The bromphenol blue is an acid/base indicator. Vinegar is acidic, as is your stomach. Bromphenol blue turns yellow in the presence of excess acid, as is the case when the vinegar is added to the water. When too much acid builds up in your stomach, it can feel upset. Alka-Seltzer is a "buffer" which keeps your stomach from being too acidic. This is demonstrated by the fact that the Alka-Seltzer turns the solution back to blue – indicating the acid has been neutralized.

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Note: All experiments should be done using original formula effervescent Alka-Seltzer.

• Alka-Seltzer tablet
• 100ml (1/2 cup) of water
• Vinegar
• Bromphenol blue indicator

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1. To about 100ml (1/2 cup) of water add 5-10 drops of bromphenol blue indicator. Water must turn blue.

2. Add a dropper full of vinegar. The water will turn yellow.

3. Add an Alka-Seltzer tablet. As it starts to fizz, the color changes back to blue.

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