Tests

Question 1

Benedict’s test for non-reducing sugars

Answer 1

Non reducing sugars do not react with the Benedict’s solution ans stay blue

Sucrose is the most common non-rescuing sugar

1. If sucrose is first boiled with hydrochloric acid, it will then give a Positive result when warmed with Benedict’s solution
2. Because Sucrose has been hydrolysed to glucose and fructose (both are reducing sugars

Question 2

Benedict’s test for reducing sugars

Answer 2

All monosaccharides and some disaccharides (maltose + lactose) are reducing sugars
- donate electrons/ reduce another molecule

Benedict’s reagent is used (Copper(II) Sulfate solution)

1. Place sample in a boiling tube
   - if not liquid, grind or blend w water
2. Add an equal volume of Benedict’s reagent
3. Heat the mixture gently in a boiling water bath for 5 mins

Question 3

Benedict’s test for reducing sugars - results

Answer 3

Reducing sugars react with copper ions, electrons are added to the blue Cu2+ ions, reducing them to brick red Cu+ ions. When warmed, brick red precipitate is formed = positive result

More reducing sugar = more precipitate

Test is qualitative
green = low conc. of sugar
Orange = medium conc. of sugar
Red = high conc. of sugar

Question 4

Reagent strips

Answer 4

Manufactured reagent strips can be used to test for presence of reducing sugars Eg glucose

Advantage = using colour coded chart, conc of sugar can be determined

Question 5

Iodine test for Starch

Answer 5

Few drops of iodine dissolved in potassium iodide solution are mixed w a sample

solution changes from yellow/brown to purple/black = starch is present

Else, negative result

Question 6

Emulsion test for lipids

Answer 6

sample is mixed with ethanol
Resulting solution is mixed with water and shaken
White emulsion appears as a layer on top of the solution = lipid is present
Solution is clear = negative result

Question 7

Biuret test for proteins

Answer 7

Peptide bonds form violet coloured complexes with copper ions in alkaline solutions

1. 3cm^3 of liquid sample mixed with an equal volume of 10% sodium hydroxide solution
2. 1% copper Sulfate was then added, a few drops at a time until the sample solution turned blue
3. Solution is then mixed and left to stand for 5 mins
4. If violet colour is seen, protein is present

• mixture of an alkali and copper Sulfate solution is known as a biuret reagent and can be used instead of adding individually