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non reducing/reducing sugars

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    Ok im very confused here.

    Are all sugars either NON reducing or reducing?

    I have a table with lots of substances and results of tests. Most of them turn blue in benedict's solution.. does that mean they aren't sugars at all and are a different substance? OR does it mean that they might be sugars, just non reducing ones?

    With the 'benedict's test' (i presume that means without hydrolising first)
    substance a, b and d is blue. substance c however is brick red- so does that mean its a reducing sugar?

    what happens if you do the reducing sugars test, and the solution is blue, and then you do the non reducing sugars test, and the solution is STILL blue? does that mean its not a sugar at all? thanks!
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    Basically all monosccharides and most disaccharides are reducing sugars. Sucrose is a non reducing sugar. If you add iodine in potasium iodide to starch you will get a blue-black colour (non reducing sugar) . The test for a reducing sugar will give a yellow, orange or brick red precipitate. if this test for a reducing sugar is negative and you retest a sample with hcl and add weak alkali and you get a yellow orange or brick red precipitate again you have a non reducing sugar. umm this may be a bit confusing but i HOPE you understand lol
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    Try this for some clarification - i have forgotten some of this stuff to be honest, lol! - www.edexcelbiology.co.uk/ReducingSugars.htm
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    (Original post by tiger_vio)
    Ok im very confused here.

    Are all sugars either NON reducing or reducing?

    I have a table with lots of substances and results of tests. Most of them turn blue in benedict's solution.. does that mean they aren't sugars at all and are a different substance? OR does it mean that they might be sugars, just non reducing ones?

    With the 'benedict's test' (i presume that means without hydrolising first)
    substance a, b and d is blue. substance c however is brick red- so does that mean its a reducing sugar?

    what happens if you do the reducing sugars test, and the solution is blue, and then you do the non reducing sugars test, and the solution is STILL blue? does that mean its not a sugar at all? thanks!
    Ok... I think you need to understand the chemistry behind the test...

    'Reducing sugars' are sugars that acts as reducing agents...

    This means that the sugars are oxidised and cause something else to be reduced...

    Glucose is an 'aldehyde' because it has the -CHO functional group on it's first carbon...

    The aldehyde group on the glucose is oxidised to -COOH, which allows the Cu2+ ions in the benedicts solution to be reduced to Cu+ ions...

    This change of ions caused a different copper oxide to form, which is why the colour of the benedicts reagent changes...

    Non-reducing sugars are slightly different... The only example I know of one is sucrose...

    Sucrose is a disaccharide, which is formed from glucose and fructose connected by a bond...

    The bond joining the sugars in sucrose can be broken by addition of acid before adding the benedict's reagent...

    The glucose that is released from the sucrose then gives a positive result for the benedict's test as before...

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Updated: September 13, 2006
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