Carbon dioxide (CO2) is odorless and colorless, so you cannot identify it by observation alone. You need to collect a sample of air (or CO2) and then perform one of the different tests to verify its presence. You can create gas bubbles in lime water or hold a lit match in the sample to see if the flame goes out in the presence of CO2.
Steps
Method 1 of 3: Prepare the Sample
Step 1. Collect a gas sample
To start the test, you need a sealed tube filled with carbon dioxide; alternatively, you can use a gas cylinder, thermal tube, or any other airtight container. Generally, the collection takes place above a beaker that contains water; carbon dioxide is denser than air, so you can "capture" it using a gas-tight syringe or diffusion tubes.
Step 2. Mix the calcium carbonate with the hydrochloric acid
The simplest way to sample CO2 is to make these two substances react. To start, pour 20 ml of acid into a conical flask, add a spoonful of calcium carbonate and when the reaction starts, cover the flask with a cap and a cannula: the gas enters the cannula and reaches an inverted tube, in turn immersed in a bowl of water. If the water in the tube moves, it means that gas is accumulating inside the container.
- You can continue to collect the sample as long as the reaction is active.
- To perform classroom demonstrations, a small amount of hydrochloric acid is sufficient; the best is that diluted to concentrations of 1 M or 2 M, but must be used with great care. The chemical equation describing the reaction is: CaCO3(s) + 2HCl (aq) ==> CaCl2(aq) + H2O (l) + CO2(g).
- Be very careful when working with hydrochloric acid - wear gloves, a lab coat, safety glasses and avoid direct contact with the substance! It would be better to trigger this reaction only if you have access to a real structured laboratory.
Step 3. Cover the tube with a cork
Place it in a support grid to keep it safe until you can perform the test. The cap is a particular model for laboratory that allows the insertion of a cannula for the transfer of the sample into other containers. It is important to seal the container so that the CO does not escape2; if you leave the tube open, the gas mixes with the air and the test is less effective.
Method 2 of 3: Insert the CO bubbles2 in the Lime Water
Step 1. Create gas bubbles in the lime water
The most effective way to check for carbon dioxide is to inject the gas through a dilute solution of calcium hydroxide (hydrated lime). When the gas enters the liquid, solid precipitates of calcium carbonate, gypsum or calcite are formed; calcium carbonate is insoluble in water. Also, if there is CO in the sample2, the calcium water becomes cloudy and milky.
Step 2. Make a calcium water solution
This is a simple procedure which consists in diluting the calcium hydroxide in water. This compound (Ca (OH)2) is a white powder that you can buy at any lab supply store. Pure lime water, once mixed, is clear, colorless, with a slight earthy smell and the alkaline, bitter taste of calcium hydroxide. Follow these instructions to make it:
- Place a teaspoon of lime hydroxide in a clean 4-liter (or smaller) jar. Lime water is a saturated solution, which means that by adding more solute, it does not dissolve. As long as you use a container no larger than 4 liters, a teaspoon of calcium hydroxide should completely saturate the liquid.
- Fill the jar with distilled or tap water. The first allows to obtain a pure solution, but the minerals present in the tap solution should not alter the test.
- Put the lid on the jar, shake the solution vigorously for 1-2 minutes and then let it sit for 2 hours.
- Pour the clearer liquid from the top of the container through an American or paper coffee filter. Be very careful not to scramble the sediments; if necessary, repeat the filtration process until you get a perfectly clear solution. Then keep it in a clean bottle or jar.
Step 3. Create gas bubbles in the lime water
Half-fill a test tube with the solution and boil the liquid. Use a cannula to transfer the contents of the CO sample tube2 directly in boiling lime water. You should use a flexible diffusion tube or, failing that, a metal cannula; let the gas "boil" in the liquid and wait for the reaction to start.
If you prefer not to boil the liquid, you can inject the gas directly into the half-filled tube of lime water using a laboratory syringe. Close the container with a cap and shake it vigorously for 1-2 minutes; if there is carbon dioxide in the sample, the liquid becomes cloudy
Step 4. Look at the cloudy water
If the gas sample contains CO2, lime water becomes milky due to suspended calcium carbonate particles. If the liquid is boiling and you enter the gas, the reaction should begin immediately; if nothing happens for a minute or so, you can confidently say that there is no carbon dioxide in the sample.
Step 5. Know the chemical reaction
Understand what is the phenomenon that occurs and that indicates the presence of CO2. The chemical equation that describes it is: Ca (OH)2 (aq) + CO2 (g) -> CaCO3 (s) + H2O (l). In other words: the union between lime water (liquid) and gas (which contains CO2) triggers the formation of solid lime (particles) and liquid water.
Method 3 of 3: with a lit match
Step 1. Try using the gas sample to put out a flame
Carbon dioxide, in high concentrations, extinguishes the fire. Just keep a small lit match inside the test tube that may contain CO2; if gas is present, the flame should go out immediately. Combustion (the process of creating fire) is the reaction between oxygen and another substance, it consists of a rapid oxidation of the organic compound and the reduction of oxygen. The fire goes out because oxygen is replaced by CO2, which is an incombustible gas.
Be aware that any gaseous compound in which oxygen is not present causes a flame to go out; therefore, this test is not reliable for expressly identifying CO2 and it could mislead you.
Step 2. Collect the gas in an inverted tube
Make sure the sample is stored properly and the vessel is hermetically sealed before proceeding. Make sure the tube does not contain flammable or explosive gases; in this case, the introduction of a lit match could be dangerous or at least very frightening.
Step 3. Insert the flame into the tube
Use a long match or a strip of wood. A common match or lighter is also fine, but the further your fingers are from the opening of the container, the safer the experiment. If the flame goes out immediately, there is a high concentration of carbon dioxide in the tube.
Step 4. Alternatively, try using a gas-tight syringe to blow out a candle
Fill the syringe with the sample. He then uses a drop of melted wax to attach a small candle to a coin; transfer everything to a cup with a large opening and light the candle. Insert the tube into the syringe and transfer the CO2 at the bottom of the cup. If you release the entire contents of the syringe within a couple of seconds, the flame should go out.
