Theory
Carbon dioxide and the carbonates are among the most important compounds of carbon in its +4 oxidation state. They connect laboratory chemistry to everyday processes: respiration, combustion, the hardness of water, the setting of cement, and the carbon cycle. This lab studies the preparation and tests of carbon dioxide and the reactions of carbonates and hydrogencarbonates.
1. Carbon dioxide
Carbon dioxide (CO₂) is a colourless, odourless gas, denser than air and slightly soluble in water, where it forms the weak acid carbonic acid (H₂CO₃). In the laboratory it is made by adding a dilute acid to a carbonate, for example dilute hydrochloric acid on calcium carbonate (marble chips). It does not support combustion and is used in fire extinguishers.
2. The limewater test
The confirmatory test for carbon dioxide is the limewater test. When CO₂ is bubbled through limewater (calcium hydroxide solution), a white precipitate of calcium carbonate forms, turning the limewater milky. If excess CO₂ is passed, the milkiness clears as the insoluble calcium carbonate is converted to soluble calcium hydrogencarbonate.
3. Carbonates and hydrogencarbonates
The carbonate ion (CO₃²⁻) and the hydrogencarbonate ion (HCO₃⁻) both react with dilute acids to release carbon dioxide with brisk effervescence. This fizzing, together with a positive limewater test on the gas, is the standard test for a carbonate or hydrogencarbonate.
4. Thermal decomposition
Most metal carbonates decompose on heating to give the metal oxide and carbon dioxide; for example, calcium carbonate decomposes to calcium oxide (quicklime) and CO₂. Group 1 carbonates (except lithium) are unusually stable and resist this decomposition. Hydrogencarbonates decompose even more easily than carbonates.
5. Hardness of water
Temporary hardness in water is caused by dissolved calcium hydrogencarbonate. Boiling decomposes it to insoluble calcium carbonate (limescale) plus water and carbon dioxide, removing the hardness. This is the laboratory basis of limescale formation in kettles and pipes.
Apparatus and Reagents
The equipment and reagents a real carbonate and carbon-dioxide experiment uses. In this simulation they are modelled for you, but the observations correspond to what each test would actually show.
- Test tubes, delivery tube, test-tube rack, and a Bunsen burner.
- Limewater (calcium hydroxide solution) for the carbon-dioxide test.
- Calcium carbonate (marble chips) and other metal carbonates.
- Sodium carbonate (Na₂CO₃) and sodium hydrogencarbonate (NaHCO₃).
- Dilute hydrochloric acid (HCl) and dilute sulfuric acid.
- Blue and red litmus paper for testing the acidity of the gas.
- A gas jar and lid for collecting carbon dioxide by downward delivery.
Instructions
This is a predict-and-check lab. In each tab you decide the answer yourself first, choose it, and only then does the simulation confirm the correct result so you can compare. Record every result in your worksheet.
Simulation — The Carbonate Bench
Team Questions
Work these out with your team, type each answer, and check it.
Example Lab Report
A worked example showing the expected format and the predict-and-check workflow.
Carbonates and Carbon Dioxide
Chemistry | Section: [Your Section] | Date: [Date]
Lab Members: [Names of all members present]
Objective — To prepare and test carbon dioxide, to carry out and interpret the identification tests for carbonates and hydrogencarbonates, and to predict the products of their reactions, comparing every prediction with the simulation.
Part A — Identification Tests (worked example)
Adding dilute hydrochloric acid to marble chips produced brisk effervescence; the gas was passed into limewater, which turned milky, confirming carbon dioxide. Passing excess CO₂ cleared the milkiness as soluble calcium hydrogencarbonate formed. A burning splint was extinguished in the gas, confirming CO₂ does not support combustion.
Part B — Reactions and Products (worked example)
Heating calcium carbonate gave calcium oxide and carbon dioxide (CaCO₃ → CaO + CO₂). Sodium carbonate, a Group 1 carbonate, did not decompose under Bunsen heating. Gently heating sodium hydrogencarbonate gave sodium carbonate, water, and CO₂ (2 NaHCO₃ → Na₂CO₃ + H₂O + CO₂). Each predicted product matched the simulation.
Part C — Carbonate vs Hydrogencarbonate (worked example)
Both ions fizzed with acid and gave a positive limewater test. They were distinguished by gentle heating: the solid hydrogencarbonate released CO₂ readily, while the carbonate required much stronger heating. The carbonate ion carries a 2- charge (CO₃²⁻); the hydrogencarbonate ion carries a 1- charge (HCO₃⁻).
Discussion and Conclusion — Every prediction agreed with the simulation. Carbon dioxide was confirmed by the limewater test and by extinguishing a flame; carbonates and hydrogencarbonates were identified by effervescence with acid; and the reactions followed from the +4 oxidation state of carbon and the relative thermal stabilities of the ions. The link to temporary water hardness and limescale was confirmed by the boiling of calcium hydrogencarbonate.
Practice Quiz
For extra practice on this topic, use the interactive Carbonates and Carbon Dioxide quiz in the Quizzes section of the site.