Carbon dioxide is the gas that turns lime water milky. This is a classic and widely recognized chemical test used to identify the presence of carbon dioxide.
The Chemistry Behind the Milkiness
Lime water is essentially a clear, aqueous solution of calcium hydroxide (Ca(OH)₂). When carbon dioxide (CO₂) gas is bubbled through this solution, a chemical reaction occurs. The carbon dioxide reacts with the calcium hydroxide to form calcium carbonate (CaCO₃) and water (H₂O).
Thus, carbon dioxide is the gas due to which the lime water turns milky.
- Reactants:
- Calcium Hydroxide (Lime Water): Ca(OH)₂(aq)
- Carbon Dioxide Gas: CO₂(g)
- Chemical Reaction:
- Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)
- Products:
- Calcium Carbonate (CaCO₃): This compound is a white, insoluble solid. It precipitates out of the solution as fine particles, creating a suspension that appears milky or cloudy. For more details on calcium carbonate, you can refer to resources like Wikipedia.
- Water: H₂O(l)
The insoluble calcium carbonate particles scatter light, which is why the clear lime water becomes opaque and takes on a milky appearance. It's important to note that if an excessive amount of carbon dioxide is passed through the milky solution, the calcium carbonate can react further to form calcium bicarbonate (Ca(HCO₃)₂), which is soluble in water. This secondary reaction causes the milky appearance to disappear, and the solution becomes clear again.
Common Applications of the Lime Water Test
The lime water test is a simple yet effective indicator for the presence of carbon dioxide and is utilized in various scientific and educational contexts:
- Biology Experiments:
- Respiration: To demonstrate that living organisms, including humans and animals, exhale carbon dioxide as a byproduct of cellular respiration. Blowing breath into lime water is a common example.
- Plant Biology: To show that plants produce carbon dioxide during respiration (especially in the dark) and consume it during photosynthesis.
- Chemistry Demonstrations:
- Combustion: To confirm that burning substances containing carbon (e.g., wood, candles, fossil fuels) produce carbon dioxide.
- Acid-Carbonate Reactions: To detect carbonate ions in a sample; when an acid is added, CO₂ is released, which can then be tested with lime water.
- Environmental Monitoring (Basic): For simple, qualitative assessments of carbon dioxide presence in certain gases.
Summary Table: Lime Water Reaction
| Original State | Reactant Gas | Product Causing Milkiness | Overall Result |
|---|---|---|---|
| Clear Solution | Carbon Dioxide | Calcium Carbonate (CaCO₃) | Solution turns milky |
This reaction provides a reliable visual confirmation of carbon dioxide.
