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Chemistry LibreTexts

Magic Breath

Concept

Phenolphthalein is an indicator, which detects whether a substance is an acid or a base.

magicbreath.GIF

Materials Needed

  • 200 mL beaker of water
  • 10 - 12 drops of phenolphthalein
  • 3 mL of lime water (saturated calcium hydroxide solution)
  • straw

Procedure

  1. Fill the beaker with 200 mL of water.
  2. Put 10 - 12 drops of phenolphthalein solution in the water.
  3. Put 3 mL of lime water into the water (you may have to experiment a bit with the amount so that you do not have to blow forever to get it to change colors.
  4. Stir the pink/purple solution.
  5. Insert the straw into the beaker and blow OUTWARD.
  6. The color will start to change back to colorless.

Introduction

"Guess what Ms. K. I have magic breath." Ms. K = "Yeah, right, well if you do, I do to, I mean we both go to the same school." Me = I don't think that you do, what do you think boys and girls?" "How many of you think I have magic breath?", "OK.. how many of you think that Ms. K. has magic breath, well lets see." Have Ms. K. have a pink/purple solution that will not change back to being colorless. My solution will change... hence the magic breath.

Explanation

Phenolphthalein is an acid/base indicator. It is colorless when it is an acid and pink when it is a base. Lime water is a calcium hydroxide solution, Ca(OH)2, which is a base. Lime water was added to the regular water in the solution. Lime water is a base, so when the phenolphthalein was added to the solution, it turned the solution pink.

When I exhaled into the solution, the carbon dioxide in a person's breath reacted with the water to form Carbonic Acid. When enough carbonic acid is formed it neutralizes the lime water, which is a base and makes the solution an acid. Therefore with the phenolphthalein, the color disappears in the now acidic solution. As you keep blowing your breath into the solution, there is now an excess of hydrogen ions or acid.

Ca(OH)2 =limewater = calcium hydroxide = base = pink color in phenolphthalein

CO2 + H2O ---> H2CO3 = carbonic acid

H2CO3 + Ca(OH)2 ---> CaCO3 + H2O

The above is the neutralization reaction - hydrogen ions from the carbonic acid react with the hydroxide ions in the calcium hydroxide to form water.

Safety

  • Wear safety goggles as the solution splashes a little as you blow into it.
  • Do not ingest any of the solution when blowing into the straw

Waste Disposal

  • Dispose of solution down the drain. Wash hands thoroughly with soap and water. Rinse out all beakers with soap and water

Reference

  1. Summerlin, L, Borgford, C, Chemical Activities, American Chemical Society, 1985, p. 15.

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