What is chemical purity ?
- substance made of just one substance, nothing else
- can either be element or compound
How can you tell if a substance is chemically pure ?
- using its fixed points (mp + bp)
- a pure substance will have fixed points that can be found in a databank
- if the fixed points of the substance don’t adhere to what they’re supposed to be, the substance is not pure and is a mixture
What effect does an impurity have on a substance’s fixed points ?
- lowers mp
- raises bp
- widens range of mps/bps (eg. an impure substance will have a range such as 180’C - 220’C for its mp, whilst the purer version may have 210’C - 220’C)
What is a formulation ?
mixtures with fixed proportions of their components designed to be useful products
- paints
- drugs
- washing up liquids
- fuels
- alloys
- fertilisers
etc. are all formulations
Explain simply how paper chromatography works.
- mobile phase (solvent chosen)
- stationary phase (paper)
- base line drawn in pencil at bottom of paper
- dots of substance being tested placed on base line
- paper submerged in solvent until just above base line
- wait
- draw solvent front in pencil
- measure distance from base to solvent front, and from base to the end of the substance you want to identify
- divide the base to substance length by base to solvent front length to get Rf value
- compare Rf value to a database to identify the substance
What is the positive test for hydrogen gas ?
burns rapidly with a squeaky pop when lit splint is applied
What is the positive test for oxygen gas ?
relights a glowing splint
What is the positive test for carbon dioxide gas ?
turns limewater milky/cloudy
What is the positive test for chlorine gas ?
DAMP blue litmus paper turns red, then is bleached white
Describe how to carry out a flame test for positive (metal) ions.
- dip nichrome wire loop in conc. hydrochloric acid
- burn to sterilise and clean
- do not set down or blow on it whilst cooling
- dip into acid again
- dip into metal compound being tested
- burn in ROARING BLUE flame of bunsen burner
- use colour of resulting flame to identify metal ion
- if it is a mixture, the colour will be muddy or may be masked by stronger colours of certain metal ions in the mixture
What colour flame do lithium ions produce ?
crimson
What colour flame do sodium ions produce ?
yellow
What colour flame do
potassium ions produce ?
lilac
What colour flame do calcium ions produce ?
orange-red
What colour flame do copper (II) ions produce ?
green
Describe how to conduct a test for positive ions using sodium hydroxide ?
- in a boiling tube, add the unknown metal compound
- add sodium hydroxide
- swirl to mix
- observe the precipitate formed
- if white precipitate, add more sodium hydroxide
- if this precipitate dissolves, positive test for aluminium
- if not, do a flame test to determine if it is magnesium or calcium
- if original precipitate is coloured, can either be copper (II), iron (III) or iron (II)
What colour flame do magnesium ions produce ?
no colour
What colour precipitate do calcium ions produce ?
white
What colour precipitate do magnesium ions produce ?
white
What colour precipitate do aluminium ions produce ?
white - dissolves in excess sodium hydroxide
What colour precipitate do copper (II) ions produce ?
light blue
What colour precipitate do iron (II) ions produce ?
light green, slowly turns brown
What colour precipitate do iron (III) ions produce ?
reddish-brown
Describe how to test for a metal carbonate.
- in one boiling tube, hydrochloric acid with the carbonate
- air tube connected to it leading to submerged in limewater
- if limewater turns milky, is carbonate
- this is because HCl + metal carbonate –> [metal] chloride + water + CO2 gas
