Properties of Glucose and Starch Lab – Iodine and Benedict’s Solution
Aim / Objective:
To investigate the properties of Glucose and its polymer Starch
The properties of glucose and starch were investigated using taste, water and Benedict’s Solution.
The word polymer means many units. A polymer consists of long chain-like molecules which are formed when several thousands of small molecules (monomers) combine chemically. Monomers are small molecules which can be linked to form long chain molecules.
There are two distinctive types of monomers: 1. Unsaturated small molecules, for example ethene and some of its derivatives. And 2, molecules containing 2 functional groups, for example ethane- 1, 2- diol and 1, 6- diamino-hexane
The typical reaction of unsaturated molecules, for e.g. alkenes is an addition reaction. Unsaturated monomers will therefore bond by addition reaction to form addition polymers.
The process by which polymers are formed is called addition polymerization. Many of the bi-functional molecules used as monomers contain two identical functional groups. Such monomers rarely bond with each other so two different bi-functional monomers are used and they bond by condensation reactions.
The polymer formed in this reaction is called a condensation polymer, and the process by which it is formed is condensation polymerization.
Polysaccharides are polymers that belong to a group of naturally occurring compounds containing carbon, hydrogen and oxygen based on the formula Cx(H2O)y. The simplest carbohydrates are the monosaccharides which have the general formula of CnH2nOn.
Examples of these simple sugars are; galactose, glucose, and fructose. Glucose molecules are the building blocks for the carbohydrate polymer and the condensation of the glucose molecule occurs to form disaccharides and eventually more complex carbohydrates called polysaccharides.
Glucose, Starch, Bunsen Burner, Test Tube, Spatula, Dropper, Tong, Iodine solution, distilled water, benedicts solution, beaker.
Method / Procedure:
- Taste a small sample of glucose and starch and note your observations.
- Add a few drops of the starch solution to a test tube and add a few drops of iodine solution to the test tube containing the starch solution. Note observations.
- Repeat step 2 using glucose solution.
- Add a half spatula of starch to a test tube of water, shake and not observations.
- Repeat step 4 with glucose
- Set up a water bath and leave to boil. Add Benedict’s solution to the solutions used in steps 4 and 5 and place the test tubes in the boiling water bath. Note observations.
- Tabulate the results.
1. Explain the difference in taste between glucose and starch
>>>Glucose has a sweet taste because it belongs to the free aldehyde group which interacts with the sweet taste buds on the tongue. Starch on the other hand, even though contains one free aldehyde group is tasteless because of the hundreds of glucose subunits attached together.
The one free aldehyde group in starch is at one end and becomes insignificant because the molecule is too big.
2. Explain the reaction of iodine on starch solution
>>> In the presence of iodine, starch solution reacts to form a starch-iodine complex. This complex has a transfer of charges causing a change in the arrangement in the atoms thus leaving spaces between which light is absorbed and gives the solution a blue-black appearance.
3. Differentiate between the solubility of glucose and starch in water.
>>> Glucose is a monosaccharide and is soluble because of its many hydroxyl (OH) groups which tend to increase the solubility of a molecule. Starch is a polysaccharide and is made up of thousands of monosaccharide bonded together therefore making the starch bonds harder to break apart and dissolve in water.
4. Explain the reaction of Benedict’s solution on glucose and starch.
>>> Glucose contains one aldehyde group (-CHO) in its structure and this reacts with the free Cu2+ in the benedicts solution to give the characteristic color. Reducing sugars such as glucose gives the Cu2+ one electron each thereby reducing it to Cu+ Oxide. In the presence of starch the Benedicts solution oxidizes the aldehyde form in carbohydrate.
The polymerization of glucose has converted all but a very small fraction of the aldehyde to acetals which are unreactive to this oxidizing agent. The few groups on the end of the starch molecules would give only a very small reaction which could not be observed, hence there was no observable color change.
5. Write the redox reaction for Benedict’s solution and Glucose.
>>> 2Cu2+ + 2OH- + 2e- = Cu2O(s) + H2O(l)
CHO + 3OH- = 2e- + COO- + 2H2O
6. Explain the series of colors seen in the reaction of Benedict’s solution with glucose
>>>The Benedict’s solution is originally blue due to Cu2+ ions, this changes to green, yellow, orange or red, because of the formation of the Cu2O solid precipitate and the disappearing Cu2+ ions. Finally the color turns into a brick-red or brown color because of the high concentration of glucose present in the solution.