section 2: biological molecules - topic 7: biochemical tests for molecules

Question 1

what test would you do for protein?

Answer 1

the biuret test.

Question 2

describe the biuret test.

Answer 2

1. the test solution needs to be alkaline, so first you add a few drops of sodium hydroxide solution.
2. then add some copper(ll) sulphate solution.

Question 3

what colour is the biuret solution if protein is present?

Answer 3

turns purple.

Question 4

what colour is the biuret solution if no protein is present?

Answer 4

stays blue.

Question 5

what test would you do for starch?

Answer 5

the iodine test.

Question 6

describe the iodine test.

Answer 6

add iodine dissolved in potassium iodine solution to the test sample.

Question 7

what colour is the iodine solution if starch is present?

Answer 7

changes from browny-orange to a dark, blue-black colour.

Question 8

what colour is the iodine solution if no starch is present?

Answer 8

stays browny-orange.

Question 9

what test would you do for lipids?

Answer 9

the emulsion test.

Question 10

describe the emulsion test.

Answer 10

shake the test substance with ethanol for approx. a minute, then pour the solution into water.

Question 11

what colour is the emulsion solution is lipids are present?

Answer 11

the solution will turn milky.
more lipid = more noticeable milkiness.

Question 12

what colour is the emulsion solution if no lipids are present?

Answer 12

the solution will stay clear.

Question 13

what test would you use for sugars?

Answer 13

the Benedict’s test.

Question 14

what are the 2 types of sugars?

Answer 14

reducing.
non-reducing.

Question 15

what do reducing sugars include?

Answer 15

all monosaccharides (i.e. glucose) and some disaccharides (i.e. maltose & lactose).

Question 16

describe the Benedict’s test.

Answer 16

add Benedict’s reagent (blue) to a sample and heat it in a water bath that has been brought to the boil.

Question 17

what happens if a reducing sugar is present?

Answer 17

a coloured precipitate will form.
the higher the concentration, the colour will form green, yellow, orange, then brick red.

Question 18

how do you test for a non-reducing sugar?

Answer 18

first, break them down into monosaccharides.
this is done by getting a new sample of the test solution, adding dilute hydrochloric acid acid & carefully heating in a water bath that’s been brought to the boil.
then neutralise it with sodium hydrogencarbonate.

Question 19

what happens if a non-reducing sugar is present?

Answer 19

sample forms green - yellow - orange - brick red precipitate.

Question 20

how else can glucose be tested for?

Answer 20

by using test strips coated in a reagent.

Question 21

what do the strips do if glucose is present?

Answer 21

they are dipped in a test solution and change colour if glucose is present.

Question 22

how do you work out the concentration of glucose present from the test strips?

Answer 22

compare the colour change to a chart to give an indication of the concentration.

Question 23

when may the test strips for glucose be useful?

Answer 23

for testing a persons urine for glucose - may indicate they have diabetes.

Question 24

what do quantitative tests tell you?

Answer 24

the amount (i.e. concentration) of a substance that is present in a sample.

Question 25

what can you use colorimetry and the Benedict’s test for?

Answer 25

to get a quantitative estimate of how much glucose (or other reducing sugar) there is in a solution.

Question 26

what is a colorimeter?

Answer 26

a device that measures the strength of a coloured solution by seeing how much light passes through it.

Question 27

what does a colorimeter measure?

Answer 27

absorbance (the amount of light absorbed by the solution).

Question 28

how do colour concentration and absorbance relate in a colorimeter?

Answer 28

the more concentrated the colour of the solution, the higher the absorbance is.

Question 29

how do you find out the glucose concentration of an unknown solution?

Answer 29

• first make up several solutions of known glucose concentrations.
• then measure the absorbance of these solutions.
• finally plot these absorbances on a graph to make a calibration curve.
• you can then use the calibration curve to estimate the concentration of glucose in the unknown solution.

Question 30

what is the relation between glucose concentration and absorbance?

Answer 30

it’s easiest to measure the concentration of the blue Benedict’s solution that’s left after the test.

the paler the solution, the more glucose there was.

the higher the glucose concentration, the lower the absorbance of the solution.