing and contains the reaction.
An example of diluting a strong acid:
Sulfuric acid is the most dangerous common acid to dilute, this is because it reacts so violently with skin and clothing. Sulfuric acid quickly dehydrates proteins and carbohydrates in skin and muscle. The acid is much heavier than water, so water added to it reacts with the top layer first. A lot of heat gets generated, even when sulfuric acid and water are mixed properly. Mixing $100ml$ of concentrated sulfuric acid and $100ml$ of water at $19^\circ C$ reaches a temperature over $131^\circ C$ in under a minute.
It’s the hydration reaction that generates all that heat:
${H_2}S{O_4} + {H_2}O \to {H_3}{O^ + } + HS{O_4}^ - $
Similarly, sulfuric acid readily strips water from organic molecules, dehydrating them.

Note :
There are various safety tips for diluting an acid with water:
$ \bullet $ The higher the concentration of the acid, the greater the heat increases and higher the chances of boiling and splashing.
$ \bullet $ Be extremely careful diluting a concentrated strong acid.
$ \bullet $ Always wear proper safety gear and work under a fume hood.
$ \bullet $ Finally choose your glassware wisely. An Erlenmeyer flask or volumetric flask is a better choice than a beaker or graduated cylinder because the flask shape acts as a sort of splash shield.