Which one of the following does not represent the salt, calcium carbonate?
Lime water
Understanding Calcium Carbonate and Related Substances
The question asks us to identify which of the given options is not a representation of the salt calcium carbonate. Let's examine each option to understand its chemical composition.
What is Calcium Carbonate?
Calcium carbonate is a common inorganic salt with the chemical formula \(\text{CaCO}_3\). It is found naturally in many forms and is a principal component of rocks like limestone, chalk, and marble. It is a white, insoluble solid.
Common Forms Representing Calcium Carbonate
- Limestone: This is a sedimentary rock composed primarily of calcium carbonate (\(\text{CaCO}_3\)). It is formed over millions of years from the accumulation of shell, coral, algal, and fecal debris.
- Chalk: This is a soft, white, porous sedimentary rock, also composed primarily of calcium carbonate (\(\text{CaCO}_3\)), specifically coccoliths (microscopic fossils of marine algae). It is a form of limestone.
- Marble: This is a metamorphic rock composed primarily of recrystallized carbonate minerals, most commonly calcium carbonate (\(\text{CaCO}_3\)). It is formed when limestone is subjected to high pressure and temperatures over time.
Thus, Limestone, Chalk, and Marble are all natural forms or derivatives that represent calcium carbonate.
What is Lime Water?
Lime water is a common name for a saturated solution of calcium hydroxide. Calcium hydroxide has the chemical formula \(\text{Ca(OH)}_2\). It is formed by slaking quicklime (calcium oxide, \(\text{CaO}\)) with water. While related to calcium, lime water is chemically distinct from calcium carbonate.
Comparing Lime Water and Calcium Carbonate
The key difference lies in their chemical composition:
- Calcium Carbonate: \(\text{CaCO}_3\)
- Calcium Hydroxide (in solution, i.e., Lime Water): \(\text{Ca(OH)}_2\)
These are different chemical compounds. Calcium carbonate is a carbonate salt, whereas calcium hydroxide is a base (alkali) which dissolves slightly in water to form an alkaline solution known as lime water.
Conclusion: Identifying the Substance Not Representing Calcium Carbonate
Based on the chemical compositions, Lime water (calcium hydroxide solution) is not calcium carbonate. Limestone, Chalk, and Marble are all forms of calcium carbonate.
Therefore, Lime water does not represent the salt, calcium carbonate.
Revision Table: Calcium Compounds
| Common Name | Chemical Name | Chemical Formula | Relation to Calcium Carbonate |
|---|---|---|---|
| Limestone | Calcium Carbonate | \(\text{CaCO}_3\) | A natural form of \(\text{CaCO}_3\) |
| Chalk | Calcium Carbonate | \(\text{CaCO}_3\) | A natural form of \(\text{CaCO}_3\) |
| Marble | Calcium Carbonate | \(\text{CaCO}_3\) | A metamorphic form of \(\text{CaCO}_3\) |
| Lime Water | Calcium Hydroxide Solution | \(\text{Ca(OH)}_2\) (in solution) | Not \(\text{CaCO}_3\); distinct compound |
| Quicklime | Calcium Oxide | \(\text{CaO}\) | Used to make \(\text{Ca(OH)}_2\) |
| Slaked Lime | Calcium Hydroxide | \(\text{Ca(OH)}_2\) | Solid form dissolved to make Lime Water |
Additional Information on Calcium Compounds
Calcium compounds are very common and important in chemistry and everyday life. Here are a few related points:
- Test for Carbon Dioxide: Lime water is famously used to test for the presence of carbon dioxide gas. When \(\text{CO}_2\) gas is bubbled through lime water, it reacts with calcium hydroxide to form insoluble calcium carbonate, which appears as a milky precipitate. The reaction is: \(\text{Ca(OH)}_2\text{(aq)} + \text{CO}_2\text{(g)} \rightarrow \text{CaCO}_3\text{(s)} + \text{H}_2\text{O(l)}\).
- From Quicklime to Lime Water: Quicklime (\(\text{CaO}\)) is produced by heating calcium carbonate (like limestone) strongly. This process is called calcination: \(\text{CaCO}_3\text{(s)} \rightarrow \text{CaO(s)} + \text{CO}_2\text{(g)}\). Quicklime reacts with water (slaking) to form calcium hydroxide: \(\text{CaO(s)} + \text{H}_2\text{O(l)} \rightarrow \text{Ca(OH)}_2\text{(s)}\). When calcium hydroxide is dissolved in water, it forms lime water.
- Hardness of Water: Calcium carbonate contributes to the temporary hardness of water when dissolved as calcium bicarbonate.
Understanding the difference between these calcium compounds is crucial for various chemical concepts.
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