Summary for acid alkali color changing demo Discussion
Acid/alkali color changing demo for Cub Scouts: Sandra House wanted a safe, simple three-step pH demonstration using thymol blue to show clear/orange, then yellow, then blue as part of an advancement ceremony. The reply from A. suggested several reasonably safe, obtainable materials: vinegar as the acid (about pH 2.4), pure water as neutral (pH 7.0), baking soda as a mild base (about pH 8.5), and milk of magnesia as a stronger but slower-acting base (about pH 10.5). Sandra then noted thymol blue should give the right sequence of colors. A. later added that aqua ammonia (ammonia dissolved in water) is another inexpensive option from a supermarket, and mentioned a neat Cub Scout-friendly trick: holding damp litmus or pH paper above it shows the alkaline color change from ammonia vapor. A. also recalled that mixing vinegar and aqua ammonia produces heat.
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 16, 2017
A. Hi Sandra. vinegar
⇦ in bulk on
or
Amazon [affil link] 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.
You’ll also need a pH indicator, of which there are many. Good luck.
Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com – Pine Beach, New Jersey
Need quick confidential answers? $25
Need project assistance? $100/hr.
February 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] 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 ⇦ on eBay or
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