The solubility of a solute is 35 at 60°C. If 200g of solvent is made to saturate with the solute at 60°C, what is the amount of precipitate formed at 30°C?
Answers
A saturated solution is one where no more solute can be dissolved in thesolvent. In other words, the rates at which the solvent is dissolving into solution and re-crystalizing are equal, so it appears as if no more is dissolving, though it is a dynamic state.
Anything you try to dissolve can be a saturated solution, it just depends on theKsp/solubility of the solute and what solvent you are using. Soda, for example, is a saturated solution of CO2 and water.
Basically, you will get excess precipitate at the bottom of your solution which will not dissolve unless you alter the conditions by doing something like increasing the temperature or pressure.
A supersaturated solution contains more solute at a given temperature than is needed to form a saturated solution.
Increased temperature usually increases the solubility of solids in liquids.
For example, the solubility of glucose at 25 °C is 91 g/100 mL of water. The solubility at 50 °C is 244 g/100 mL of water.
If we add 100 g of glucose to 100 mL water at 25 °C, 91 g dissolve. Nine grams of solid remain on the bottom. We have a saturated solution.
If we now heat the mixture to 50 °C, the remaining 9 g of glucose will dissolve. At the new temperature, the solubility limit in 100 mL of water is 244 g glucose. With only 100 g of glucose dissolved, the solution is now unsaturated.
If we next cool the mixture back to 25 °C, 9 g of glucose should precipitate from solution.
If glucose crystals do not form, the system has more dissolved glucose (100 g) than it can hold at 25 °C (91 g). We have a supersaturated solution.
The first step in the formation of crystals is nucleation. This is when the solute molecules arrange themselves to form crystals.
Sometimes this happens at once. If it doesn't, we have a supersaturated solution. This is an unstable situation.
A piece of dust or a small crystal of the solute, a seed crystal, provides a template for crystallization of the excess solute. The excess solute starts to form crystals on the nuclei.
Once crystals start to form, their surface area increases as they grow. This attracts more molecules and promotes growth at an ever-increasing rate. Finally, the solution stabilizes and no more crystals can form.
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