4 Rhianna tested two different foods using iodine solution and Benedict's solution,
These were her results.
spaghetti – went blue-black with iodine solution, went blue with
Benedict's solution
honey went orange-brown with iodine solution, turned brick red
with Benedict's solution
a
Describe how Rhianna tested the foods with Benedict's solution.
b Construct a results table and fill it in to show Rhianna's results clearly.
What conclusions can Rhianna make from her results?
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Answers
Answer:
Core practical 3 - Food tests
There are different tests which can be used to detect carbohydrates, proteins and lipids in foods.
Aims
To investigate the use of chemical reagents to identify starch, reducing sugars, proteins and fats.
Iodine test for starch
Method:
Place one spatula of the food sample on a dish or 1 cm3 if the sample is liquid.
Using a dropper, place a few drops of iodine solution onto the food.
Record any change in the colour of the solution.
Starch is detected using iodine solution. This turns blue-black in the presence of starch.
Iodine is placed on a sample of rice
Testing rice for the presence of starch.
Benedict's test for reducing sugars
Place two spatulas of the food sample into a test tube or 1 cm3 if the sample is liquid. Add about 1 cm3 depth of water to the tube and stir to mix.
Add an equal volume of Benedict's solution and mix.
Place the tube in a water bath at about 95°C for a few minutes.
Record the colour of the solution.
Reducing sugars are detected using Benedict's solution.
Reducing sugars include:
monosaccharides – such as glucose and fructose
disaccharides – such as maltose.
Benedict's solution gradually turns from blue to cloudy orange or brick red when heated with a reducing sugar. Sucrose is a non-reducing sugar and does not react with Benedict's solution.
Biuret test for proteins
Place one-two spatulas of the food sample into a test tube or 1 cm3 if the sample is liquid. Add about 1 cm3 depth of water to the tube and stir to mix.
Add an equal volume of potassium hydroxide solution to the tube and stir.
Add two drops of copper sulfate solution and stir for two minutes.
Record the colour of the solution.
Proteins are detected using Biuret reagent. This turns a mauve or purple colour when mixed with protein.
Emulsion test for lipids
Place two spatulas of the food sample into a test tube or 1 cm3 if the sample is liquid.
Add 2 cm3 of ethanol to the tube. Cover the end of the tube and shake the tube vigorously.
Allow the contents to settle.
Pour the liquid from the top of the mixture into a test tube half-filled with water.
Record the level of the food and whether the water is cloudy or clear.
Lipids are detected using the emulsion test. This is what happens:
The test substance is mixed with 2 cm3 of ethanol.
An equal volume of distilled water is added.
A milky-whiteemulsion forms if the test substance contains lipids.
Risks
Potassium hydroxide can be harmful to skin and eyes.
Hot water can scald.
Control measures
Wear eye protection.
Do not taste any of the foods.
Take care with hot water.
Wash off any splashes immediately.
Results
Colour observed at end of the test:
BeefOrange-brownBluePurpleMilky white
VegetableOrange-brownBlueBlueMilky white
RiceBlue-blackBlueBlueColourless
Breakfast cerealBlue-blackOrangeBlueMilky white
Conclusions
The colour changes show that beef is the only food tested that contains protein. Beef, vegetable oil and breakfast cereal contain lipids. Both rice and breakfast cereal contain starch. Breakfast cereal is the only food tested that contains reducing sugars.
Answer:
Spaghetti is monosaccharide starch that does not contain any sugar and Honey is a carbohydrate having sugar.
Explanation:
Starch gives blue-black with iodine solution and dark blue in Benedict's reagent shows the presence of monosaccharides or no sugar.
A solution of iodine and potassium iodide has a light orange-brown color, this shows it has sugar, and brick red Benedict's reagent shows the presence of simple carbohydrates, it has no starch. This reaction is caused by the reducing property of simple carbohydrates and turns Benedict's solution to brick red.
The conclusion table is attached.
Hence, it can be concluded that Spaghetti is a monosaccharide starch and Honey is a carbohydrate with the presence of potassium iodide.
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