Testing for Carbon Dioxide Gas

The question asks about a specific chemical test used to identify a gas that turns lime water milky. This is a common and important test in chemistry.

What is Lime Water?

Lime water is simply the common name for a dilute aqueous solution of calcium hydroxide, with the chemical formula \( \text{Ca(OH)}_2 \). It is a clear, colorless liquid.

The Reaction with Carbon Dioxide

When carbon dioxide gas (\( \text{CO}_2 \)) is passed through lime water, a chemical reaction occurs. Carbon dioxide reacts with the calcium hydroxide to form calcium carbonate (\( \text{CaCO}_3 \)). Calcium carbonate is a solid substance that does not dissolve in water; it is insoluble. This insoluble solid forms tiny particles that are suspended in the solution, making it appear cloudy or milky.

The chemical equation for this reaction is:

\( \text{Ca(OH)}_2\text{(aq)} + \text{CO}_2\text{(g)} \rightarrow \text{CaCO}_3\text{(s)} + \text{H}_2\text{O(l)} \)

Here:

• \( \text{Ca(OH)}_2\text{(aq)} \) represents calcium hydroxide solution (lime water).
• \( \text{CO}_2\text{(g)} \) represents carbon dioxide gas.
• \( \text{CaCO}_3\text{(s)} \) represents solid calcium carbonate (the white precipitate).
• \( \text{H}_2\text{O(l)} \) represents water.

The formation of the solid calcium carbonate is what causes the milky appearance in the lime water.

Why Other Gases Don't Turn Lime Water Milky

• Nitrogen dioxide (\( \text{NO}_2 \)): Nitrogen dioxide is a reddish-brown gas. While it reacts with water, it does not produce insoluble calcium carbonate when bubbled through lime water.
• Carbon monoxide (\( \text{CO} \)): Carbon monoxide is a colorless, odorless gas. It is much less reactive than carbon dioxide and does not react with lime water to form calcium carbonate.
• Oxygen (\( \text{O}_2 \)): Oxygen is a colorless, odorless gas essential for life. It is relatively unreactive and does not react with lime water.

Therefore, among the given options, only carbon dioxide gas reacts with lime water in a way that produces an insoluble precipitate (calcium carbonate), leading to the characteristic milky appearance.

Conclusion

The gas that turns lime water milky is carbon dioxide.

Revision Table: Common Gas Tests

Additional Information on Carbon Dioxide and Lime Water

It's important to note what happens if you pass an excess amount of carbon dioxide through the milky lime water. Initially, the milky precipitate of calcium carbonate forms:

\( \text{CaCO}_3\text{(s)} + \text{H}_2\text{O(l)} + \text{CO}_2\text{(g)} \rightarrow \text{Ca(HCO}_3\text{)}_2\text{(aq)} \)

When excess carbon dioxide is passed, the insoluble calcium carbonate reacts further with water and carbon dioxide to form calcium bicarbonate (\( \text{Ca(HCO}_3\text{)}_2 \)), which is soluble in water. This reaction causes the milky solution to turn clear again.

So, the milky appearance due to carbon dioxide is temporary if the gas is passed in excess.