How to Test for Carbon Dioxide Gas

Written by john brennan
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How to Test for Carbon Dioxide Gas
Many scientists believe the levels of CO2 in Earth's atmosphere are rising as a result of human activity. (CO2 image by photlook from

Carbon dioxide is a common gas that, together with water vapour, makes up about 1 per cent of Earth's atmosphere. In recent years, many scientists have become concerned about the possible impact of fossil fuel combustion and other activities that release CO2. Testing for the presence of CO2 in a sample is a relatively straightforward experiment you can carry out at home. Since carbon dioxide reacts with calcium hydroxide, bubbling CO2 through a solution of calcium hydroxide dissolved in water will turn the water cloudy. You can buy calcium hydroxide at aquarium supply shops and many home improvement stores.

Skill level:

Things you need

  • Calcium hydroxide (also called slaked lime or pickling lime)
  • Clean glass jar
  • Water
  • Test tube
  • Rubber syringe
  • Test tube stopper
  • Short plastic tube

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  1. 1

    Add 1 tsp of calcium hydroxide to the clean glass jar, then fill the jar with water. Screw the lid on tight and shake the jar to dissolve the calcium hydroxide in the water, then allow it to stand for several hours. The solution should be clear and transparent.

  2. 2

    Pour some of the solution from the glass jar into the test tube until it's about one-fourth full. Insert one end of the plastic tube into the test tube.

  3. 3

    Pump the sample of gas you want to test into the plastic tube using the rubber syringe so that it bubbles through the solution. Cap the test tube with the stopper and shake it carefully.

  4. 4

    Observe the test tube. If carbon dioxide is present in the sample, it will have reacted with the calcium hydroxide to form calcium carbonate, which is insoluble in water; the water will turn a cloudy colour. If no carbon dioxide was present the water will remain transparent as before.

Tips and warnings

  • Calcium hydroxide is an eye, skin and respiratory tract irritant. If ingested it can potentially cause vomiting, diarrhoea and collapse. Calcium hydroxide poisoning can potentially be fatal. Do not eat calcium hydroxide; do not drink solutions containing calcium hydroxide; do not bring it into contact with your eyes or skin.
  • Calcium hydroxide is a strong base that can react vigorously with acids; it should always be kept separate from them.

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