SC2 - Methods of Separating and Purifying Substances ✓

Question 1

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

Answer 1

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

Question 2

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

Answer 2

The composition:

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

Question 3

SC2a - What is a mixture?

Answer 3

• A mixture contains multiple elements/compounds that aren’t chemically joined together.
• You can use physical processes to separate mixtures into different substances

Question 4

SC2b - Describe the steps in crystallisation

Answer 4

You would first warm the solution in an open container, allowing the solvent to evaporate, leaving a saturated solution

Then, allow this solution to cool

The solid will come out of the solution and crystals will start to grow, these can then be collected and allowed to dry

Question 5

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

Answer 5

• 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 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 - What is filtration?

Answer 7

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

Question 8

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

Answer 8

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

Question 9

SC2c - What can paper chromotography be used for?

Answer 9

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

Question 10

SC2c - What is paper chromotography?

Answer 10

• A method of separation carried out by spotting drops of the same samples onto paper and then allowing a solvent to move up the paper.
• Different components travel up the paper in the solvent at different rates, separating the compounds

Question 11

SC2c- What is the stationary phase and the mobile phase in paper chromatography?

Answer 11

1. The stationary phase is the paper
2. The mobile phase is the solvent

Question 12

SC2d - Describe a simple distillation set-up.

Answer 12

• 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 13

SC2d - Describe how nitrogen and oxygen can be separated in air (6 marks)

Answer 13

• First, the air is cooled to -200 degrees, so all components are liquid
• At this temperature, some components of air, like carbon dioxide and water, will become solid so they can be filtered out, preventing them from affecting our results
• We now have a liquid solution of Nitrogen and Oxygen.
• This mixture can be piped into a fractionating column, which should be warmer at the bottom and cooler at the top.
• Nitrogen will boil adn rise up the column, since the temperature is above -196, the oxygen will remain as a liquid
• Nitrogen can be removed as a gas from the top, oxygen removed as a liquid through the bottom

Question 14

SC2d - Describe what happens as hot vapour rises up the column in fractional distillation

Answer 14

• The liquid with the lower boiling point evaporates first. The vapour, at first, condenses when it hits the cool glass and drips back down into the flask.
• As the column gradually heats up, there will be a temperature gradient - hottest at the bottom, coolest at the top.

Question 15

SC2d - What 3 things can fractional distillation be used for?

Answer 15

1. To separate the different products in crude oil
2. To make alcoholic drinks such as whisky and vodka
3. To separate out the gases in the air, after the air has been colled and turned into a liquid at -200 degrees

Question 16

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

Answer 16

They have to have boiling points that are at least 25° C apart. The solute will have a much higher melting point that solvent anyways.

Question 17

SC2d - What is fractional distillation?

Answer 17

• Distillation used tos eparate two or more liquids. It works because some liquids boil more easily than others, so a fractionating column is used tos plit the original mixture into several parts.

Question 18

SC2d - What is simple distillation?

Answer 18

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

Question 19

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

Answer 19

• Paper chromotography
• Simple distillation

Question 20

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

Answer 20

• 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 sterlilize microbes.

Question 21

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

Answer 21

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

Question 22

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

Answer 22

It takes up too much energy.

Question 23

SC2e - What is desalination and how can this be achieved?

Answer 23

• Producing pure water from sea water is called desalination and can be achieved using simple distillation