The most effective way to test for CO2 is to bubble the gas through "lime water", a diluted solution of calcium hydroxide (slaked lime). When you bubble carbon dioxide through the solution, it forms a solid precipitate of calcium carbonate – chalk or limestone. Calcium carbonate is insoluble in water.Thus, if there is CO2 present in the sample, the limewater will turn milky, cloudy white., The process is simple: dilute calcium hydroxide with water. Calcium hydroxide (Ca(OH)2) is a colorless white powder that you can buy from most chemistry suppliers. Pure limewater, once mixed, is clear and colorless, with a slight earthy smell and the bitter, alkaline taste of calcium hydroxide. Follow these steps to make your own limewater:


Put 1 teaspoon of calcium hydroxide into a clean one-gallon (or smaller) glass jar. Limewater is a saturated solution, which means there will be some extra chemical that doesn't dissolve. A teaspoon will result in a fully saturated solution, whether you use a gallon jar or a smaller container.Fill the jar with distilled water or tap water. Distilled water will always give you a purer solution, but the minerals present in tap water should not hinder your test.
Put a lid on the jar. Shake the solution vigorously for 1-2 minutes, then let it stand for 24 hours.
Pour the clearer solution off the top of the jar through a clean coffee filter or filter paper. Be very careful not to stir up the sediment. If necessary, repeat this filtering step until you obtain a clear limewater solution. Store in a clean jar or bottle.
 Half-fill a test tube with limewater – then boil it. Use a delivery tube to pipe the contents of the CO2 sample test tube directly into the boiling limewater. You can use a flexible pipe or a (metal) straw as a delivery tube, if nothing better is around. Let the captured gas "bubble" through the liquid, and wait for the reaction to take place.


If you don't want to boil anything, you can use a gas syringe to discharge the CO2 gas directly into the half-filled limewater test tube. Stopper the test tube, then shake vigorously for 1-2 minutes. If there is carbon dioxide in the sample, then the solution should grow cloudy., If CO2 is present, the lime water will turn milky white with calcium carbonate particulates. If the limewater is boiling, and the gas is piped directly into the limewater, then the reaction should begin immediately. If nothing happens after a minute or so, you can safely assume that there is no carbon dioxide in your sample., Understand what exactly is taking place to indicate the presence of CO2. The equation for the test is: Ca(OH)2 (aq) + CO2 (g) -> CaCO3 (s) + H2O (l). In non-chemistry language: liquid limewater + gas (which contains CO2) react to solid lime (the particles) and liquid water.
Bubble the gas through limewater.
Make a limewater solution.
Bubble the gas through the limewater.
Look for cloudy water.
Know the chemical reaction.
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