BIOLOGY: Test for Starch

 Experiment 1. The test for starch

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

                                                         10% glucose solution into tube 1

                                                         1% starch solution into tube 2

(b) Put about 20 mm (depth) of

                                                         1% albumen solution into tube 3

                                                          water into tube 4

(c) To each tube, using a dropping pipette, add three drops of iodine solution.  Shake the tube (sideways, not up and down) to mix the contents. Look for any colour changes apart from the yellow colour of iodine itself. Copy the table below and record the results in your notebook.

Substance	Colour change after adding iodine
10% glucose solution
1% starch solution
1% albumen solution
water

Experiment 1. Discussion

I The substances selected for testing are examples of three of the principle chemical substances in cells, sugar (glucose), starch, protein (albumen). With which of these substances did iodine react to give a colour change ?

2 Does your result indicate that there is, for example, no sugar and no protein that will

give a colour change with iodine ?

3 What experiments would you have to carry out in order to give a confident answer to

question 2 ?

4 Does the result indicate that starch will always react with iodine solution to give a

colour change?

5 What was the point of the water in tube 4 ?

Experiment 1. Discussion - answers

I Only the starch should give a blue colour. The glucose and albumen solutions may take up the colour of the iodine.

2 The point is being made that the results from a single sample do not justify a sweeping generalization.

3 One would have to test samples of all sugars, all proteins and all fats.

4 The results permit confident prediction only about starch as a 1% aqueous solution.

5 Students often say that the unreactive samples have 'turned yellow'. This control shows that the yellow colour is simply the diluted iodine.

TEACHERS NOTES

Experiment 1. The test for starch - preparation

Outline The experiment establishes that iodine gives a blue colour with starch, but not with glucose or albumen.

Prior knowledge Use of test-tubes and dropping pipettes.

Advance preparation and materials *

     10% glucose solution

                                         

     1% starch solution                 10 cm³ per group

                                  

     1% albumen solution

     iodine solution                        5 cm³ per group

Apparatus-per group

test-tube rack  and 4 test-tubes                        

4 labels or spirit marker                                   

dropping pipette (for iodine)

                  

* Instruction for preparing reagents are given on p. 01