chemical analysis

Question 1

how to check a substance is pure

Answer 1

check boiling and melting point if impure: there will be a larger range of temperatures boiling point rise melting points lower

Question 2

pure substance

Answer 2

made up of only one element/compound

Question 3

formulation

Answer 3

mixtures of chemicals that are designed to create useful products, each chemical component must be present in a precisely measured quantity

Question 4

examples of formulations

Answer 4

fuel, medicine, fertilizers, metal alloys, cleaning agents

Question 5

stages of chromatography

Answer 5

stationary phase (e.g. chromatography paper) mobile phase (e.g. solvent)

Question 6

separation of substance in chromatography

Answer 6

more attracted to stationary phase = moves little more attracted to mobile phase = moves far

Question 7

steps in paper chromatography

Answer 7

1) draw line on paper IN PENCIL 2) place equal amounts of each dye on start line (label in pencil) 3) place solvent in beaker 4) place paper in beaker LEVEL OF SOLVENT MUST BE BELOW THE START LINE 5) allow solvent to travel upwards 6) take out paper and let dry 7) measure distance from start line to solvent front measure distance from start line to end of dye dye length / solvent length = Rf value

Question 8

reference substance

Answer 8

pure sample that can provide indication (not evidence) of a substance present

Question 9

test for hydrogen

Answer 9

lit splint in a test tube of hydrogen produces a squeaky pop

Question 10

test for oxygen

Answer 10

a glowing splint is relit

Question 11

test for chlorine

Answer 11

bleaches litmus paper white

Question 12

test for carbon dioxde

Answer 12

bubble through or shake in calcium hydroxide (limewater) and turns the solution from clear to cloudy

Question 13

results for the flame test

Answer 13

Last Christmas Santa Yelled Please Leave Carrots OR Cut Grass lithium = crimson sodium = yellow potassium = lilac calcium = orangey-red copper = green

Question 14

steps of flame test

Answer 14

1) Clean a nichrome wire loop by submerging (completely covering) it in dilute hydrochloric acid. 2) Dip the wire loop into the sample to be tested. 3) Hold the wire loop in the blue flame of a Bunsen burner. 4) Record the colour of the Bunsen flame.

Question 15

metals + sodium hydroxide

Answer 15

produces coloured metal hydroxide precipitates: white = calcium, magnesium, aluminium (dissolves in excess to form colourless solution) iron (ii) = green iron (iii) = brown copper = blue

Question 16

how to test for carbonates

Answer 16

carbonate + dilute acid = salt + carbon dioxide + water carbon dioxide can be identified with limewater

Question 17

how to test for halides

Answer 17

1) add dilute nitric acid (to remove carbonates) 2) add silver nitrate

Question 18

results for halide test

Answer 18

silver CHLORIDE = white silver BROMIDE = cream silver IODIDE = yellow

Question 19

test for sulfates

Answer 19

1) add dilute hydrochloric acid (to remove carbonates) 2) add barium chloride

Question 20

result for sulfate test

Answer 20

if sulfates are present a white precipitate forms

Ba²⁺ + SO₄^2- = BaSO4

Question 21

advanages of instrumental methods

Answer 21

more accurate

quicker

more sensitive (require smaller amounts of samples)

Question 22

disadvantages of instrumental methods

Answer 22

expensive

need training to use

(often only useful with predetermined sets of data)

Question 23

how flame emisson spectroscopy works

Answer 23

1) sample is put into a flame
2) light given out is passed through a spectroscope output is a line spectrum

(spectroscope can differenciate between different wavelengths of light to produce line spectrum)

Question 24

uses of flame emisson spectroscopy

Answer 24

line spectrum produced can be analysed to

1) identify the metal ions in the solution
2) measure their concentrations

(Unlike flame tests, flame emission spectroscopy permits the analysis of mixtures by comparison with reference spectra)

Question 25

what do reference substances in chromatography tell us

Answer 25

it is likely that that substance is present if sample match it but is is NOT PROOF

Question 26

what happens if the start line in chromatography is in pen?

Answer 26

Pen has ink in it. This ink will also move with the solvent and so the start line will not remain in the same place but rather it will move and may spread into the dye samples.

Question 27

what happens if the solvent level is above the start line?

Answer 27

If the solvent is covering the dyes, the dyes will diffuse into the pool of solvent rather than up the chromatography paper. This means that all the dyes will mix together and the dyes will not be separated into their constituents

Question 28

why add dilute acids for the halide tests?

Answer 28

If there were carbonate ions present, the nitric acid would cause CO2 to be evolved This would be easily identifiable. If you do not test for carbonate ions, the silver nitrate solution could form silver carbonate This is another precipitate which could give a false impression of the composition of the sample