separation techniques

Question 1

what apparatus do you measure time with?

Answer 1

digital stopwatch

Question 2

what apparatus do you measure temperature with?

Answer 2

laboratory thermometer

Question 3

what apparatus do you measure mass with?

Answer 3

electronic balance

Question 4

what apparatus do you measure an approximate volume of liquid/solution with?

Answer 4

marked/graduated beaker

Question 5

what apparatus do you measure an accurate volume of liquid/solution with?

Answer 5

measuring cylinder

to nearest 0.5 cm3

Question 6

what is the use of a pipette?

Answer 6

measures a very accurate fixed volume

Question 7

what is the use of a burette?

Answer 7

measures very accurate variable volumes

to nearest 0.05cm3

Question 8

what apparatus do you use to measure volume of gas?

Answer 8

marked/graduated gas syringe

Question 9

what is the use of fractional distillation?

Answer 9

to separate two or more miscible liquids with different boiling points, with the use of a fractionating column

Question 10

what are miscible liquids?

Answer 10

liquids that dissolve in each other to form a solution

Question 11

what is the procedure/method for fractional distillation?

Answer 11

1. set up the apparatus for fractional distillation
2. heat the mixture in a round-bottomed flask
3. check the temperature when the liquid starts to distil over. the liquid with the lowest boiling point distils over first. the temperature remains constant at that liquid’s boiling point until all the liquid has been distilled over.
4. when the temperature starts to rise again, change the conical flask (receiver). this is because another liquid with a higher boiling point will start to distil over once its boiling point is reached. the temperature remains constant at that liquid’s boiling point until all the liquid has been distilled over.
5. repeat step 4 until all the different liquids in the solution are separated.

Question 12

how does a fractionating column work?

Answer 12

• glass beads/glass folds in the column provide a large surface area for vapour to condense on
• when a flask containing two miscible liquids is heated, a mixture of vapours is obtained and they rise up the fractionating column
• the liquid with the lower boiling point is more volatile, so it evaporates more easily. the vapours will contain a higher proportion of this liquid.
• vapour of the liquid with the higher boiling point condenses more readily on the cooler surfaces of the fractionating column. some of the liquid flows back into the round-bottomed flask.

Question 13

applications of fractional distillation

Answer 13

crude oil, liquid air

Question 14

what are the three methods that test the purity of a substance or identify the substance?

Answer 14

1. determine the melting point (used for solids)
2. determine the boiling point (used for liquids)
3. paper chromatography

Question 15

what is the procedure/method to determine the melting point of a solid?

Answer 15

1. put a sample of solid into a melting point tube, and attach the tube to the bulb of a thermometer
2. place the stirrer (holding the thermometer and melting point tube) into a beaker of paraffin oil
3. heat gradually, with constant stirring to keep the temperature uniform
4. record the temperature as the solid sample starts to melt
5. record the temperature as the solid sample has fully melted

Question 16

what are the defining criteria for pure and impure solids?

Answer 16

pure solid:
1. has a fixed melting point/melts at a constant temperature.

impure solid:
1. melting point is lowered
2. melts over a range of temperatures

Question 17

what is the procedure/method to determine the boiling point of a liquid?

Answer 17

1. pour approximately 5cm3 of a liquid sample into a boiling tube with a side arm
2. heat gradually until the liquid starts to boil

notes:
* for ethanol, a water bath is preferred over direct heating since ethanol is flammable
* the boiling tube’s side arm allows vapour to escape, to prevent pressure from building up in the tube

Question 18

what are the defining criteria for pure and impure liquids?

Answer 18

pure liquid:
1. has a fixed boiling point/boils at a fixed temperature

impure liquid:
1. boiling point is raised
2. boils over a range of temperatures

Question 19

when do you use paper chromatography to test for the purity of a substance?

Answer 19

• the substance cannot be melted or boiled easily
• the amount of substance is very small

Question 20

what defines pure and impure substances based on a chromatogram?

Answer 20

pure substance:
* always gives a single spot on a chromatogram

impure substance:
* gives more than one spot on a chromatogram