•Aqueous solutions may be classified as saturated, unsaturated or supersaturated.
•Saturated- the maximum mass of solute that can be dissolved in a certain volume of solvent at a certain temperature.
•Unsaturated- solution contains less than the mass stated above.
•Supersaturated- contains more than the maximum mass (after heating)- results in crystallisation.
•The solubility of a substance in water is measured in g per 100 g’s of water.
Solubility curves can be drawn to show the maximum amount of solute which can be dissolved at different temperatures.
Solubility curve diagram courtesy of By TheKiteGuy - Template:Https://commons.wikimedia.org/wiki/File:SolubilityVsTemperature.png, CC0, https://commons.wikimedia.org/w/index.php?curid=51036796
•Saturated- the maximum mass of solute that can be dissolved in a certain volume of solvent at a certain temperature.
•Unsaturated- solution contains less than the mass stated above.
•Supersaturated- contains more than the maximum mass (after heating)- results in crystallisation.
•The solubility of a substance in water is measured in g per 100 g’s of water.
Solubility curves can be drawn to show the maximum amount of solute which can be dissolved at different temperatures.
Solubility curve diagram courtesy of By TheKiteGuy - Template:Https://commons.wikimedia.org/wiki/File:SolubilityVsTemperature.png, CC0, https://commons.wikimedia.org/w/index.php?curid=51036796
Looking at the solubility curve above for Barium nitrate, at 60 degrees, 20 g of solid will dissolve in 100g of water (saturated). Below 20g at this temperature the solution will be unsaturated.
Note that with heating more solid can generally dissolve (graphs increase). This is the opposite for gases. Worked example using solubility ratios. A saturated solution of sodium nitrate contains 50g of sodium nitrate in 40g of water at 60 oC. What is the solubility of sodium nitrate at this temperature? 50 g dissolves in 40g of water x g dissolves in 100g of water. x = 100 x = 100 x50 50 40 40 x= 125 g/100g water. Crystallisation Crystallisation occurs when a solution is cooled to the point where it becomes saturated with solute and the solute begins to form solid crystals. Two main methods: 1.Cooling a solution- salts are less soluble at lower temperatures. 2.Evaporation- removing solvent away from the solution resulting in a saturated solution. Variables affecting crystal growth Several factors affect the formation of crystals from solutions as well as their size: 1.Cooling rate- fast cooling results in small crystals and slow cooling results in large crystals. 2.Evaporation rate of the solvent- the faster the evaporation the smaller the crystals. 3.Nucleation-crystals form more rapidly if a nucleus (something onwhich the crystals can grow) is present in the solution on which new crystals can form. 4.Type of compound- different compounds form different-shaped crystals. |
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