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This is a cool and colorful demonstration of liquid density/diffusion/pH/acid-base indicators




7, 8, 9, 10, 11, 12  

Title – Density – pH – Diffusion Demonstration
By – Frank Virzi
Primary Subject – Science
Secondary Subjects – Science
Grade Level – 7 – 12

This is an easy-to-set-up and very colorful demonstration of density of liquids, that could also be extended to a discussion of acid-base indicators, pH, and diffusion along a concentration gradient.


  • Two 2-liter plastic soda bottles
  • 0.04% Bromothymol Blue solution (4 liters)
  • NaOH solution – dissolve 4.0 g in 20 mL of tap water
  • Phosphoric acid
  • Disposable plastic pipettes


1. Fill the two 2-L soda bottles with 0.04% Bromothymol blue almost all the way to the top.

2. Add 6 drops of phosphoric acid to one bottle, screw the cap back on, and shake. It turns yellow!

3. Add 3 drops of the NaOH solution to the other bottle, screw the cap back on, and shake. It turns blue!

4. Using a disposable plastic pipette (or an eyedropper), add 1 mL of the NaOH solution slowly into the yellow solution. Do this by placing the tip of the pipette just below the surface of the water and then slowly squeezing out the base. After a minute or two, the bottom half (approximately) of the liquid in the bottle will turn blue, and the top half will remain yellow (you may see a thin band of green in between the two regions).

5. To the all-blue bottle, add 1 mL of phosphoric acid in the same way that you just added the base to the other bottle. You will see the bottom of this bottle turn yellow, and the top remains blue.

6. Let the bottles sit undisturbed, and follow the rate of the merging of two layers into one because of diffusion of the liquids over time.


These nifty color changes happen because of the settling of the more dense phosphoric acid (causing the blue bottle to turn yellow as the more dense acid initially concentrates in the bottom half of the bottle) and sodium hydroxide (causing the yellow bottle to turn blue as the more dense NaOH solution initially concentrates towards the bottom). Bromothymol blue [76-59-5} is an acid – base indicator with a transition level of pH 6.0 (yellow) to pH 7.6 (blue). Other acid-base indicators would probably also work just as well.

E-Mail Frank Virzi !

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