topic 3

• a solution is a homogenous mixture. Properties are uniform throughout but the composition varies
• solute + solvent = solution. solution is formed by dissolving a solute in a solvent which does the dissolving
• solutions can be:

  • a mixture of gas e.g. air
  • solid dissolved in liquid e.g. sea water, sugar in water
  • liquid dissolved in liquid e.g. wine (ethanol in water), vinegar (acetic acid in water)
  • gas dissolved in liquid e.g. fizzy drinks
  • solid with another solid e.g. bronze = alloy of zinc and copper
• a saturated solution is a solution in which no more solute can be dissolved at a particular temperature and pressure for gases.

  • when distinguishing, adding a crystal of the solute in a saturated solution there is no change. Crystal does not dissolve
• an unsaturated solution is a solution that contains less solute than is needed to make the solution saturated at a particular temperature and pressure for gases.

  • when distinguishing, adding a crystal of the solute to an unsaturated solution will result in the crystal being dissolved.
• a supersaturated solution is an unstable solution that contains more solute than a saturated solution could normally hold at a particular temperature and pressure for gases.

  • if disturbed, excess solute will separate from solvent as solid. This is achieved by adding a crystal of solute which acts as crystallisation point (crystallises out of solution)
• forming a supersaturated solution:

  • warm solvent and saturate for that temperature; add solute until no more dissolves
  • cool down with no disturbance to supersaturate it
• concentrated: a solution with a large amount of dissolved solute per litre of solvent.

  
• dilute: solution with a small amount of dissolved solute per litre of solvent

  
• solubility: how much of the solute will dissolve in a specific amount of solvent
• solubility of solute depends on temperature and pressure:

  • solubility of solids increase = increasing temperature
  • solubility of gases decrease = increasing temperature
  • solubility of a gas in a liquid = directly proportional to the pressure of the gas above the solution
• solubility of a substance is the mass of the substance dissolved in 100g of water
• in formation of a solution a rearrangement of particles occurs. Solids dissolve in water molecules or ions that make up solid are surrounded by water molecules. They are attracted to the solvent and move away from solid
• sugar dissolved in water = crystal structure of sugar broken down + sugar molecules become distributed throughout solution
• dissolving sugar:

  • intermolecular forces between sugar molecules must be overcome as crystal dissolves
  • intermolecular forces between some water molecules must be overcome to make space for sugar molecules
  • intermolecular forces must form between the sugar molecules and surrounding water molecules
• ionic solid:

  • water molecules are attracted to ions
  • electrically negative region (oxygen) and electrically positive region (hydrogen)
  • Water molecules negative oxygen end attracted to crystal's positive ions
  • positive hydrogen ends of other water molecules attracted to negative ions
  • when ionic crystal dissolves in water, ions leave the crystal and become surrounded by water molecules (dissolving), ions evenly distributed
  • if attractions between ions in crystal = sufficiently strong, ionic substance may only be slightly soluble in water
• a solution may start unsaturated at a high temperature and as it cools becomes saturated, any further cooling once saturated will result in formation of crystals.
• solubility curve = maximum amount of solute that can dissolve in a certain amount of solvent (usually 100g of water) at different temperatures

  • lines = saturated solutions
  • data point below line = unsaturated, no more solute can be added
  • data point above line = supersaturated, assuming solution undisturbed

  
• Electrolyte: substance when dissolved in water produces ions.
• Dissociation: separation of ions in water.

  • ionic compounds
  • dissolving = ions in lattice separate as ionic bonds are broken
  • 100% dissociates

  NaCl(s) --> Na+ (aq) + Cl- (aq)
• Ionisation: formation of ions through a reaction with water.

  • covalently bonded substances, acids + bases
  • complete/partial

  HCl(g) + H2O(l) --> H3*O+ (aq) + Cl- (aq)

  *Hydronium
• Strong electrolytes: substances where all of particles dissolved exist as ions

  • all ionic compounds
  • strong acids + few covalent compounds (HCl, HBr, HI, H2SO4, HNO3)
  • high conductivity
  • ionisation and dissociation
• weak electrolytes: substances which exist in solution mainly in their molecular form. Only some molecules will ionise.

  • ethanoic acid and ammonia (base)
  • low conductivity
  • ionisation
  • low conductivity
• Non electrolytes: substances that remain as molecules in solution so that no charged species are present

  • covalent molecular compounds
  • sugar and ethanol
  • non conductor
• the movement of ions through the solution is the electric current. The greater the concentration of ions the greater conductivity of the solution. More ions = higher conductivity