SC2 - Methods of Separating and Purifying Substances ✓ Flashcards Preview

Edexcel GCSE Chemistry COPY > SC2 - Methods of Separating and Purifying Substances ✓ > Flashcards

Flashcards in SC2 - Methods of Separating and Purifying Substances ✓ Deck (17)
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SC2a - What are the two key features of a pure substance?

The composition:

  • Cannot be changed by physical means
  • Is the same in all parts of a piece of the substance


SC2a - What is a mixture?

  • A mixture contains multiple elements/compounds that aren't chamically joined together.
  • The composition cannot be changed and so it isnt a pure substance.


SC2a - How will the heating curve of a mixture look different to a pure substance and why?

  • Instead of straight lines there will be curves.
  • This is because there are multiple different elements which will have different melting points.


SC2b - What is filtration?

Using a filter to trap larger insoluble substances from a mixture.


SC2b - What is cystallisation?

  • When a solution is evapourated, meaning that the solvent turns to gas leaving behin dthe solute which forms crystals.
  • The size of these crystals depend on the time the crystallisation takes
  • (Longer time taken = larger crystals)


SC2b - What is a saturated solution and how can this be changed?

  • A saturated solution is a solution in which a solvent contains the highest amount of dissolved solute as possible.
  • This limit can be increased by increasing the temperature.


SC2b - Descirbe what a risk assesment is and how it is used when crystallising in a lab.

  • Identifying and evaluating the hazards and considering ways of reducing the risk of harmful effects.
  • In crystallisation this would be indentifying:
    •  The solution can spit: wearing eye protection and removing from the bunsen burner before the solution is completely evapourated
    • You can get burnt from the bunsen burner: keep your hands away from the flames at all times


SC2c - What is paper chromotography?

  • A method of speration that involves having a piece of paper with dots of ink slightly dipped in the water (so that the water doesnt reach the pencil line that the ink is one).
  • The solvent is called the mobile phase and the paper contains the stationary phase.
  • If an ink moves less up the page it is less attracted to the stationary phase.
  • Different inks have different levels of attraction to the stationary phase and so will get sperated out at different distances.


SC2c - How do you measure the Rƒ value of an ink?

Rƒ = Distance travelled by spot ÷ Total distance travelled by solvent


SC2c - What can paper chromotography be used for?

  • Distinguishing between pure/impure substances
  • Identifying substances by comparing their chromotogram with known substances
  • Identifying substances by calculating their Rƒ value


SC2d - What is simple distillation?

The separation of a liquid from its dissloved solids, by evapourating it and condensing the gas formed.


SC2d - What conditions do the components of the solution have to meet for simple distillation to work?

They have to have boiling points that are at least 25° C apart.


SC2d - Describe a simple distillation set-up.

  • A side-arm flask on a tripod over a bunsen burner.
  • It contains the solution and anti-bumping granules to help the liquid boil more smoothly.
  • A thermometer is connected to the top.
  • A delivery tube, leading to a conical flask, is surrounded with a cooling jacket (condenser) which takes cold water in from the bottom and puts it out at the top.
  • The conical flask is in a bath of ice water.
  • The bunsen burner is ona heat resistant mat.


SC2d CP - Which two methods can you use to seperate solvents in an ink?

  • Paper chromotography
  • Simple distillation


SC2e - Why can't water be purified by distillation on a large scale?

It takes up too much energy.


SC2e - Why can't tap water be used for chemical analysis?

Tap water contains small amounts of dissolved salts and minerals which may react and alter the resuslts


SC2e - Describe the stages river water goes through before it can be used in homes.

  • First, it is screened with a seive to get rid of large objects such as leaves and twigs
  • It is then sent to a sedimentation tank where small particles will settle out at the bottom
  • The water is then filtered through layers of sand and gravel
  • Finally it is treated with chlorine to kill microbes