Topic 8- Chemical Analysis

Question 1

what is meant by a pure substance

Answer 1

something that only contains one compound or element throughout

Question 2

how can you test the purity of a sample

Answer 2

by measuring its melting or boiling point and comparing it with the melting/boiling point of the pure substance (which can be found from a data book)

the closer your value is to the actual bp or mp, the purer your sample is

Question 3

what do impurities do to a sample

Answer 3

lower the mp
increase the melting range of the substance

increase the bp
may result in your sample boiling at a range of temperatures

Question 4

what is a formulation

Answer 4

a useful mixture with a precise purpose which is made by following a formula ‘recipe’.
example… paint
pigment solvent binder and additives are added in specific, measured quantities

Question 5

when are formulations useful

Answer 5

pharmaceutical industries have to...
make sure it delivers drug to correct part of the body
right concentration
consumable 
long shelf life

cleaning products
fuels 
cosmetics
fertilisers 
metal alloys
even food and drink

Question 6

what is chromatography

Answer 6

an analytical method used to separate the substances in a mixture

Question 7

what is the mobile phase

Answer 7

where the molecules can move (liquid or a gas)

Question 8

what is the stationary phase

Answer 8

where the molecules can’t move (solid or a really thick liquid)

Question 9

explain what is meant by equilibrium in chromatography

Answer 9

the substances in the sample are constantly moving between both phases

Question 10

how does the mobile phase move

Answer 10

it moves quickly through the stationary phase, and anything else dissolved in the mobile phase moves with it.

Question 11

what does the speed of a chemical depend on in chromatography

Answer 11

how it is distributed between the two phrases- whether it spends more time in the mobile phase or the stationary phase

Question 12

what will a pure substance look like on a chromatogram

Answer 12

it will only ever form one spot in any solvent as there is only one substance in the sample

Question 13

what is used as the the stationary phase

Answer 13

chromatography paper

Question 14

what is used as the mobile phase

Answer 14

the solvent (eg. ethanol or water)

Question 15

what does the amount of time spent in each phase depend on

Answer 15

how soluble they are in the solvent

how attracted they are to the paper

Question 16

what will happen when a molecule with a high solubility in the solvent is used in chromatography

Answer 16

will spend more time in the mobile phase, and they’ll be carried further up the paper

Question 17

what’s the name of the result of the chromatography?

Answer 17

chromatogram

Question 18

what’s the rf value equation

Answer 18

distance traveled by substance divided by distance traveled by solvent

Question 19

Different compounds have different what

Answer 19

rf values in different solvents

Question 20

what can the different rf values be useful for?

Answer 20

can be used to help identify the compounds

Question 21

what happens to the spots in a mixture

Answer 21

The compounds
in a mixture may separate into different spots depending on the
solvent

Question 22

what happens in the hydrogen test

Answer 22

The test for hydrogen uses a burning splint held at the open end of a
test tube of the gas. Hydrogen burns rapidly with a pop sound.

Question 23

what happens in the oxygen test

Answer 23

The test for oxygen uses a glowing splint inserted into a test tube of
the gas. The splint relights in oxygen.

Question 24

what happens in the CO2 test

Answer 24

The test for carbon dioxide uses an aqueous solution of calcium
hydroxide (lime water). When carbon dioxide is shaken with or
bubbled through limewater the limewater turns milky (cloudy).

Question 25

what happens in the chlorine test

Answer 25

The test for chlorine uses litmus paper. When damp litmus paper is
put into chlorine gas the litmus paper is bleached and turns white.

Question 26

lithium compounds result in a

Answer 26

crimson flame

Question 27

sodium compounds result in a

Answer 27

yellow flame

Question 28

potassium compounds result in a

Answer 28

lilac flame

Question 29

calcium compounds result in an

Answer 29

orange-red flame

Question 30

copper compounds result in a

Answer 30

green flame

Question 31

what happens if a sample containing a mixture of ions is used

Answer 31

some flame colours

can be masked

Question 32

what can be used to identify some metal ions

cations

Answer 32

Sodium hydroxide solution

Question 33

which ions form white

precipitates when sodium hydroxide solution is added

Answer 33

Solutions of aluminium, calcium and magnesium ions

Question 34

what happens when sodium hydroxide solution is added to aluminium hydroxide

Answer 34

precipitate dissolves in excess sodium hydroxide

solution, producing a colourless solution

Question 35

which ions form coloured precipitates and what are these colours?

Answer 35

Copper(II) forms a blue precipitate
iron(II) a green precipitate and
iron(III) a brown precipitate

(calcium, white)

Question 36

how can you test for carbonates

Answer 36

Carbonates react with dilute acids to form carbon dioxide gas, so add a few drops of this. Carbon
dioxide can be identified with limewater.

Question 37

how do you test for halides

Answer 37

add a couple of drops of dilute nitric acid, then a couple of drops of silver nitrate solution

Question 38

what colours are the halides

Answer 38

silver chloride is white
silver bromide is cream
silver iodide is yellow

Question 39

how do you test for sulfates

Answer 39

add dilute HCl acid
and a few drops of barium chloride solution

if they are present, then white precipitate of barium sulfate will form

Question 40

what is good about instrumental methods

Answer 40

accurate, sensitive

and rapid

Question 41

what does a flame emission spectroscopy experiment’s outcome look like?

Answer 41

a line spectrum that can be
analysed to identify the metal ions in the solution and measure their
concentrations.