sugars

monosaccharides

- all carbohydrates contain the elements c, h and o. the monomers that carbohydrates are made from are monosaccharides, e.g. glucose, fructose and galactose. - glucose is a hexose sugar; a monosaccharide with six carbon atoms in each molecule. there are two types of glucose, alpha (α) and beta (β) glucose - they're isomers (molecules with the same molecular formula as eachother, but with atoms connected in a different way). 

disaccharide formation

- a disaccharide is formed when two monosaccharides join together. monosaccharides are joined together by condensation reactions; a glycosidic bond forms between the two monosaccharides as a molecue of water is released. - two alpha glucose molecules are joined together by a glycosidic bond to form maltose. sucrose is a disaccharide formed from a condensation reaction between a glucose molecule and a fructose molecule. lactose is another disaccharide, formed from a glucose molecule and a galactose molecule. 

the benedict's test for sugars

- all sugars can be classified as reducing sugars or non-reducing sugars . to test for sugars you use the benedict's test. the test differs depending on they type of sugar you're testing for.

reducing sugars

- reducing sugars include all monosaccharides and some disaccharides e.g. maltose and lactose. you add benedict's reagent (which is blue) to a sample and heat it in a water bath that's been brought to the boil. if the test is positive it will form a coloured precipitate - solid particles suspended in the solution. if the sample stays blue then there is no reducing sugar present. - the higher the concentration of reducing sugar, the further the colour change goes (from green to yellow to orange to red); you can use this to compare the amount of reducing sugar in different solutions. a more accurate way of doing this is to filter the solution and weigh the precipitate, or to remove the precipitate and use a colorimiter to measure the absorbance of the remaining benedict's reagent. 

non-reducing sugars

- if the result of the reducing sugars test is negative, the could still be a non-reducing sugar present. to test for non-reducing sugars, like sucrose, first you have to break them down into monosaccharides. you do this by getting  a new sample of the test solution, adding dilute hydrochloric acid and carefully heating it in a water bath that's been brought to the boil. then you neutralise it by adding sodium hydrogencarbonate. finally, just carry out the benedict's test as you would for a reducing sugar.