Qualitative tests for food Flashcards
1
Q
Explain the steps of the experiment to test for reducing sugars (6)
A
- dissolve glucose in water in test tube.
- Add an equal amount of benedicts solution.
- In a 2nd test rube mix equal volumes of water and benedicts solution. This is the control
- Heat both test tubes in a boiling water bath
- If reducing sugar is present, the solution turns brick red
- If it isn’t present the solution remains blue
2
Q
What colour is bendict’s solution and what colour does it turn
In the presence of what does this colour change happen?
A
Blue to brick red when reducing sugar is present
3
Q
Explain the test for starch (4)
A
- Add a few drops of iodine solution to some starch dissolved in water.
- Add a few drops of iodine solution to some water. This acts as the control.
- If starch is present the colour turns from red-yellow to blue-black or purple.
- If it is absent the solution stays red-yellow
4
Q
What colour does iodine change from and to?
In the presence of what does this happen?
A
red-yellow to blue-black or purple in the presence of starch
5
Q
Explain the test for lipids (6)
A
- label a piece of brown paper as lipid.
- Rub a small piece of butter or cooking oil on the paper.
- Repeat process using a few drops of water on a piece of paper labeled water. This acts as a control.
- Leave the 2 pieces of paper over a radiator to dry
- Lipid produces a permanent stain or translucent spot on the paper
- The water stain dries out
6
Q
Explain the test for protein (6)
A
- Dissolve a smaple of soluble protein (egg white or milk) in water.
- Add sodium hydroxide (colourless) until the solution clears.
- Then add a few drops of dilute copper sulfate (blue).
- As a control, add sodium hydroxide and copper sulfate ( or biuret solution) to water.
- The appearance of a purple-violet colour shows that proteins are present.
- If protein isn’t present the colour remains blue.
7
Q
What is a negative result and positive result of a protein test using biuret solution?
A
A negative result is blue and a positive result is purple-violet