chemistry - analysis

pure substance

a substance only containing one element or compound

how can you test for the purity of a substance?

boiling and melting point and comparison to a data book

will impurities lower or raise the melting point of a substance?

lower

will impurities lower or raise the boiling point of your sample?

raise

formulations

mixtures with a precise purpose

chromatography is a ...... method

analytical

why do we do paper chromatography?

separate the substances in a mixture / is it pure?

mobile phase in chromatography

water (always a liquid or gas) - molecules can move

stationary phase in chromatography

paper (solid) - molecules can't move

Rf value calculation

distance travelled by substance / distance travelled by solute

why would an Rf value change?

different temperatures or different solvent

test for chlorine

Bleaches damp litmus paper

test for oxygen

Relights a glowing splint

Test for carbon dioxide

Turns limewater cloudy

test for hydrogen

Lighted splint gives a squeaky pop

what is an anion

A negatively charged ion

examples of anions

carbonates (CO3 2-) , Sulphates (SO4 2-) , Halides (Cl- , Br- , I-)

Test for carbonates

First add dilute acid (hydrochloric acid)

If carbonate ions (CO3 2-) are present carbon dioxide will be released and will turn limewater cloudy

test for sulphates

add drops of dilute hydrochloric acid and drops of barium chloride, if white precipitate if present

test for halides

Add dilute nitric acid and then silver nitrate solution, coloured precipitates form

why can't you use hydrochloric acid in test for halides

contains chloride ions - would give a false positive

chloride precipitate

White precipitate of silver chloride

bromide precipitate

cream precipitate of silver bromide

iodide pecipitate

yellow precipitate or silver iodide

What is the first step of the flame test method?

Dip nichrome/platinum wire into dilute HCl to sterilize it and burn it in a blue flame in a Bunsen burner until it burns without any color.

What is the second step of the flame test method?

dip loop in the sample and record the colour of the flame produced when the sample is burned.

disadvantages of flame test

only works for samples containing a single metal ion, if a mixture then some colours could be hidden by others

Lithium ions flame test

crimson flame

Sodium ions flame test

yellow flame

Potassium ions flame test

Lilac flame

Calcium ions flame test

Orange-red flame

Copper ions flame test

Green flame

what do you add in a metal precipitate test (cations)

sodium hydroxide

calcium precipitate

white

Copper precipitate

blue

Iron (II) precipitate

green

Iron (III) precipitate

brown

Aluminium precipitate

white - redissolves in excess NaOH

Magnesium precipitate

white