When carbon dioxide is blown into lime water, the lime water turns milky.
This phenomenon occurs due to a chemical reaction. Lime water is a common name for a saturated solution of calcium hydroxide (Ca(OH)₂). When carbon dioxide (CO₂) is bubbled through it, the CO₂ reacts with the calcium hydroxide to form calcium carbonate (CaCO₃), which is insoluble in water. This calcium carbonate precipitates out of the solution as very fine particles, causing the solution to appear milky.
The Chemical Reaction
The reaction can be represented by the following equation:
Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)
Where:
- Ca(OH)₂(aq) represents calcium hydroxide in aqueous solution (lime water)
- CO₂(g) represents carbon dioxide gas
- CaCO₃(s) represents solid calcium carbonate (the precipitate)
- H₂O(l) represents liquid water
What Happens with Excess Carbon Dioxide?
If you continue to bubble carbon dioxide through the milky solution, the milkiness will eventually disappear. This is because the calcium carbonate reacts with more carbon dioxide and water to form calcium bicarbonate (Ca(HCO₃)₂), which is soluble in water.
The reaction for this is:
CaCO₃(s) + H₂O(l) + CO₂(g) → Ca(HCO₃)₂(aq)
Summary of Changes
| Stage | Observation | Explanation |
|---|---|---|
| Initial blowing of carbon dioxide | Lime water turns milky | Formation of insoluble calcium carbonate (CaCO₃) precipitate. |
| Continued blowing of carbon dioxide | Solution clears up | Calcium carbonate reacts with more carbon dioxide and water to form soluble calcium bicarbonate (Ca(HCO₃)₂). |
This reaction is a classic test for the presence of carbon dioxide.
