What we need: reagents
Reagent means a chemical used in a reaction. For CO₂ we use two.
- Marble chips (CaCO₃) or limestone. Chalk also works. Small chips react at a steady speed.
- Dilute hydrochloric acid (HCl). "Dilute" means mixed with plenty of water.
The reaction is: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂↑. Words: calcium carbonate + hydrochloric acid → calcium chloride + water + carbon dioxide.
Any carbonate or hydrogencarbonate reacts with an acid in the same way, so baking soda and vinegar also make CO₂.
Why these chemicals and not others?
Why not dilute sulphuric acid? It makes calcium sulphate, CaSO₄. This salt is almost insoluble. It coats the marble like a thin skin and the acid cannot reach the marble again. The reaction stops after a short time.
Why not powder? Powdered carbonate has a very large surface, so it reacts too fast and the froth can rush up the tube.
Why not strong acid? A strong acid also gives HCl fumes that mix with the gas.
The apparatus
The common set-up has five parts.
- Flask (or a conical flask / Woulfe bottle) that holds the chips.
- Thistle funnel to pour acid in. Its lower end must dip below the acid so gas cannot escape upward.
- Cork that seals the flask.
- Delivery tube bent twice. It carries gas to the jar.
- Gas jar kept upright. The tube reaches almost to its bottom.
A Kipp's apparatus does the same job and lets you stop and start the gas by closing a tap.
Collecting the gas
CO₂ is about 1.5 times heavier than air (relative mass 44 against about 29). It also dissolves a little in water. So we collect it by upward delivery of air, also called downward displacement of air: the gas settles at the bottom of an upright jar and pushes air out of the top.
Collecting over water is not good, because some gas dissolves and the gas stays wet. To get dry gas, pass it through a bottle of concentrated sulphuric acid first.
To know the jar is full, hold a burning splint at its mouth: it goes out.
Testing for carbon dioxide
Flame test: a burning splint goes out in CO₂, because CO₂ does not burn and does not help burning.
Limewater test: shake the gas with limewater, which is calcium hydroxide solution. It turns milky: Ca(OH)₂ + CO₂ → CaCO₃↓ + H₂O. The milk is tiny solid calcium carbonate.
If you keep passing more CO₂, the milk goes clear again: CaCO₃ + H₂O + CO₂ → Ca(HCO₃)₂, which dissolves.
Blue litmus turns faintly red in water with dissolved CO₂, because carbonic acid is weak.
Try it: fizz at home
Put a pinch of baking soda in a glass and add a little vinegar. See the bubbles. Hold a burning splint over the glass (with an adult near): the flame dims or goes out, because the heavy gas sits in the glass. First predict, then check.
Key formulas and definitions
- CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂
- Ca(OH)₂ + CO₂ → CaCO₃ (milky) + H₂O
- CaCO₃ + H₂O + CO₂ → Ca(HCO₃)₂ (milk clears)
- 100 g CaCO₃ gives 44 g CO₂
- 1 mole of gas at STP = 22.4 L
Worked examples
1. Name the two chemicals used to make CO₂ in the lab and write the equation.
Marble chips (CaCO₃) and dilute hydrochloric acid. CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂.
2. Why is the lower end of the thistle funnel kept below the acid?
So that the acid seals the funnel and the gas cannot leave through it. All the gas must go through the delivery tube.
3. How is CO₂ collected and why?
By upward delivery of air in an upright gas jar, because CO₂ is heavier than air and dissolves a little in water.
4. How much CO₂ comes from 25 g of CaCO₃ (enough acid)?
100 g CaCO₃ gives 44 g CO₂. So 25 g gives 25 × 44 / 100 = 11 g.
5. A gas turns limewater milky. A student says it must be CO₂. Is that enough proof?
Not fully. Sulphur dioxide also turns limewater milky. Add the burning splint test and the way the gas was made. For CO₂ the splint goes out and the gas came from a carbonate and an acid.
6. Find the volume of CO₂ at STP from 10 g of CaCO₃.
Moles of CaCO₃ = 10 / 100 = 0.1 mol. Each mole gives one mole CO₂, so 0.1 mol. Volume = 0.1 × 22.4 = 2.24 L.
Common mistakes
- Using dilute sulphuric acid: it coats the marble with CaSO₄ and the reaction stops.
- Collecting CO₂ by downward delivery with the jar upside down: the heavy gas falls out. The jar must be upright.
- Calling any gas that puts out a flame "CO₂". Nitrogen also puts it out. Use the limewater test too.
- Putting the thistle funnel above the acid, so gas escapes through it.