Chemical tests

Question 1

Why is the positive result for reducing sugars a red precipitate?

Answer 1

It is from blue Cu 2+ ions becoming red Cu + ions because they are reduced by reducing sugars and the mir concentrated the glucose solution, the ,ore Cu ions are reduced

Question 2

What is the test for reducing sugars?

Answer 2

1. Add 2cm of a food sample (in liquid form) to a test tube
2. Add 2cm of Benedict’s solution
3. Heat mixture in 80° water bath for 2 minutes

Question 3

What is the result for reducing sugars?

Answer 3

If the solution has changed colour from a blue colourless solution up to a red precipitate, reducing sugars are present

Question 4

What is the test for non-reducing sugars?

Answer 4

1. Carry out the test for reducing sugars to ensure the sample is not a reducing sugar
2. Add 2cm of the sample (in liquid form) to 2cm of dilute hydrochloric acid and heat in an 80° water bath for 5 minutes.
3. Let cool and then slowly add sodium hydrogencarbonate until pH is neutral
4. Re-test this solution now with Benedict’s

Question 5

What is the result for non-reducing sugars?

Answer 5

When the substance is retested for reducing sugars after it is tested for non-reducing sugars, it should produce a positive result of a green- red precipitate

Question 6

What is the test for starch?

Answer 6

1. Add iodine dissolved in potassium iodide solution to the sample
2. If starch is present, sample changes from brown-orange to blue-black

Question 7

What is the test for lipids?

Answer 7

1. Add ethanol to the lipid and mix
2. Pour the mixture into a tube of water
3. Shake the mixture
4. If lipid is present, a cloudy white emulsion forms