separating + purifying

• what is a mixture?

  • 2 or more elements/ compounds that are not chemically combined together
  • chemical properties of each substance in the mixture = unchanged
• what is a pure substance?

  • a single element/ compound
  • not mixed with any other substance
• how can you use melting point data to distinguish between pure substances and mixtures?

  • pure substances= sharp exact melting point
  • mixtures melt over a range of temps: consist of several elements / compounds
• when is simple distillation used?

  • separate a solvent from a solution
  • useful for producing water from a salt solution
• how can ethanol be separated from water?

  • ethanol= lower boiling point than water: can use simple distillation
• explain how ethanol can be separated from water using simple distillation

  • distillation apparatus set up
  • mixture boiled
  • ethanol evaporates first
  • ethanol vapour cools in condenser
  • condensing back to a liquid before being collected
• when is fractional distillation used to separate mixtures?

  • separate all the elements/compounds in a mixture
  • these chemicals must have different boiling points
• what is the difference between fractional + simple distillation?

  • simple distillation: separate solvent from a solution
  • fractional distillation: separate different liquids from a mixture of liquids-> using different boiling points
• how does fractional distillation work?

  • oil = heated until evaporates into the fractionating column
  • vapours rise up fractionating column + condense at different fractions
  • depends on relative boiling point of each substance
• what sort of mixtures can filtration be used to separate?

  • separate an insoluble.substance from a solution
• describe how to separate an insoluble substance from a solution

  • place filter paper in a funnel
  • pour solution containing an insoluble substance through funnel into conical flask
  • insoluble substance will collect on the filter paper
  • solution will collect in conical flask
• when is the process of crystallisation used to separate a mixture?

  • separate a soluble solid from a solution if the slid decomposes when heated
• if the solid decomposes when heated, how could you separate a soluble solid from a solution?

  • crystallisation:
  • pour the solution into a evaporation g dish + heat gently
  • when crystals start to form, remove dish from heat + leave to cool
  • once cold, filter crystals out of solution + leave in a warm place to dry
• what process can be used to identify substances in a mixture?

  chromatography
• how does paper chromatography work to separate a mixture?

  • mobile phase (solvent) moves through stationary phase (paper)- anything dissolved in mobile phase will move up the paper
  • compounds interact differently with each phase so will move different distances through the stationary phase meaning they will be separated
• how can chromatography show the composition of a mixture?

  • different coloured substances in the mixture will separate as they have different solubilities in the solvent
  • will travel at different rates
• why should pencil be used to draw the line along the bottom of the chromatography paper?

  • wil not affect experiment: insoluble in the solvent
• why should the water (solvent) in the beaker for paper chromatography be no deeper than 1 cm deep?

  if deeper = wash away the substances placed on line on chromatography paper
• why should you use a lid when carrying out paper chromatography?

  to prevent the solvent evaporating
• how many pure spots will a pure substance produce on a chromatogram?

  one spot
• how would the one spot produced by a pure substance differ from an impure substance on a chromatogram?

  • impure substance = more than 1 compound: will produce more spots
  • one spot for each chemical
• what is the Rf value?

  • ratio between the distance travelled by dissolved substance (Solute) and the distance travelled by the solvent
• how do you calculate the Rf value?

  • distance travelled by substance/ distance travelled by solvent
• where should you measure between when measuring the distance moved by the solution/substance on chromatography paper?

  • measure from the pencil baseline to the middle of the spot.of the substance
• how can you use chromatography to see if a certain substance is present in a mixture?

  • run pure sample of this substance alongside the unknown mixture
  • if Rf value of pure substance = matches the value of one of the spots from the mixture: likely to be present
• True or false? 'substances with a higher solubility in the solvent will travel further up the chromatography paper'

  • true
  • they will stay dissolved in the solvent (mobile phase) for longer
• how could you separate salt from a mixture of salt and sand?

  • salt = soluble in water- sand isn't
  • add water to mixture
  • filter solution to remove the insoluble sand
  • evaporate water to collect the salt crystals
• how can liquids be separated if they have different densities?

  • different densities = naturally separate into two layers
  • to separate these layers: use a flask with a tap on the bottom
  • open tap to collect bottom liquid (has highest density)
• what does potable mean?

  safe to drink
• how can waste + groundwater be made potable?

  1. sedimentation: large insoluble particles sink to bottom after water left still for a while
  2. filtration: removes small insoluble particles by passing water through layers of sand and filters
  3. chlorination: kills bacteria + microorganisms which = too small to be removed by filtration
• what are the three steps to making waste and groundwater potable?

  1. sedimentation
  2. filtration
  3. chlorination
• how can seawater be made potable?

  distillation:

  1. filter to remove insoluble particles
  2. boil
  3. Cool + condense the water vapour
• what issues surround the process of making seawater potable?

  • very expensive: requires a lot of energy to boil large volumes of water
  • wastewater = toxic due to high concentration of salt -> must be disposed of carefully
• what is deionised water?

  • water that has had metallic ions (e.g copper/ calcium ions) removed
• why is deionised water used in experimental analysis?

  • deionised water = prevents ions in water interacting with substances under analysis
  • if water = not deionised: false positive results may be produced