The rainbow tubes demonstration was reported by Bob Becker in the 1990 December Issue of Chem 13 News. Two stoppers are needed that will fit a 40-80 cm clear plastic (or glass) tube. The contents of the tube is 1 part of 5% universal indicator in 20 parts of water. By keeping the tube vertical you can add 4-5 drops of 0.1 M HCl at one end, then stopper and invert the tube. Next remove the stopper at the opposite end and add 4-5 stopper of 0.1 M NaOH. By slowly rotating the tube a mixture will occur. This will show the colors of the entire range of pH colors offered by the universal indicator. Often students can be tricked by asking a question such as what is the pH of a solution containing 1 x 10^-9 HCl? The answer is 7, since it is not possible to dilute an acid to the point of becoming a base. By only adding acid this demonstration can also be used to show that a strong acid cannot be diluted to a pH above 7. Alternatively it could be used to show that a strong base cannot be diluted to a pH below 7.