chapter 3: separation and purification

Question 1

what is a mixture?

Answer 1

a mixture is made up of two or more substances that are not chemically combined

Question 2

what is a pure substance?

Answer 2

• it is made up of a single element or a compound and is not fixed with any other substances

Question 3

what is filtration used for? and how does it work?

Answer 3

• used to separate insoluble solid particles from a liquid
  > can do so because the liquid particles are small enough to pass through the pores of the filter paper but a solid cannot
• solid that remains on the filter paper is called the residue and the liquid that passes through is called the filtrate

Question 4

what is evaporation to dryness used for?

Answer 4

• used to obtain a soluble solid ( thermally stable) from a solution by heating the solution until all the water has boiled off
• solid obtain by evaporation to dryness is not always pure
  > when all the water has been removed, any soluble impurities will be left together with the solid

Question 5

what is crystallisation and what is it used for?

Answer 5

• obtaining a pure, soluble solid (thermally unstable) sample from a solution
• water is removed by heating the solution and heating is stopped when a saturated solution is formed
• if the resulting solution is allowed to cool to room temp
  > dissolved solid crystal will form pure crystals
• crystals are the rinsed with cold distilled water to remove water soluble impurities
  > blot dry between two sheets of filter paper

Question 6

what is sublimation used to separate? and what are some examples?

Answer 6

• it is used for separating a solid that sublimes from one that does not
• eg. iodine, carbon dioxide, ammonium chloride

Question 7

what is using a magnet for and what are some example of metals that are magnetic?

Answer 7

• used to separate a magnetic substance from a non magnetic substance
• eg. iron, nickle, cobalt

Question 8

what is simple distillation used for?

Answer 8

• used to separate a pure solvent from a soluble solute in a solution

Question 9

fill in the blanks on the steps of simple distillation:

1. in distillation flask, solution boils
   > __i__ added to ensure ___ii__ boiling
   > water __iii__, rises and enters the _iv___
2. in the condenser, water vapour is cooled
   > vapour condenses and cooled into __i__ water
3. pure water collected as a __i__ in the conical flask
4. remaining salt solution becomes concentrated
   > if distillation allowed to carry on, a solid residue of salt will be left in the flask

Answer 9

1i) boiling chips
1 ii)smooth
1 iii)vapourises
1 iv) condenser

2i) pure
3i) distillate

Question 10

distillation:

why should the bulb of the thermometer be placed beside the side arm of the distillation flask and not inside the solution?

Answer 10

• ensures that the thermometer is measuring the boiling point of the substance being distilled .

Question 11

what is the separating funnel used for?

Answer 11

it is used for separating immiscible liquids

Question 12

what needs to be done for a volatile liquid to be distilled and why?

Answer 12

• if distillate is volatile, the receiver can be put into a large container filled with ice
  > helps keep the temperature of the distillate low so that it remains in a liquid state

Question 13

how does the temperature change as salt solution undergoes distillation?

Answer 13

-when the salt solution is heated, the temperature increases
- when solution boils > thermometer records a temperature of 100°C which is the temperature of the steam
>temperature remains unchanged until all the water has boiled off

Question 14

what is fractional distillation used for? and give an example

Answer 14

• used to separate two miscible liquids with different boiling points
  eg. ethanol and water

Question 15

what are industrial applications of fractional distillation?

Answer 15

• obtain nitrogen, argon and oxygen from air
• separate petroleum into useful fractions
• obtain ethanol produced by the fermentation of glucose solution

Question 16

what is chromatography used for? and what is a chromatography paper with the separated components called?

Answer 16

• a method of separating components of a mixture that dissolves in the solvent
• chromatogram

Question 17

what does Rf value stand for?

Answer 17

• retention factor

Question 18

just remember this :)

• when chromatography is allowed to proceed for a short time and a longer time, Rf value is obtained
• Rf value of substance does not change as long as chromatography is carried out under the same conditions

Answer 18

tehe patrick dempsey is hot

Question 19

what happens if we need to run a chromatogram on colourless substances such as amino acids?

Answer 19

we spray a locating agent

> causes colourless substances to become coloured spots

Question 20

what are the uses of chromatography?

Answer 20

1. separate the components in a sample
   > dyes in ink, pigments in plants and amino acids obtained from plants
2. identify components present in a sample
   >traces of banned substances in food
3. identify substances
   > poisons pesticides and drugs
4. determine whether a sample is pure

Question 21

how to determine if a solid is pure?

Answer 21

• if it has an exact and fixed melting point

Question 22

what are the effects of impurities on melting point?

Answer 22

• impurities lowers the melting point of a solid
  >greater amount of impurities, the lower the melting point
• cause melting point to take place over a range of temperature

Question 23

what are the effects of impurities on boiling point?

Answer 23

• increases the boiling point of a liquid
  > the greater amount of impurities, the higher the boiling point
• also causes boiling point to take over a range of temperatures.

Question 24

describe the process and the steps of separating immiscible liquids using the separating funnel. use oil and water for your explanation.

Answer 24

1. *make sure that the tap of it is closed
   pour the mixture of oil and water into the separating funnel
2. support the separating funnel using the retort stand and place a clean beaker below the separating funnel
3. allow liquids to separate completely ( may take some time)
   > denser liquid (water) will be the bottom layer
4. open the tap of the funnel & allow the bottom layer to drain into the beaker
   > close tap before top layer liquid runs out
5. place another beaker below the separating funnel and open the tap and allow a little of the top layer of liquid into the beaker > dispose it

> funnel contains oil and beaker contains water

Question 25

describe the process and steps of fractional distillation. use water and ethanol as an example.

Answer 25

1. ethanol vapour and water vapour rises up the column as the solution is heated
2. water vapour condenses in the fractionating column and falls back into the flask
3. ethanol which has a lower boiling point reaches the top part of the fractionation column and is distilled over
4. thermometer shows 78°C ( boiling point of ethanol)
5. in the condenser, hot ethanol vapour condenses as cold water cools it
   > liquid ethanol flows down the inner tube of the condenser into the receiver
6. ethanol collected as a distillate