Separation techniques

• when can you use filtration to separate a mixture?

  • if the product is an insoluble solid that needs to be separated from a liquid reaction mixture
  • in purification. For example, solid impurities in the reaction mixture can be separated out using filtration
• How can you separate soluble solids from solutions?

  • Evaporation
  • crystallisation
• Explain evaporation

  1. Pour the solution into an evaporating dish
  2. slowly heat the solution. The solvent will evaporate and the solution will get more concentrated. Eventually, crystals will start to form
  3. keep heating the evaporating dish until all you have left are dry crystal.
• Explain crystallisation

  1. Pour the solution into an evaporating dish and gently heat the solution. Some of the solvent will get more concentrated.
  2. once some of the solvent has evaporated, or when you see crystals start to form (the point of crystallisation), remove the dish from the heat and leave the solution to cool.
  3. the salt should start to form crystals as it becomes insoluble in the cold, highly concentrated solution.
  4. filter the crystals out of the solution, and leave them in a warm place to dry. You could use a drying oven or a desiccator.
• How do you know which way to separate a soluble solid from a solution?

  • If the salt decomposes (breaks down) when it’s heated, you’ll have to use crystallisation.
  • if the salt doesn’t decompose when heated, use evaporation as it is really quick.
• Which separation techniques are used to separate rock salt?

  • Filtration
  • crystallisation
• What is rock salt made out of?

  Salt and sand (what they put on roads when it’s icy)
• What makes rock salt unique?

  • Salt and sand are both compounds but salt dissolves in water and salt doesn’t.
• Explain the separation of rock salt

  1. Grind the mixture to make sure the salt crystals are small, so will dissolve easily.
  2. put the mixture in water and stir. The salt will dissolve but the sand won’t.
  3. filter the mixture. The grains of sand won’t fit through the tiny holes in the filter paper, so they collect on the paper instead. The salt passes through the filter paper as it is part of the solution.
  4. Evaporate the water from the salt so that is forms dry crystals.
• What is distillation used to separate?

  • Mixtures that contain liquids.
• What are the two types of distillation?

  • Simple distillation
  • fractional distillation
• When do you use simple distillation?

  • Separating out a liquid from a solution.
• Explain simple distillation

  1. The solution is heated. The part of the solution that has the lowest boiling point evaporates first.
  2. the vapour is then cooled, condenses (turns back into a liquid) and is collected
  3. the rest of the solution is left behind in the flask.
• When is simple distillation used in real life?

  • to purify water from sea water
• What is a drawback of using simple distillation?

  • You can only use it to separate things with very different boiling points- if the temperature goes higher than the boiling point of the substance with the higher boiling point, they will mix again.
• When do you use fractional distillation?

  • If you have a mixture of liquids with similar boiling points
• When if fractional distillation used in real life?

  • fractional distillation of crude oil at a refinery.
• Explain fractional distillation

  1. Put mixture in a flask and stick a fractioning column on top. Heat it.
  2. different liquids will all have different boiling points so will evaporate at different temps.
  3. liquid with lowest bp evaporates first. When temp on thermometer matches bp of this liquid, it will reach the top of the column.
  4. liquids with higher bp’s might also start to evaporate but column is cooler towards top so will only get part of the way up before condensing and running back down towards flask.
  5. when the first liquid has been collected, raise temp until next one reaches the top.