When air is blown from mouth into a test tube containing limewater, the limewater turns milky. This is due to the presence of
carbon dioxide
Understanding the Limewater Test with Exhaled Air
When you blow air from your mouth into a test tube containing limewater, you observe that the limewater turns milky or cloudy. This phenomenon is a classic chemical test used to detect the presence of a specific gas in the exhaled air.
What is Limewater?
Limewater is essentially a dilute aqueous solution of calcium hydroxide, with the chemical formula \(\text{Ca(OH)}_2\). It is a clear, colourless solution.
The Reaction with Exhaled Air
Exhaled air contains a higher concentration of carbon dioxide compared to the air we inhale. When this exhaled air is bubbled through the limewater solution, the carbon dioxide (\(\text{CO}_2\)) reacts with the calcium hydroxide (\(\text{Ca(OH)}_2\)).
This reaction produces calcium carbonate (\(\text{CaCO}_3\)) and water (\(\text{H}_2\text{O}\)). The chemical equation for this reaction is:
\[ \text{Ca(OH)}_2\text{(aq)} + \text{CO}_2\text{(g)} \rightarrow \text{CaCO}_3\text{(s)} + \text{H}_2\text{O}\text{(l)} \]
The calcium carbonate (\(\text{CaCO}_3\)) formed is an insoluble solid. These tiny solid particles remain suspended in the solution, causing the limewater to appear milky or cloudy.
Identifying the Responsible Gas
Based on the chemical reaction described, the gas responsible for turning the limewater milky is carbon dioxide. The presence of carbon dioxide in the exhaled air reacts with the calcium hydroxide in limewater to form insoluble calcium carbonate.
Why Other Options are Incorrect
- Water Vapour: Exhaled air does contain water vapour, which might condense on the cooler parts of the test tube, but water vapour itself does not react with calcium hydroxide to produce an insoluble substance that would turn the limewater milky.
- Oxygen: Exhaled air still contains a significant amount of oxygen. However, oxygen does not react with calcium hydroxide in this manner to cause cloudiness.
- Carbon Monoxide: Carbon monoxide is a toxic gas. While it is a carbon compound, it is not a significant component of normal exhaled air, and it does not react with limewater to form calcium carbonate.
Therefore, the milky appearance of limewater when air from the mouth is blown into it is specifically due to the presence of carbon dioxide in the exhaled air.
Revision Table: Limewater Test Summary
| Substance | Component in Exhaled Air | Reaction with Limewater (\(\text{Ca(OH)}_2\)) | Effect on Limewater |
|---|---|---|---|
| Carbon Dioxide (\(\text{CO}_2\)) | Present (higher concentration than inhaled air) | Reacts to form \(\text{CaCO}_3\) (s) and \(\text{H}_2\text{O}\) | Turns milky/cloudy |
| Oxygen (\(\text{O}_2\)) | Present | No reaction | No change |
| Water Vapour (\(\text{H}_2\text{O}\)) | Present | No reaction (condensation may occur) | No change in clarity due to reaction |
| Carbon Monoxide (\(\text{CO}\)) | Not significant in normal exhaled air | No reaction to form \(\text{CaCO}_3\) | No change |
Additional Information: Carbon Dioxide and Respiration
The carbon dioxide present in exhaled air is a waste product of cellular respiration. During cellular respiration, our bodies use glucose (a sugar) and oxygen to produce energy, water, and carbon dioxide. This carbon dioxide is transported by the blood to the lungs and then expelled from the body during exhalation.
The limewater test is a simple and effective way to demonstrate that carbon dioxide is present in the air we breathe out, confirming one aspect of the process of respiration.
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