Experiment 2. The test for glucose

(a) Half fill a beaker with tap water and place it on a tripod and gauze. Heat the water with a Bunsen burner. While waiting for the water to boil, carry on with instructions (b) to (d).

(b) Label four test-tubes 1-4.

1% starch solution into tube 1

10% glucose solution into tube 2
(c) Put 20 mm (depth) of 1% albumen solution into tube 3

water into tube 4

(d) To each tube add about 10 mm Benedict's solution.

(e) Place the test-tubes in the beaker of hot water (see Figure on p. 2), and adjust the flame to keep the water just boiling and then copy the table below into your notebook

(f) After about 5 minutes, turn out the flame. Place the four tubes in a test-tube rack and compare the colours. Record the results in the form of a table in your notebook and match the final colours as nearly as possible with crayons.

| |Solution |Colour change on heating with Benedict’s reagent |Final colour (crayon) |
|1 | 1% starch | | |
|2 |10% glucose | | |
|3 |1% albumen | | |
|4 | water | | |

CLEANING THE TUBES. If a coloured deposit sticks to the inside of the tube even after rinsing with water, it can be removed by pouring in a little dilute hydrochloric acid (HCl).
Rinse the tube with water afterwards.

Experiment 2. Discussion

I What colour changes took place when Benedict's solution was added to each liquid ?

2 The solutions selected for testing are examples of three of the principal chemical substances found in cells: glucose, starch, protein (albumen). With which of these food materials did Benedict's solution give a decisive change on heating?

3 Apart from the colour, what change took place in the consistency of the Benedict's solution ?

4 Do your results indicate that any sugar (e.g. sucrose, fructose and maltose) will give the same colour as glucose did when tested with Benedict's solution?

5 Do your results allow you to say that (a) no protein will give a colour change when heated with Benedict's solution, (b) albumen never reacts with Benedict's solution to give a colour change ?

6 Can you predict that glucose will always give the same result with Benedict's solution as it did in your experiment ?

7 Why was water included in the test ?

Experiment 2. Discussion - answers

I No colour change should occur in the cold.

2 The only 'decisive' change, i.e. a red/orange colour, is with the glucose solution. The 1% starch may give a cloudy green colour .

3 Students may have observed the change