Carbon dioxide (CO²) is colorless and odorless, so you can't detect it through direct observation. You'll need to collect an air sample (or a CO² sample), then run one of several simple tests to identify the presence of the gas. You can bubble the gas through limewater, or you can hold a lit splint into the sample to see if it is extinguished by the presence of CO².

Preparing a Sample
1. Collect a CO² sample. To begin your test, you will need a sealed test tube filled with collected gas. You can collect carbon dioxide in a gas jar, a boiling tube, or another airtight container. Collection is usually performed over water in a beaker. CO² gas is denser than air, so you can collect it using "downward delivery" or a gas syringe.

2. Mix calcium carbonate with Hydrochloric acid (HCl). The simplest way to collect carbon dioxide is to react calcium carbonate (or limestone chips) with Hydrochloric acid. First, pour 20ml of HCl into a conical flask. Add a spoonful of calcium carbonate (or limestone chips) to the HCL. When the reaction starts, cover the conical flask with a bung and delivery tube: you will collect the gas through the delivery tube and into an upturned test tube (which is immersed in bowl of water). If the water in the test tube is displaced, then gas is being collected.

3. Cover the test tube with a bung. Put the tube on a rack to keep it safe until you perform the test. The "bung" is essentially a small cork or cap that allows you to pipe the contents of the test tube elsewhere through a connected delivery tube. It's important to seal the CO² gas into the container. If you leave it open, the gas will mix with the air, and your test will be much less effective.



Bubbling Through Limewater
1. Bubble the gas through limewater. 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.

2. Make a limewater solution. 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.

3. Bubble the gas through the limewater. 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.

4. Look for cloudy water. 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.

5. Know the chemical reaction. 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.