Science project

The Limewater Carbon Dioxide Test


  • Container with lid (a plastic food container would work fine)
  • Distilled water
  • Tap water
  • Calcium oxide (lime)
  • Teaspoon
  • Spoon
  • ¼ cup measuring cup
  • White vinegar
  • Small shallow dish (a baby food jar or a salad dressing cup work great)
  • 2 clear water bottles
  • Modeling clay
  • 2 bendy straws
  • Scissors
  • Ruler
  • Toilet paper
  • Baking soda


  1. Take the container with a lid and add 1L of distilled water and 1 teaspoon of calcium oxide. Stir with the spoon.
  2. Let the solution sit overnight. This will be your limewater. Why do we refer to it as limewater?
  3. Fill the small, shallow dish with limewater.
  4. Take modeling clay and mold it into a ring just below the bendable part of your straw.
  5. Add ¼ cup of water and ¼ cup of white vinegar into the water bottle.
  6. Take 1 square of toilet paper and spread some baking soda in the middle of the paper. Roll it up and twist the ends of the toilet paper so the baking soda does not spill out.
  7. Drop of the baking soda in the toilet paper into the water bottle.
  8. Immediately insert the end of the straw into the water bottle, making sure that it is submerged in the liquid. Use the modeling clay to seal the straw into the neck of water bottle.
  9. Bend the end of the straw and submerge it in your container of limewater.
  10. Observe what happens. Record your observations.
  11. Add ¼-cup of limewater to another clear water bottle.
  12. Make a second modeling clay plug around your second bendy straw and insert the straw and plug into the water bottle.
  13. Take a deep breath and exhale into the straw so your breath goes into the limewater. Be careful not to suck any lime water up!
  14. Record your observations.


When carbon dioxide, CO2, enters the limewater, the limewater becomes cloudy. When you exhale into the bottle, the limewater will turn cloudy.


Chemists refer to baking soda as sodium bicarbonate, a compound with the chemical formula NaHCO3. Vinegar is a mixture of acetic acid (CH3COOH) and water (H20). When the two are combined, the following reaction takes place:

NaHCO3 + HC2H3O2 → NaC2H3O2 + H2O + CO2

But this is known to happen in two steps:

NaHCO3 + HC2H3O2 → NaC2H3O2 + H2CO3,

...where carbonic acid is formed, and finally

H2CO3 → H2O + CO2,

...where carbonic acid breaks down into water and CO2. The CO2 then bubbles up through the straw and into the limewater.

Limewater is created with calcium hydroxide, or Ca(OH)2. Named for the mineral, not the fruit, lime reacts with CO2 in water to form calcium carbonate, which is white and does not dissolve in water, causing the water to turn cloudy.

Similarly, when we exhale we are removing CO2 from our bodies, so breathing CO2 into the lime water will produce the same reaction, though probably not as quickly due to the smaller amount of CO2 in your breath.

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