You have been provided with three test tubes. One of them contains distilled water and the other two contain an acidic solution and a basic solution, respectively. If you are given only red litmus paper, how will you identify the contents of each test tube?
Hint. Red litmus only ever does one useful thing on its own — think about what you can do with it once it's already changed colour once.
Step 1 — What red litmus alone can tell you directly. Red litmus turns blue in a basic solution, but stays red in both an acidic solution and in neutral distilled water. So dipping red litmus into all three test tubes immediately identifies the basic one — it's the only test tube where the paper turns blue.
Step 2 — Reusing that same strip to tell the other two apart. The strip that just turned blue (from touching the base) is now, in effect, a piece of blue litmus paper. Dip this now-blue strip into each of the remaining two test tubes: in the acidic solution it turns back to red, since acids turn blue litmus red; in distilled water it stays blue, since neutral water doesn't affect litmus at all.
✦ Answer: Dip red litmus into all three — the one where it turns blue is the base. Then dip that same (now blue) strip into the other two — it turns red again in the acid, and stays blue in the distilled water.
Where students slip. Assuming you're stuck once you've used up your only piece of litmus paper — the trick is that the base itself converts your red litmus into blue litmus, which you can then reuse on the remaining two solutions.
