How do you tell acid from a base?

Answer 1

There are a number of ways to tell an acid from a base. By definition, any solution with a pH lower than 7 is an acid, and if the pH is higher than 7 it is a base (if it is precisely 7 it is neutral and probably pure water). pH measures the level of hydrogen ions in a solution, but not in the way you might expect; a lower pH number means a higher amount of hydrogen ions and a lower amount of hydroxide ions, whereas a higher pH means less hydrogen ions and more hydroxide ions. You will observe that acids have hydrogen in their molecular formula; hydrochloric acid is HCl, sulfuric acid is H2SO4, and so forth, there is always an H. Bases have hydroxide, which is OH, in their molecular formula, so sodium hydroxide is NaOH. I realize that this could be confusing because hydroxide also contains hydrogen, so if you only look at the hydrogen, both acids and bases have it. But in a base, the hydrogen is connected to an oxygen, in the form of a hydroxide radical.

Answer 2

You can test it with a universal indicator, litmus paper or pH paper.