P2 8 Chemical analysis

Question 1

define pure substance

Answer 1

a single element/compound that hasn’t been mixed with another substance

Question 2

describe how to identify the purity of a substance using boiling points

Answer 2

• if another substance is mixed into a pure substance its boiling point will change
• the purer the sample, the closer its boiling point to that of the pure substance

Question 3

define a formulation +examples

Answer 3

• a mixture designed to be a useful substance
• components are mixed in measured quantities to ensure the formulation has the desired properties for its use
• eg. paint, medicines, cleaning agents, alloys, fertilisers, fuels

Question 4

describe the test for hydrogen

Answer 4

• lit splint held at the mouth of the test tube containing the gas
• burns rapidly with a squeaky pop if positive

Question 5

describe the test for oxygen

Answer 5

• glowing splint inserted into the tube containing the gas
• splint will relight if positive

Question 6

describe the test for carbon dioxide

Answer 6

• gas is bubbled through limewater
• limewater turns cloudy/milky if positive

Question 7

describe the test for chlorine

Answer 7

• damp blue litmus paper placed into the tube containing the gas
• paper turns bleached white if positive

Question 8

state uses of chromatography (3)

Answer 8

• separating (coloured) mixtures
• testing purity - pure substance will produce one spot whereas compounds in a mixture will separate into different spots
• identifying substances - compounds in a mixture identified using their Rf value

Question 9

state equation for calculating the Rf of a compound in a solvent from chromatography

Answer 9

distance travelled by substance (ink) / distance travelled by solvent

Question 10

describe what the Rf value of a compound in chromatography is

Answer 10

• distance travelled by substance (ink) / distance travelled by solvent
• measure distance between the starting pencil line and MIDDLE of the spot for substance
• measure distance between starting+end pencil line for solvent
• different compounds have different Rf values in a specific solvent
• Rf used to identify compounds in a mixture

Question 11

define stationary+mobile phase in chromatography

Answer 11

stationary - the paper
-> less soluble substances
mobile - the solvent moving up the paper eg. water
-> more soluble substances

Question 12

describe how to identify more soluble substances in chromatography

Answer 12

• less soluble substances: stronger attraction to the paper so spend longer in the stationary phase - move slowly+ travel a short distance up the paper
• more soluble substances: stronger attraction to the water (solvent) so spend longer in the mobile phase - move quickly+ travel further up the paper

Question 13

describe steps for chromatography RP

Answer 13

• draw horizontal pencil line 2cm from short edge of chromatography paper
• use a pipette to put small spots of each ink on the pencil line (label under)
• pour around 1cm3 water in beaker
• suspend paper in beaker so bottom edge of paper dips in water
• allow water to travel up paper (3/4) , remove paper then draw another line on the dry part beside where the water stopped
• hang paper to dry fully
• calculate distance travelled by water (solvent) and distances travelled by each ink (substances)
• find Rf value of each ink spot (distance of substance / distance of solvent)