Mixtures

• What are mixtures?

  The chemical properties of each substance in the mixture are unchanged. This means they act the same, as they are simply mixed.
• What are the physical processes of separation?

  • Filtration
  • Evaporation
  • Crystallisation
  • Distillation
  • Chromatography
• Filtration-

  If we want to separate an insoluble solid (one that does not dissolve) from a liquid mixture we use filtration.

  E.g., sand and water.
• What is the process of filtration?

  1. Fold a piece of filter paper into a cone shape.
  2. Place the filter paper into a funnel, then put that into the neck of a conical flask.
  3. Pour the mixture into the funnel. The liquid will pass through but the solid won't.
  4. The solid will be left behind in the funnel, separated from the liquid (filtrate) in the flask.
• Evaporation and Crystallisation

  If a soluble solid has been dissolved in a liquid we use either evaporation or crystallisation to separate the mixture.

  E.g. salt water.
• What is the process of evaporation?

  1. Pour the solution into an evaporating bowl
  2. Heat the solution over a Bunsen burner
  3. The liquid will evaporate and the solution will become more concentrated with the solid.
  4. Turn the heat off when all of the liquid has evaporated and the solid is left behind.
• What is the process of crystallisation?

  1. To produce crystals, we turn off the heat when some of the solvent has evaporated/when we see crystals start to form.
  2. We then leave the dish out for the water to slowly evaporate over time, leaving crystals behind.
  3. If there is any liquid left, we can filter the crystals or pat them dry using filter paper.
• Solute - the substance we dissolve (eg, salt)

  Solvent - the substance we put the solute in (eg, water)

  Solution - the mixture made (eg, salt water)
• Chromatography

  Chromatography can be used to separate mixtures made up of liquids of different colours.

  One example is to use paper chromatography to separate different dyes in an ink.
• Why must the line drawn on the filter paper in paper chromatography be made using a pencil?

  It is insoluble in solvent
• The ink spot in paper chromatography should touch the solvent to dissolve properly.

  False
• The solvent used in paper chromatography depends on the compounds being tested.
• A lid must be placed on the container during paper chromatography to prevent solvent evaporation.
• What causes the different dyes in the ink to separate on the paper during paper chromatography?

  Their solubility in solvent
• Each spot in a different position on the chromatogram represents a different dye.
• Dyes that are insoluble in the chosen solvent will stay at the baseline.
• What is the line indicating the furthest point the solvent reached called in paper chromatography?

  Solvent front
• The final result of paper chromatography is a pattern of spots known as a chromatogram.
• Steps in the process of paper chromatography

  1️⃣ Draw a line near the bottom of filter paper in pencil

  2️⃣ Add a spot of ink above the pencil line

  3️⃣ Place the paper in a beaker of solvent, ensuring the pencil line is above the solvent

  4️⃣ Cover the beaker with a lid

  5️⃣ Allow the solvent to move up the paper

  6️⃣ Remove the paper when the solvent is near the top and let it dry

  7️⃣ Mark the solvent front
• Why is ethanol sometimes used as a solvent instead of water in paper chromatography?

  Some compounds dissolve better
• The rate at which a dye moves up the paper depends on its solubility in the solvent.
• What should be done when the solvent reaches near the top of the paper in paper chromatography?

  Remove the paper to dry
• The solvent front is marked when the solvent has reached near the top of the paper.
• $R_f$value = $\frac{distAnce\ moved\ by\ substAnce}{distAnce\ moved\ by\ solvent}$
• Distillation

  Distillation is used to separate mixtures that contain liquids.

  It does this using heating and cooling, using the different boiling points of different liquids.

  There are two types of distillation: simple distillation and fractional distillation.
• Simple Distillation

  • The mixture is heated, and the substance with the lowest boiling point evaporates.
  • As the vapour rises it passes into the condenser, where it is cooled and condenses.
  • It then runs down the condenser into a container below.
  • Substances with high  boiling points are left behind  in the flask.

  A) condenser

  
• Simple distillation only works with substances that have very different boiling points.

  We use fractional distillation with substances that have similar boiling points.
• Fractional Distillation

  • The liquid with the lowest boiling point will evaporate first.

  When the temperature of the thermometer matches the boiling point of the liquid it will reach the top of the column, enter the condenser and condense into a container below.

  • Liquids with higher boiling points may begin to evaporate, but will only get part of the way up before condensing and running back down the column.

  When the first liquid has been collected, we can alter the temperature to the next boiling point until the next liquid reaches the top and can be collected.