Question

Give scientific reasons (any two):
When the gas formed on heating limestone is passed through freshly prepared lime water, the lime water turns milky.

Hint:

To test for carbon dioxide gas, pass it through freshly prepared lime water. If the solution turns milky, it confirms the presence of \(CO_2\). Excess \(CO_2\) dissolves the precipitate, making the solution clear again.

Answer 1

Step 1: Identifying the gas produced
- Limestone (\(\text{CaCO}_3\)) is heated to produce carbon dioxide gas. - The reaction is: \[ \text{CaCO}_3 \xrightarrow{\text{heat}} \text{CaO} + \text{CO}_2 \]
Step 2: Reaction with lime water
- The released \(\text{CO}_2\) is bubbled through freshly prepared lime water (\(\text{Ca(OH)}_2\)). - A chemical reaction occurs: \[ \text{Ca(OH)}_2 + \text{CO}_2 \rightarrow \text{CaCO}_3 \downarrow + \text{H}_2\text{O} \] - Calcium carbonate (\(\text{CaCO}_3\)) is insoluble in water and forms a white precipitate, making lime water appear milky.
Step 3: Conclusion
- The milkiness confirms the presence of \(\text{CO}_2\). - If excess \(\text{CO}_2\) is passed, the milkiness disappears due to the formation of soluble calcium bicarbonate (\(\text{Ca(HCO}_3\text{)}_2\)). Thus, the lime water turns milky due to the formation of insoluble calcium carbonate (\(\text{CaCO}_3\)).