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Acid/alkali color changing demo for Cub Scouts

February 16, 2017

Q. I am a Scout leader who wants to do a color changing demo as part of one of our advancement ceremonies. This is what I want to show:

Clear liquid to orange (can be reddish orange) liquid to yellow liquid to blue liquid

I was thinking that starting with an acid with a pH of 2.8 or so would give the clear liquid to orange by adding thymol blue. Increasing the pH would change the color to yellow. Continuing to increase the pH to 8.0 should change it to blue.

My question is, what are some reasonably safe, obtainable acids and bases to make this happen? I am envisioning adding the indicator to beaker 1 for orange. Adding the contents of beaker 2 to beaker 1 for yellow; then adding the contents of beaker 3 to beaker 1 for blue.

This symbolized the Scout before attaining rank; attaining the rank of Tiger; attaining the rank of Wolf; and finally attaining the rank of Bear.

Any help would be most appreciated!

Sandra House
- Phoenix, Arizona USA


February 2017

A. Hi Sandra. Vinegar is a safe acid at a pH of about 2.4; pure water is neutral at pH 7.0; baking soda is a safe alkaline material at about pH 8.5; milk of magnesia is safe at pH of about 10.5 but may be a bit slow acting since it's dissolvable but not really dissolved.

wikipedia
pH Indicator

You'll also need a pH indicator, of which there are many. Good luck.

Regards,

pic of Ted Mooney
Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


simultaneous February 16, 2017

Q. Thanks. I am looking at thymol blue for the indicator because it has the right colors as the solution goes from acid to neutral to base.

Sandra House [returning]
- Phoenix, Arizona USA


February 21, 2017

A. Aqua Ammonia (ammonia gas dissolved in water) -- sometimes called ammonium hydroxide -- is a good choice for this sort of experimentation. It's cheap at your local supermarket (get the clear, not the soapy). And (neat trick for Cub Scouts) if you hold damp litmus or pH paper above it you can see the color change from neutral to alkaline with the volatile ammonia absorbed by the water in the indicator paper. I still remember (60+ years ago) my Dad mixing vinegar and aqua ammonia to show me that heat was generated when acids and alkalies mix.

tom_rochester
Tom Rochester
Plating Systems & Technologies, Inc.  

Jackson, Michigan, USA


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