Biology Forums - Study Force

I know in chemistry lab, we are told to add water before adding acid. What is the chemistry behind this?

What are you referring to? Is your question, why is acid always added to water, and not the reverse? If so, a large amount of heat is released when strong acids are mixed with water. Adding more acid releases more heat. If you add water to acid, you form an extremely concentrated solution of acid initially. So much heat is released that the solution may boil very violently, splashing concentrated acid out of the container! If you add acid to water, the solution that forms is very dilute and the small amount of heat released is not enough to vaporize and spatter it. So Always Add Acid to water, and never the reverse.

Now, if you're referring to adding water to acid to dilute it, that's fine. It decreases the strength of the acid. pH is defined as the -log of the concentration of hydrogen ions in a solution, or -log[H+]. As you add water to an acid, you are diluting it, thus lowering the concentration of H+. Putting a lower value in for [H+] in the equation -log[H+] will give you a great value than you originally had. This means the pH has increased, making the solution more basic.