Magic liquid

Make water change its color!

Scientific name: The thymol blue indicator changes the color of the solution depending on its acidity level

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Safety

Put protective gloves and goggles on.

Always follow general safety recommendations. Please note that conducting chemistry experiments you must comply with the relevant legal procedures in your country.

Perform this experiment

Reaction formula

H2Ind → HInd- → Ind2-

H2Ind ← HInd- ← Ind2-

Step-by-step instruction

  1. Arrange 5 plastic cups in line.
  2. Add 5 drops of 2M sodium hydrosulfate solution Na2SO4 into the first one.
  3. Add 5 drops of 2M citric acid C6H8O7 solution into the second plastic cup.
  4. Add 5 drops of 2M citric acid C6H8O7 solution and 2M sodium carbonate Na2CO3 solution into the third cup.
  5. Add 5 drops of 2M sodium chloride NaCl solution into the fourth cup.
  6. Add 5 drops of 2M sodium carbonate Na2CO3 solution into the fifth cup.
  7. Take one more plastic cup. Add 10 drops of thymol blue and dilute it with regular water (still mineral water with no additives would be the best option).
  8. Stir the thymol blue solution with a black stirring rod.
  9. Distribute the thymol blue solution obtained in step 7 among the five cups you have prepared earlier.
  10. Thymol blue dye changes the color of solution depending on its acidity level.

Scientific background

Why does the liquid change color?

We have previously prepared five glasses, each with a few drops of different substances, and then added the solution of thymol blue to each of them. In each glass we get a different solution; molecules of thymol blue have fallen into diverse neighborhoods, and therefore they have behaved differently.

Let us analyze in detail what happened in each glass. We will mark the thymol as “Ind” (Indicator).

Glass #1 (red)

In the first glass we put 5 drops of sodium hydrogen sulfate NaHSO4 water solution. In water it dissociates cheerfully into three charged particles (called ions):

NaHSO4 → Na+ + H+ + SO42-

In the presence of a large quantity of protons H+, all of the thymol blue (Ind2- – blue) becomes red H2Ind.

Glass #2 (orange)

Citric acid C6H8O7 from the second glass also dissociates in water, producing protons H+. Moreover, each of its molecules can form a whole three H+! But, unlike NaHSO4, citric acid does it so unwillingly, that as a result much less H+ appears in the solution than in the first case. Therefore, a part of thymol blue becomes red H2Ind, and another part becomes yellow HInd-. A mixture of yellow and red, just as when we mix paint colors, gives us orange.

Glass #3 (yellow)

In the third glass, a mixture of citric acid C6H8O7 and sodium carbonate Na2CO3 produces even fewer protons H+. Therefore, all the thymol blue in such a neighborhood turns into yellow HInd-.

Glass #4 (green)

Sodium chloride NaCl in water dissociates into Na+ and Cl-. But the water itself, as a matter of fact, contains a small number of protons H+ (read more on that topic in Addition to this question). Therefore, we get a slightly green colored solution from yellow HInd- and blue Ind2-.

Glass #5 (blue)

In the last glass, thymol blue is present in its own form Ind2- and has, as its name suggests, the blue color.

Danger:
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Difficulty:

Published on 17 January 2016

  • Fire
  • Heating with fire
  • Explosion
  • Poisoned gas
  • Organic
  • Electricity
  • Solution
  • Oxidation reduction
  • Color change
  • Precipitate
  • Gassing
  • Catalyst