The primary method to purify copper sulfate is through recrystallization.
Here's a detailed explanation of the recrystallization process:
Steps for Purifying Copper Sulfate by Recrystallization:
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Dissolve: Begin by dissolving the impure copper sulfate in a minimum amount of boiling water to create a saturated solution. Using the least amount of water maximizes the yield of purified crystals.
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Filter (Optional but Recommended): If there are any undissolved solid impurities, filter the hot solution using filter paper and a funnel. This removes insoluble impurities.
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Cool: Allow the hot, saturated solution to cool slowly. As the solution cools, the copper sulfate will crystallize out of the solution. Slow cooling encourages the formation of larger, purer crystals. Cooling to around 0°C (using an ice bath) maximizes the amount of copper sulfate that crystallizes, typically recovering about 5/6 of the original copper sulfate in purified form.
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Separate the Crystals: Once crystallization is complete, separate the copper sulfate crystals from the remaining solution (called the mother liquor). This can be done by decanting the solution, filtering the crystals, or using a centrifuge.
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Wash: Wash the crystals with a small amount of cold water to remove any adhering impurities from the surface. Using cold water minimizes the amount of copper sulfate that dissolves during the washing process.
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Dry: Dry the purified copper sulfate crystals. This can be done by spreading them out on filter paper and allowing them to air dry, or by using a drying oven at a low temperature.
Explanation of the Process:
- Solubility and Temperature: Copper sulfate's solubility in water increases with temperature. This is why you dissolve it in boiling water.
- Impurity Exclusion: As the copper sulfate crystallizes, the impurities tend to remain in the solution because they are present in lower concentrations and their crystal structures differ.
- Saturation and Cooling: Cooling the saturated solution forces the copper sulfate to come out of the solution as crystals because the solution can no longer hold as much copper sulfate at the lower temperature.
Example:
Imagine you have a batch of copper sulfate contaminated with iron sulfate. By dissolving the mixture in hot water, filtering (if needed), and slowly cooling, the copper sulfate will crystallize, leaving much of the iron sulfate in the solution because copper sulfate is much less soluble at lower temperatures.
In summary, purifying copper sulfate involves dissolving the impure compound in hot water, allowing it to slowly cool and crystallize, separating the purified crystals, washing them, and then drying them. This recrystallization process leverages the differences in solubility to separate copper sulfate from its impurities.
