SC2 - Methods of Separating and Purifying Substances ✓

Question 1

SC2a - What are the two key features of a pure substance?

Answer 1

The composition:

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

Question 2

SC2a - What is a mixture?

Answer 2

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

Question 3

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

Answer 3

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

Question 4

SC2b - What is filtration?

Answer 4

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

Question 5

SC2b - What is cystallisation?

Answer 5

• 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)

Question 6

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

Answer 6

• 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.

Question 7

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

Answer 7

• 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

Question 8

SC2c - What is paper chromotography?

Answer 8

• 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.

Question 9

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

Answer 9

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

Question 10

SC2c - What can paper chromotography be used for?

Answer 10

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

Question 11

SC2d - What is simple distillation?

Answer 11

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

Question 12

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

Answer 12

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

Question 13

SC2d - Describe a simple distillation set-up.

Answer 13

• 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.

Question 14

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

Answer 14

• Paper chromotography
• Simple distillation

Question 15

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

Answer 15

It takes up too much energy.

Question 16

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

Answer 16

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

Question 17

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

Answer 17

• 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