Solubility

Solubility

1. Proper definitions

   • The solubility of a substance in a given solvent is the number of grams of the solute(usually a solid) that dissolve in 100g of the solvent(the liquid) at a particular temperature
     • Eg. at room temperature (20 degrees), about 36g of sodium chloride (NaCl) will dissolve in 100g of water
   • The solubility of a (solid) solutes usually increases with temperature
     • Eg. at 60 degrees, about 37g of sodium chloride (NaCl) will dissolve in 100g of water
   • A saturated solution is one that cannot hold any more solid at that temperature - and you have to be able to see solid on the bottom to be certain that its saturated

2. Solubility Curves show when a solution is saturated

   • 1 - A solubility curve plots the mass of solute dissolved in a saturated solution at various temperatures
   • 2 - The solubility of most solids increases as the temperature increases
   • 3 - This means that cooling a saturated solution will usually cause some solid to crystallise out - that means it separates from the solution
   • 4 - The mass of crystals formed by cooling a solution a certain amount can be calculated from a solubility curve...
     • Draw lines perpendicular to both axes through the temperatures in the question, then subtract the smaller mass from the larger - that difference will precipitate out on cooling.
     • Ex - What mass of solid copper sulphate will crystallise when a saturated solution containing 100g of water is cooled from 100 degrees to 20 degrees?
       • Answer - 75g - 20g = 55g

3. All gases are soluble - to some extent, anyway

   • Chlorine water is a solution of chlorine gas in water
     • Used as a bleach in paper and textiles industries
       • Also to sterilise water supplies - kills bacteria
   • The amount of gas that dissolves depends on the pressure of the gas above it - the higher the pressure, the more gas that dissolves
     • Fizzy drinks initially contain a lot of CO2 dissolved in water - pressure released when lid removed, CO2 fizzes out of solution
   • But gases become less soluble as the temperature of the solvent increases - opposite of solids
     • Aquatic life needs dissolved oxygen, but levels in rivers can be lowered by pollution and a rise in temperature (caused by warm water discharged from towns and industries) causing problems

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