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Digestion In Human (Food Tests)

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Practical 12: Digestion in Humans

Aim: To distinguish between 5 liquids to come to a conclusion from which region of the alimentary canal it was extracted from.


In this practical, we have been given 5 unknown solutions hence it is not possible for us without any logical analysis to come up with a conclusion regarding the positioning along the alimentary canal. We cannot come up with a proper answer or predict results without running through the tests.

I though can base my analysis on the following hypothesis:-

The more the starch, the higher up the alimentary canal the liquid is.

The more the reducing sugar, the lower down the alimentary canal the liquid is.

The more the protein, the higher up the alimentary canal the liquid is.

The more the fats, the more the higher up alimentary canal the liquid is.

Description of the appearance of liquids at the beginning:-

D-33: Light White cloudy Emulsion

D-34: Clear transparent liquid

D-35: White cloudy emulsion

D-36: Clear transparent liquid

D-37: White cloudy emulsion

Materials: Benedict solution, Iodine solution, Ethanol, Copper (II) Sulphate Liquids (D33, D34, D35, D36, D37)

Apparatus: 250 ml glass beaker, Test tube holder, Test tube rack, 5 test tubes, White tile, 10 ml measuring cylinder, Bunsen burner and other heating apparatus


The digestible carbohydrates are broken into simpler components by enzymes in the saliva (a juice produced by the pancreas) and in the lining of the small intestine. Starch is digested in two steps: First, an enzyme amylase in the saliva and pancreatic juice breaks down the Amylose in the starch into molecules called maltose; then an enzyme in the lining of the small intestine, maltase, splits the maltose into glucose molecules that can be absorbed into the blood. Glucose is carried through the bloodstream to the liver, where it is stored or used to provide energy for the work of the body. Through this piece of information, we can roughly come to a conclusion on the origin of the extract.

Iodine Test

The Amylose present in starch is responsible for the formation of a blue-black

color in the presence of iodine. The iodine molecule slips inside of the Amylose coil. Iodine is not very soluble in water; hence the iodine reagent is made by dissolving iodine in water in the presence of potassium iodide. This makes a linear tri-iodide ion complex along with it soluble. The tri-iodide ion slips into the coil of the starch causing an intense blue-black color.

Chemical formula: I2 - I - => I3 -


1. Align the 5 test tubes in the rack and then number them parallel to the five liquids.

2. Measure exactly 2 cm3 of its liquid and pour it into its respective test tubes.

3. Now add Iodine solution to the liquid drop-by-drop (max 3 drops)

4. And now record your observation.


Blue Black: ++ starch present is high quantity

Dirty green + starch present in minute quantity

Yellow-Brown - starch not present


Type of Liquid D-33 D-34 D-35 D-36 D-37

Observations Turns blue-black in a single drop

(Blue more prominent) Turns blue-black in a single drop

(Black more prominent) Iodine solution remains yellowish- brown in colour Turns blue-black in a single drop

(Black more prominent) Turns dirty green in colour after 2 drops. Has a tinge of blue

Starch present







The starch was not at all present in D-35 but it was in the rest of the test liquids. This makes it evident that D-35 is the liquid that went the most far down the alimentary canal as all its starch was broken down totally.

Both liquids D-34 and D-36 have acquired the blue black colour after just a single drop. We are unable to distinguish them as of yet but we can say that they are both extracted from near the start of the alimentary canal.

Liquid D-33 also gives the blue-black colour after the first drop but the colour is more bluish than black. This shows that some of the starch has already been digested and the concentration of starch is lower than those in D-34 and D-36. We can hence conclude that D-33 is extracted from the alimentary canal further down as compared to D-34 and D-36.

When added to D-37 the colour if the iodine solution becomes dirty green. This indicates a slight concentration of starch present in D-37. It place in the alimentary canal is further down as compared to D-33, D-34 and D-36.

From this we can conclude the order of the test liquids in the alimentary canal (from the top): -

D-34 / D-36, D-33, D-37, D-35

(These were the derived results from the Iodine solution test)

Benedict solution test

It is used as a test for reducing sugars or for the presence of aldehydes. The principal reagent is copper(II) sulphate. Benedict's solution contains Copper, which can accept electrons from some sugars and consequently change color. One litre of Benedict's solution contains 173 grams sodium citrate, 100



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