# Test for carbon dioxide ## What you'll learn - Which chemical is used to test for carbon dioxide gas. - How to carry out the test by bubbling or shaking with limewater. - Why limewater turns milky when carbon dioxide is present. - How to describe the test accurately in GCSE exam answers. ## Why do we need a gas test? Carbon dioxide, CO₂, is a **colourless gas**. That means you usually cannot identify it just by looking at it. In chemistry, you use a **test** that gives a clear, observable result. For carbon dioxide, the test uses **limewater**. A **gas test** is a practical method used to identify a gas by producing a distinctive observation, such as a colour change, a sound, or a cloudy solution. Carbon dioxide is commonly produced when a **carbonate** reacts with an acid. For example, marble chips are calcium carbonate, CaCO₃, and they make carbon dioxide when reacted with dilute hydrochloric acid. ## The reagent: limewater The **reagent** is the chemical you use to carry out a test. The reagent for carbon dioxide is **limewater**. **Limewater** is an aqueous solution of calcium hydroxide, Ca(OH)₂(aq). **Aqueous** means dissolved in water, so the state symbol is (aq). Limewater starts off **clear and colourless**. If carbon dioxide is present, it turns **milky** or **cloudy**. In GCSE Chemistry, **limewater** means calcium hydroxide solution, not the juice from a lime fruit. Always name it as **limewater** or **aqueous calcium hydroxide**. ## The positive result A **positive result** means the test has shown the gas you were looking for is present. To test for carbon dioxide, bubble the gas through limewater or shake it with limewater. If carbon dioxide is present, the limewater turns **milky/cloudy**. This is the exact observation examiners expect. “Turns cloudy” and “turns milky” both describe the correct result. ## What causes the milkiness? Carbon dioxide reacts with calcium hydroxide in the limewater. The reaction forms **calcium carbonate**, CaCO₃. Calcium carbonate is **insoluble**, meaning it does not dissolve in water. Instead, tiny solid particles form in the liquid, making the limewater look cloudy. A **precipitate** is an insoluble solid that forms during a reaction in a solution. The word equation is: **carbon dioxide + calcium hydroxide → calcium carbonate + water** The balanced symbol equation, including state symbols, is: **CO₂(g) + Ca(OH)₂(aq) → CaCO₃(s) + H₂O(l)** The state symbols mean: - **(g)** = gas - **(aq)** = aqueous, dissolved in water - **(s)** = solid - **(l)** = liquid The important product is **CaCO₃(s)**. This solid calcium carbonate is the precipitate that makes the limewater appear milky. ## The practical setup You can test carbon dioxide by passing the gas into a test tube of limewater using a delivery tube. The end of the delivery tube should go below the surface of the limewater so the gas bubbles through it. ![Schematic of carbon dioxide gas being bubbled through limewater and turning it cloudy](https://assets.yesgenie.com/notes/fba6a03f-5a5e-4e97-9f45-516473985b71.png) ### Method 1: bubbling through limewater Use this method when carbon dioxide is being produced in a reaction, such as an acid reacting with a carbonate. 1. Put a small amount of limewater into a clean test tube. 2. Use a delivery tube to bubble the gas through the limewater. 3. Watch for the limewater turning milky or cloudy. ### Method 2: shaking with limewater Use this method if the gas has already been collected in a test tube or gas jar. 1. Add a small amount of limewater to the container of gas. 2. Stopper the container. 3. Shake gently. 4. A positive result is limewater turning milky/cloudy. For a two-mark exam answer, include both the **m