When sulphur dioxide (SO₂) reacts with lime water, which is a solution of calcium hydroxide ($Ca(OH)_2$), a distinct two-stage chemical process occurs, leading to observable changes in the solution's appearance.
Initially, the reaction forms calcium sulphite, a product that causes the lime water to turn a milky white color. If more sulphur dioxide is passed through the solution, this milkiness eventually disappears. This disappearance is due to the formation of calcium hydrogen sulphite, a compound that is soluble in water.
Initial Reaction: Formation of Calcium Sulphite
Upon the introduction of sulphur dioxide gas into clear lime water, the first chemical reaction takes place:
- Reactants: Sulphur dioxide ($SO_2$) gas and Calcium hydroxide ($Ca(OH)_2$), which is the chemical name for lime water.
- Product Formed: Calcium sulphite ($CaSO_3$) is produced as a solid.
- Observation: Calcium sulphite is a milky white precipitate. Its formation causes the previously clear lime water to turn cloudy or milky.
This reaction can be represented by the following chemical equation:
$SO_2(g) + Ca(OH)_2(aq) \rightarrow CaSO_3(s) + H_2O(l)$
Subsequent Reaction: Disappearance of Milkiness
If the supply of sulphur dioxide continues beyond the point where calcium sulphite has formed, the milky white precipitate reacts further. This secondary reaction involves the calcium sulphite, excess sulphur dioxide, and water present in the solution.
- Reactants: Calcium sulphite ($CaSO_3$), excess Sulphur dioxide ($SO_2$), and Water ($H_2O$).
- Product Formed: Calcium hydrogen sulphite ($Ca(HSO_3)_2$).
- Observation: Unlike calcium sulphite, calcium hydrogen sulphite is soluble in water. As this soluble compound forms, the milky white calcium sulphite precipitate dissolves, causing the solution to become clear again and the milkiness to disappear.
This subsequent reaction can be represented as:
$CaSO_3(s) + SO_2(g) + H_2O(l) \rightarrow Ca(HSO_3)_2(aq)$
Summary of Reactions and Observations
The table below summarizes the key aspects of sulphur dioxide's reaction with lime water:
| Stage | Reactant(s) Used | Product(s) Formed | Observable Change | Product Solubility |
|---|---|---|---|---|
| Initial Reaction | Sulphur dioxide & Lime water (Calcium hydroxide) | Calcium sulphite ($CaSO_3$) | Clear solution turns milky white | Insoluble |
| Subsequent Reaction | Calcium sulphite, Excess Sulphur dioxide & Water | Calcium hydrogen sulphite ($Ca(HSO_3)_2$) | Milkiness disappears, solution becomes clear | Soluble |
This two-step process is a classic chemical test used to detect the presence of sulphur dioxide gas, demonstrating how its concentration can lead to different observable outcomes.
