Chemical tests Flashcards
Why is the positive result for reducing sugars a red precipitate?
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
What is the test for reducing sugars?
- Add 2cm of a food sample (in liquid form) to a test tube
- Add 2cm of Benedict’s solution
- Heat mixture in 80° water bath for 2 minutes
What is the result for reducing sugars?
If the solution has changed colour from a blue colourless solution up to a red precipitate, reducing sugars are present
What is the test for non-reducing sugars?
- Carry out the test for reducing sugars to ensure the sample is not a reducing sugar
- Add 2cm of the sample (in liquid form) to 2cm of dilute hydrochloric acid and heat in an 80° water bath for 5 minutes.
- Let cool and then slowly add sodium hydrogencarbonate until pH is neutral
- Re-test this solution now with Benedict’s
What is the result for non-reducing sugars?
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
What is the test for starch?
- Add iodine dissolved in potassium iodide solution to the sample
- If starch is present, sample changes from brown-orange to blue-black
What is the test for lipids?
- Add ethanol to the lipid and mix
- Pour the mixture into a tube of water
- Shake the mixture
- If lipid is present, a cloudy white emulsion forms