This article is from the Scouting FAQ, by Bill Nelson email@example.com, Soaring Golden Eagle firstname.lastname@example.org and Alan Houser email@example.com with numerous contributions by others.
3. Color-changing liquids (yellow-green-blue)
Materials: yellow and green food coloring, pitcher of water, 4 clear
plastic glasses, liquid bleach
Setup: Four clear glasses: one empty, one with a drop of yellow food
coloring, one with a drop of green food coloring, one with 1/4 teaspoon
of liquid bleach. As in above, be sure audience cannot see the
preparation or the bottom of the glasses during the trick.
Effect: Water changes colors from clear to yellow to green to blue as
the liquid is poured into successive glasses.
Scenario: Water from a spring at a nearby Boy Scout camp has the
special properties of changing into Cub Scout colors if boys have
completed all of their requirements for their badge of rank. Water is
poured from a special receptacle into the first glass. Water turns
yellow when poured from first glass into second; water turns green when
poured from second glass into third ("Are you boys sure that you
completed all of the requirements for the ______ badge?"). Water turns
blue when poured into last glass, indicating the boys have in fact
completed the final test for their badge of rank.
How it's done: First glass is empty so water is clear when poured into
the first glass. Water turns yellow when added to the glass with the
drop of yellow food coloring; yellow water turns green when added to the
glass with a drop of green food coloring; green water turns blue when
added to the glass with the liquid bleach (the yellow color is bleached
our of the green water, leaving the water blue in color.)
4. Multi-colored liquids. This trick requires the use of chemicals
normally available at scientific supply stores; in fact, most chemistry
sets will contain the required chemicals.
Materials: 6 clear plastic glasses, sodium carbonate, yellow and blue
food coloring, white vinegar, phenolphthalein solution, liquid bleach
Setup: prepare the glasses as follows:
glass 1: pinch of sodium carbonate dissolved in 6 ounces of water
glass 2: 1 drop yellow food coloring
glass 3: liquid phenolphthalein (made from powdered phenolphthalein and
rubbing alcohol); keep covered as solution evaporates.
glass 4: 1 drop blue food color
glass 5: 1/4 full white vinegar (be sure to palm the bottom half of the
glass until you begin pouring.
glass 6: 1/4 full liquid bleach
Effect: "Water" is poured into different glasses, each time changing to
a different color. Color sequence is clear, yellow, red, purple, green,
Scenario: Similar to the above except more glasses with more colors.
How it's done: Same as above. Reactions between chemicals in solution
produce the different colors. Be careful to dispose of the last glass
quickly as it does look like water but is really heavily laced with
chlorine bleach (the last thing you want to happen is for a scout to
come up and taste the "water")