2. Experimental Techniques

Question 1

suggest apparatus that is suitable of measuring time, temperature and mass

Answer 1

time - stopwatch
temperature - thermometer
mass - digital mass balance

Question 2

suggest suitable apparatus for measuring volume

Answer 2

measuring cylinder

burette

pipette with pipette filler

Question 3

what is chromatography used for

Answer 3

separating and identifying a mixture of substances

Question 4

how does separation occur in paper chromatography

Answer 4

there is a stationary phase and a mobile phase

substances have different solubilities in the mobile phase so will travel at different rate causing separation. more soluble substance travel further

Question 5

describe how you could use a paper chromatography to separate mixture of food colorings

Answer 5

• draw a pencil line 2 cm from the bottom of chromatography paper
• place one dot of 3 known food coloring’s and one dot of the unknown mixture along the line. label each line
• place the chromatography paper in a beaker containing 1 cm of water
• wait for the water to travel up most of the paper and then remove the paper from the beaker and mark the height reached by the solvent. dry the paper
• observe the chromatogram and record results

Question 6

why should pencil be used to draw the line along the bottom of the chromatography paper

Answer 6

it is insoluble in the solvent so will not affect the experiment

Question 7

why should the solvent in the beaker be no deeper than 1 cm for paper chromatography

Answer 7

if it is deeper, it will wash away the substances on the chromatography paper

Question 8

in paper chromatography, what is the stationary phase

Answer 8

the chromatography paper

Question 9

in paper chromatography, what is the mobile phase

Answer 9

the solvent

Question 10

what two things affect how long the molecules spend in each phase in paper chromatography

Answer 10

• their solubility in the mobile phase
• their attraction to the chromatography paper

Question 11

what is an Rf value

Answer 11

A Rf value is the ratio between the distance travelled by the dissolved substance and the distance travelled by the solvent

Question 12

how do you calculate Rf value

Answer 12

Rf = distance travelled by substance / distance travelled by solvent

Question 13

in paper chromatography, what affects the Rf value of a substance

Answer 13

the solvent

repeating the experiment with a different solvent will change the Rf value

Question 14

when measuring the distance moved by a substance on the chromatography paper, where should you measure between

Answer 14

from the pencil baseline to the middle of the spot of the substance

Question 15

how many spots will be observed on a chromatogram of a pure substance

Answer 15

one

Question 16

how can you identify that two mixtures contain a substance which is the same using chromatogram

Answer 16

both mixtures will produce different chromatograms but the position of one spot will match exactly

Question 17

how does solubility affect the distance a substance travels in paper chromatography

Answer 17

a substance that is more soluble in the mobile phase will travel further up the chromatography paper

Question 18

how can paper chromatography be used if a mixture contains colorless substances

Answer 18

using locating agents

after the chromatogram has been produced, it is treated with a locating agent to make the spots visible

Question 19

what is a mixture

Answer 19

a combination of two or more elements or compounds that are not chemically joined together

Question 20

what is a pure substance

Answer 20

a single element or compound not mixed with any other substance

Question 21

how can a pure substance be identified using melting or boiling points

Answer 21

pure substances have a sharp, exact melting point whereas impure substances will melt/boil over a range of temperatures

Question 22

why is the purity of substances important in everyday life

Answer 22

impurities in drugs may cause dangerous side effects

impurities in food and drink may cause health problems if ingested

Question 23

what method can be used to separate an insoluble salt from a solution? describe the process

Answer 23

filtration:
- put a piece of filter paper into a funnel and place over a conical flask
- pour the mixture into the funnel so that the liquid collects in the beaker. the insoluble salt is left on the filter paper
- pour demonized water into the funnel to wash any of the solution from the salt
- leave the salt to dry on the filter paper

Question 24

how can a soluble salt be separated from a solution? describe the process

Answer 24

crystallization:
- place the solution in an evaporating basin
- warm the solution gently so that the solvent starts to evaporate and the concentration of the solution increases
- remove from the heat and allow the mixture to cool before all the solvent evaporates
- leave to evaporate without heating. dry and collect the crystals

Question 25

when is simple distillation used as a separating technique

Answer 25

to separate and purify a liquid from a mixture of liquids. it is suitable when the liquids have different boiling points

Question 26

describe how to separate a mixture of water and ethanol using simple distillation

Answer 26

- pour the mixture into a round bottomed flask and connected to a condenser. place a beaker at the outlet - slowly heat the flask until the ethanol starts to vaporize. since ethanol has a lower boiling point than water, the ethanol evaporates first then condenses in the condenser before being collected in the beaker

Question 27

what mixture is fractional distillation commonly used to separate

Answer 27

crude oil

Question 28

describe how fractional distillation separates crude oil

Answer 28

- the crude oil is heated until it evaporates - the vapors enter a fractionating column. the column has a temperature gradient - the vapours rise up the column and substances condense at different fractions depending on their boiling points