Alkyne Lab: Making Ethyne
React calcium carbide with water
What this lesson covers
Why it matters
Crush a grey rock into water and watch it hiss, releasing a gas that smells faintly of garlic.
The idea in plain words
Match the reactants to products: CaC₂ + 2H₂O → Ca(OH)₂ + C₂H₂.
Predict first
What gas is released when calcium carbide reacts with water?
The reaction produces ethyne gas and a white precipitate of calcium hydroxide.
- Ethyne (C₂H₂) — correct
- Ethene (C₂H₄)
- Carbon dioxide (CO₂)
- Hydrogen (H₂)
What you do
Add the calcium carbide to the water. Observe the vigorous bubbling and the white residue left behind.
Check yourself
What is the visible sign of the reaction occurring in the beaker?
Calcium hydroxide is only sparingly soluble in water, so here it appears as a white suspension/precipitate.
Which equation correctly represents this preparation?
The reaction is a double decomposition yielding calcium hydroxide and ethyne.
Why is this reaction classified as double decomposition?
CaC₂ and H₂O exchange parts to form Ca(OH)₂ and C₂H₂.
- A white precipitate forms — correct
- The solution turns blue
- The solution boils violently without gas
- CaC₂ + H₂O → CaO + C₂H₂
- CaC₂ + 2H₂O → Ca(OH)₂ + C₂H₂ — correct
- CaC₂ + 2H₂O → CaH₂ + C₂O₂
- Two compounds exchange ions to form two new compounds — correct
- One compound breaks down into simpler substances
- Two elements combine to form a compound