GCSE Chemistry AQA C8 Chemical Analysis (Paper 2 - Triple)

purity definition

a pure substance is something that only contains one compound or element throughout.

how to tell the purity of a substance

a pure substance will boil/melt at a specific temperature.

you can test the purity of a sample by measuring its melting and boiling point and comparing it with data from a book

the impurer a substance, the lower the melting point and higher the boiling point.

formulations definition

formulations are useful mixtures with a precise purpose that are made by a formula

the importance of formulations

formulations are used in cleaning products, metal alloys, cosmetics, fertilisers etc.

chromatography definition

chromatography is an analytical method used to separate the substances in a mixture.

mobile phase

where the molecules can move. this is always a liquid or gas.

stationary phase

where the molecules can't move. usually a solid or thick liquid.

chromatography experiment

during chromatography, the mobile phase(ink) moves up stationary phase(paper) and it separates the substance. the solvent is the liquid the paper is dipped into.

the amount of time the molecules spend in each phase depends on...

- how soluble they are in the water

- how attracted they are to the paper.

Rf value equation

Rf= distance travelled by substance/distance travelled by solvent

test for gases: chlorine

bleaches damp litmus paper, turning it white

tests for gases: oxygen

if you put a glowing splint inside a test tube containing, oxygen will relight the splint.

tests for gases: carbon dioxide

bubbling carbon dioxide through an aqueous solution of calcium hydroxide(limewater) causes it to go cloudy.

tests for gases: hydrogen

if you hold a lit splint at the end of a test tube containing hydrogen, you will hear a squeaky pop.

test for sulfates

add hydrochloric acid followed by barium chloride to get a WHITE PRECIPITATE of barium sulfate

test for halides

add dilute nitric acid and silver nitrate solution to the solution

test for halides: chloride

a white precipitate of silver chloride

result of tests for halides - bromide

cream precipitate of silver bromide

tests for halides: iodide

yellow precipitate of silver iodide.

flame tests

used to identify metal ions

flame tests: lithium

crimson/red flame

flame tests: sodium

yellow flame

flame tests: potassium

lilac flame

flame tests: calcium

orange/red flame

flame tests: copper

green flame

metal hydroxide test with NaOH

metal hydroxides are mixed with NaOH and produce a specific colour

metal hydroxide test: calcium

white precipitate formed

metal hydroxide test: copper(II)

a blue precipitate is formed

metal hydroxide test: iron (II)

a green precipitate is formed

metal hydroxide test: iron (III)

a brown precipitate is formed

metal hydroxide test: Aluminium

white precipitate that redissolves in excess NaOH to form a colourless solution.

metal hydroxide test: magnesium

a white precipitate is formed

flame emission spectroscopy

used to identify metal ions

now flame emission spectroscopy works

1.) a sample is placed in a flame and the ions get excited and heat up.

2.) when the electrons drop back to their original energy levels, they transfer light energy

3.) the light passes through a spectroscope which detects different wave lengths of light to produce a line spectrum.

4.) the combination of wave lengths emitted depends on the ions charge and electron arrangement

5.) they give off certain colours for each ion which can be identified

flame emission spectroscopy works for mixtures

it can be used to identify different ions in mixtures. this makes it more useful than flame tests because they can only work for a single metal ion.

machine analysis

chemists often use instrumental analysis (tests that use machines) such as f.e.s instead of conducting tests

advantages of using machines

very sensitive whilst detecting things

very fast

tests can be automated

very accurate