What is the colour of the precipitate obtained by passing CO 2gas through lime water ?
White
Understanding the Reaction of Carbon Dioxide with Lime Water
The question asks about the color of the precipitate formed when carbon dioxide gas is passed through lime water. To answer this, we need to understand what lime water is and how it reacts with carbon dioxide.
What is Lime Water?
Lime water is the common name for a dilute solution of calcium hydroxide, chemical formula Ca(OH)₂. It is a clear, colorless liquid.
The Reaction with Carbon Dioxide
When carbon dioxide (CO₂) gas is bubbled through lime water (Ca(OH)₂ solution), a chemical reaction occurs. Carbon dioxide reacts with calcium hydroxide to form calcium carbonate (CaCO₃) and water (H₂O).
The chemical equation for this reaction is:
\( \text{Ca(OH)}_2\text{(aq)} + \text{CO}_2\text{(g)} \rightarrow \text{CaCO}_3\text{(s)} + \text{H}_2\text{O(l)} \)
In this reaction:
- Ca(OH)₂(aq) is Calcium Hydroxide (lime water)
- CO₂(g) is Carbon Dioxide gas
- CaCO₃(s) is Calcium Carbonate (precipitate)
- H₂O(l) is Water
The Precipitate Formation and Color
Calcium carbonate (CaCO₃) is a substance that is practically insoluble in water. When it is formed in the reaction, it does not dissolve but instead forms a solid suspension in the water. This suspension appears cloudy or milky, and it is called a precipitate. The solid calcium carbonate appears white.
Therefore, passing carbon dioxide through lime water results in the formation of a white precipitate of calcium carbonate, making the lime water turn milky.
| Reactants | Products | Precipitate & Color |
|---|---|---|
| Calcium Hydroxide (Lime Water) + Carbon Dioxide | Calcium Carbonate + Water | Calcium Carbonate (White Precipitate) |
What Happens with Excess Carbon Dioxide?
It's worth noting that if carbon dioxide gas is passed through the lime water for a longer time (in excess), the white precipitate of calcium carbonate will eventually dissolve. This happens because the excess carbon dioxide reacts with calcium carbonate and water to form soluble calcium bicarbonate (Ca(HCO₃)₂).
\( \text{CaCO}_3\text{(s)} + \text{H}_2\text{O(l)} + \text{CO}_2\text{(g)} \rightarrow \text{Ca(HCO}_3)_2\text{(aq)} \)
However, the initial precipitate formed when CO₂ is first passed through lime water is white calcium carbonate.
Conclusion on Precipitate Color
Based on the reaction and the nature of calcium carbonate, the color of the precipitate obtained by passing CO₂ gas through lime water is white.
Revision Table: Chemical Reactions
| Term | Definition/Description |
|---|---|
| Lime Water | Solution of Calcium Hydroxide (Ca(OH)₂) |
| Carbon Dioxide | CO₂ gas |
| Precipitate | Solid that forms from a solution during a chemical reaction |
| Calcium Carbonate | CaCO₃, the main component of limestone, chalk, and marble. Forms a white precipitate. |
| Chemical Equation | Symbolic representation of a chemical reaction |
Additional Information: Calcium Compounds
Calcium is an important element in many chemical compounds. Some related compounds include:
- Calcium Oxide (CaO): Also known as quicklime. Formed by heating calcium carbonate. Reacts with water to form calcium hydroxide.
- Calcium Hydroxide (Ca(OH)₂): Also known as slaked lime. Formed by adding water to calcium oxide. Used to make lime water.
- Calcium Bicarbonate (Ca(HCO₃)₂): Formed when carbon dioxide dissolves calcium carbonate in water. It is soluble and is found in hard water.
These calcium compounds are involved in various industrial processes and natural phenomena, including the carbon cycle and the formation of geological structures like stalactites and stalagmites in caves.
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