separation techniques

Question 1

how do you decide what separation method to use

Answer 1

depending on whether the solid is dissolved, and how its solubility changes with temperature.

Question 2

what is the name of the insoluble solid

Answer 2

residue

Question 3

what is the name of the soluble liquid

Answer 3

flitrate

Question 4

why does crystallisation work

Answer 4

because soluble solids tend to be less soluble at lower temperatures.

Question 5

name the steps of crystallisation in order

Answer 5

1. heat the solution to evaporate some water so that it is more concentrated
2. eventually the solution will become saturated. when it cools, crystals will start to form
3. check that it’s ready by placing a drop on a microscope slide, crystals will form quickly on the cool glass.
4. leave the solution to cool, crystals will start to form in it as the temperature falls.
5. remove crystals by filtration. rinse with distilled water and dry with filter paper.

Question 5

why doesn’t crystallisation always work

Answer 5

because some substances, the solubility changes very little as the temperature falls.

Question 5

define solvent

Answer 5

the dissolving medium (liquid)

Question 5

define solute

Answer 5

the substance being dissolved in (solid)

Question 6

how does ‘evaporating all the solvent’ work

Answer 6

1. keep heating solution to evaporate the solvent
2. when there is only a little solvent left, the solute will start to appear. heat carefully until dry.

Question 7

how do you separate a mixture of two solids

Answer 7

choose a solvent that will dissolve into only one of them

Question 7

name the steps of ‘separating a mixture of two solids’ in order

Answer 7

1. add a solvent to the mixture that would dissolve one of the solutes and stir. The solute dissolves.
2. filter the mixture. the second solute is trapped in the filter paper, but the first solute solution passes through.
3. rinse the second solute with water, and dry it in an oven.
4. evaporate the solvent from the first solute solution, to give a dry solute.

Question 7

if a solvent is flammable, how do you evaporate it

Answer 7

using a water bath

Question 8

define filtrate

Answer 8

the liquid that passed through a filter paper

Question 8

define simple distillation

Answer 8

a way to obtain the solvent from a solution.

Question 8

name the steps of simple distillation in order

Answer 8

1. heat the solution in the flask. as it boils, water vapour rises into the condenser, leaving salt behind.
2. the condenser is cold, so the vapour condenses to water in it.
3. the water drips into the beaker. (it is called distilled water. It is almost pure)

Question 8

where do people commonly use simple distillation, and for what reason

Answer 8

the middle east, many obtain drinking water through simple distillation by distilling seawater in giant distillation plants.

Question 8

define residue

Answer 8

the trapped solid in a filter paper

Question 8

define fractional distillation

Answer 8

used to separate a mixture of liquids from each other.

Question 8

what factor does fractional distillation rely on

Answer 8

the different boiling points

Question 8

name the steps of frational distillation in order

Answer 8

1. heat the mixture in the flask. at a certain temperature, the first solute begins to boil. some of the second solute evaporates too. so a mixture of the two solutes vapours rises up the column.
2. the vapours condense on the glass beads in the column, making the beads hot.
3. when the beads reach the boiling point of the first solute, the solute’s vapour no longer condenses on them. only the second solute’s vapour does. so the first solute drips back into the flask. the second solute vapour goes into the condenser.
4. there it condenses. pure liquid of solute one drips into the beaker. eventually, the thermometer reading rises above the boiling point of solute one – a sign that all the ethanol has gone. so you can stop heating.

Question 8

what industry commonly uses fractional distillation and what do they use it for

Answer 8

the petroleum industry, to refine crude oil into petrol and other groups of compounds.

Question 8

how does paper chromatography work

Answer 8

dyes in the ink have different solubilities in water. so they travel across the paper at different rates. (The most soluble one travels fastest.)

Question 9

what is a chromatogram

Answer 9

the result of paper chromatography

Question 9

how does the solubility of a solute affect the chromatography results

Answer 9

the more soluble a substance is in the solvent, the further it will travel up the chromatography paper.

Question 9

what is the formula for the Rf value

Answer 9

distance moved by amino acid/distance moved by solvent

Question 9

what are the different uses for paper chromatography

Answer 9

• identify a substance
• separate mixtures of substances
• purify a substance, by separating it from its impurities.

Question 10

define ‘locating agent’

Answer 10

a solution used to locate the position of a colourless solution

Question 10

why does the Rf table method work

Answer 10

method works because the Rf value of a compound is always the same for a given solvent, under the same conditions.

Question 11

define ‘saturation’

Answer 11

the condition in which one substance takes up as much space as possible within another substance and it can take no more molecules