Iodine solution is used to test for starch; a dark blue colour indicates the presence of starch.
Hypothesis:
The longer starch and saliva are mixed, the more starch is broken down and the iodine will become brighter. As the solution is mixed with Benedict’s solution and heated in a water bath, it will change it’s colour, indicating that the starch in the solution has been broken down to reducing sugars by the amylase in the saliva.
- The longer the amylase is reacting with the starch solution, the less starch will be present
Therefore towards the end, the iodine does not turn
blue/black anymore, because there is no starch
anymore.
Method
- Test one drop of the starch suspension provided with iodine solution, on a white dish or spotting tile.
-
Rinse mouth with 20cm3 of distilled water: spit this back into the sink.
-
Rinse mouth a second time with 20cm3 of distilled water. This time keep the water in the mouth for 30 seconds, moving it around with the tongue. Collect this diluted saliva in a beaker.
- Using a pipette place one drop of iodine solution in each of the depressions on the spotting tile.
-
Use a clean 5cm3 syringe to take up 2cm2 of the starch suspension. Rinse the outside of the syringe with distilled water.
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Now take up 2cm3 of dilute saliva into same syringe. Draw in a little air and quickly and thoroughly mix the contents of the syringe by inverting it two or three times.
- Hold the syringe vertically and immediately squeeze the plunger carefully to expel one drop of the mixture into the first drop of iodine solution on the tile. Do not let the tip of the syringe touch the iodine solution. Note the time.
- Continue to test single drops of the mixture at intervals of 30 seconds until the iodine solution shows no further change in colour.
-
Now expel 1cm3 of the remaining mixture in the syringe into a test-tube. Test this for the presence if reducing sugars using Benedict’s solution. Put it into a water bath and leave it for 10 minutes. Then take it out.
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Ten expel another 1cm3 of the remaining mixture in the syringe into a test-tube. Test this for the presence of reducing sugars, using Benedict’s solution. Put it into a water bath and leave it for 10 minutes. Then take it out again.
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Place about 5cm3 of the remaining dilute saliva in a test-tube and boil it gently for about 10seconds.Leave it to cool.
- Repeat steps 4 to 7, but this time mix boiled saliva with starch suspension in a clean syringe. Continue to test single drops of this mixture for as long as you did with the unboiled saliva and starch.
Variables
(i) Independent variable:
The time for the reaction of amylase upon the starch to take place: 30 seconds for each result.
Use a watch/stopwatch to check the time before dropping another drop of the mixture onto the spotting tile.
(ii) Dependent variable:
The colour changes when iodine solution is added to the mixture of saliva and starch
(iii) Controlled variable:
Results
After heating the remaining solution (with the added Benedict’s solution) in a water bath for 10 minutes, it turns brick red.
That means that the starch in the solution has been broken down to reducing sugars by the amylase in the saliva.
Questions-Answers
- 1) What appears to have happened to the starch during the course of this experiment?
-The starch broke down the longer it was left
- 2) What do you think was the purpose if rinsing out your mouth the first time?
-To stimulate the salivary glands to produce saliva, which is important for the experiment
OR
-To remove food residues which could influence the experiment’s outcome.
- 3) What conclusion can you draw from the results of the tests in steps 9 and 10?
- The solution turns brick red.
That means that the starch in the solution has been broken
down to reducing sugars by the amylase in the saliva.
- 4) What test should you perform on the original starch suspension before you can be confident of your answer to question 3?
-Iodine test to see if starch has completely been broken down. If it turns black blue, starch is still present!
- 5) How are the properties of saliva affected by boiling?
- The enzyme amylase in saliva is denatured and so the
starch in the saliva stays unchanged
- 6) Saliva normally acts on starch at body temperature (37°C). If you had carried out the whole experiment at this temperature, what difference would you expect this to have made to the results you obtained?
-The results would have been much faster. The colour would change much faster for example from blue/black directly to navy blue or dark purple instead of changing much slower. (as shown in the result table) The experiment would be much faster to do and to finish.