What we want to do and why this method
In this lab activity we make carbon dioxide (CO₂), collect it and test its properties. The easiest way is to react a carbonate with an acid.
Reagents: marble chips (calcium carbonate, CaCO₃) and dilute hydrochloric acid (HCl).
Equation: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂
Why not dilute sulfuric acid? It makes calcium sulfate, which is hard to dissolve. It coats the marble and the reaction stops soon. Hydrochloric acid gives calcium chloride, which dissolves, so the bubbling goes on.
Apparatus and steps
Apparatus: conical flask, one-hole cork, thistle funnel, bent delivery tube, gas jar, glass cover plate, limewater, a wooden splint, matches.
- Put a few marble chips in the flask.
- Fit the cork with the funnel. The funnel end must dip below the acid level later, so gas cannot escape upward.
- Pour dilute HCl through the funnel.
- Lead the gas through the tube into a dry gas jar.
- When the jar is full, cover it with the plate.
Safety: wear goggles, keep the acid dilute, and never smell gas directly.
Collecting the gas: upward delivery
CO₂ is about 1.5 times as heavy as air. It sinks and pushes air up and out. So we keep the jar mouth up and let the gas run to the bottom. This is called upward delivery (downward displacement of air).
We do not collect it over water because CO₂ dissolves in water a little. We do not use the downward-facing jar used for light gases such as hydrogen.
Tests and properties of CO₂
- Limewater test: CO₂ turns limewater (calcium hydroxide solution) milky. White calcium carbonate forms: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O. If you keep passing the gas, the milk clears, because soluble calcium hydrogencarbonate forms.
- Burning splint: a burning splint goes out. CO₂ does not support burning.
- Moist blue litmus: turns red, because CO₂ with water makes a weak acid, carbonic acid (H₂CO₃).
- Other facts: colourless, no smell, heavier than air, a little soluble in water.
Observe and write the report
Write: aim, apparatus, steps, what you saw, equations and conclusion. A good record has a table with Test, Observation and Inference. For example: limewater + gas → milky → CO₂ present. Add one safety line and one source of error (for example, a leak at the cork).
Key formulas and definitions
- CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂
- Ca(OH)₂ + CO₂ → CaCO₃ + H₂O (milky)
- CaCO₃ + H₂O + CO₂ → Ca(HCO₃)₂ (milk clears)
- CO₂ + H₂O ⇌ H₂CO₃ (weak acid)
Worked examples
1. A student passes the gas from marble and acid into limewater. It turns milky. What does this show?
The gas is carbon dioxide. It reacted with calcium hydroxide to give white calcium carbonate.
2. Why is the gas jar kept with its mouth upward?
CO₂ is heavier than air. It sinks to the bottom of the jar and pushes the air out through the top.
3. The bubbling slows down and stops after a minute when sulfuric acid is used. Give a reason.
Calcium sulfate forms and sticks on the marble as a layer. The acid cannot reach the marble any more.
Common mistakes
- Thinking limewater always stays milky. With too much CO₂ it turns clear again.
- Using a downward-facing jar for CO₂. That is for light gases; CO₂ is heavy.
- Using strong acid or powder. The reaction becomes too fast and foams out of the flask.
- Saying a burning splint test proves CO₂. Other gases such as nitrogen also put it out, so use limewater to be sure.