Biology – Carbohydrates and lipids | e-Consult
Carbohydrates and lipids (1 questions)
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The test for reducing sugars involves reacting the sugar with copper(II) ions (usually provided as copper sulfate). If a reducing sugar is present, it will reduce the copper(II) ions to copper(I) ions, forming a reddish-brown complex. The intensity of the colour is proportional to the concentration of the reducing sugar.
Here's the predicted colour change for each solution:
- Glucose: A positive test – the solution will turn reddish-brown. This is because glucose is a reducing sugar and readily reduces copper(II) ions.
- Fructose: A positive test – the solution will turn reddish-brown. Fructose is also a reducing sugar and will undergo the same reaction as glucose.
- Sucrose: A negative test – the solution will remain blue. Sucrose is a non-reducing sugar, so it does not contain a free aldehyde or ketone group and will not reduce copper(II) ions. The blue colour of the copper(II) ions will persist.