4.8 chemical analysis

chemistry gcse aqa

what is a pure substance?

a single element or compound that has not been mixed with any other substance

how can you determine if a substance is pure or not?

by finding its melting/boiling points, as pure substances have fixed specific m/b points

what is a formulation

a complex mixture that’s been designed as a useful product

give 3 examples of formulations

fuels, paints, medicines

what does paper chromatography do?

separate substances based on their different solubilities

what is chromatography used for

to identify substances

what is the stationary phase

the paper as it does not move

what is the mobile phase

the solvent as it does move

why is the starting line drawn in pencil

if it was drawn in pen, the pen ink would move up the paper with the solvent

how is chromatography used to identify unknown substances?

measure the distance moved by the unknown chemical (pencil line to spot), measure distance moved by solvent, find Rf value, look up Rf value in a database, may have to repeat with different solvents

how do you find the Rf value

distance moved by substance/distance moved by solvent

rp6: chromatography (16)

label unknown food colouring U and label 4 known colourings A-D, use ruler to draw horizontal pencil line on chromatography paper roughly 2cm from bottom, mark 5 pencil spots at equal spaces across the line, use a capillary tube to place a small spot of each food colour onto spots and label, pour water into a beaker at depth of 1cm³, attach paper to glass rod, lower paper into beaker and dip bottom of paper into water, pencil line must be above the surface of the water and sides of paper must not touch the side of the beaker, place lid on beaker to reduce evaporation, leave beaker still, remove paper when water has travelled ¾ up paper, use pencil to mark the point reached by water, hang paper up to dry, measure distance from pencil line to center of each spot, find Rf values

what units do Rf values use?

no units!!!

how do you test for hydrogen?

collect gas in test tube, remove bung, insert burning splint, if squeaky pop sound produced it is hydrogen

how do you test for oxygen?

collect gas in test tube, place glowing splint in test tube, if splint relights it is oxygen

how do you test for carbon dioxide?

draw some of the gas into a plastic pipette, bubble gas in limewater, repeat, if limewater goes cloudy it is carbon dioxide

what is limewater?

an aqueous solution of calcium hydroxide

how do you test for chlorine gas?

hold damp litmus paper into test tube with gas, if paper is bleached/turns white it is chlorine

how do you carry out flame tests?

place a small amount of chemical onto a wire mounted in a handle, then place this into the end of a blue bunsen burner flame

what colour is the flame for lithium ions

crimson red

what colour is the flame for sodium ions

yellow

what colour is the flame for potassium ions

lilac flame

what colour is the flame for calcium ions

orange-red

what colour is the flame for copper ions

green

what are the problems with using flame tests

colour can be difficult to distinguish, especially if there is a low concentration of metal ions, colour may be masked if sample contains a mixture of metal ions

how is flame emission spectroscopy carried out

sample of metal ion put into flame, light produced is passed into a spectroscope, converts light into a line spectrum, positions of lines are specific to metal ions, concentration of ion shown by how intense lines are

what is a instrumental method

a method carried out by a machine

what are the 3 main advantages of instrumental methods

rapid, accurate (no human error etc), sensitive (will work even on tiny sample)

how can you identify metal ions?

flame tests or testing with sodium hydroxide solution

how do aluminium, calcium, and magnesium ions react with sodium hydroxide solution?

form white precipitates of sodium hydroxide solution

what happens to the aluminium hydroxide precipitate when excess sodium hydroxide is added?

it dissolves

what colour precipitate do copper (II) ions form with sodium hydroxide solution?

blue

what colour precipitate do iron (II) ions form with sodium hydroxide solution?

g