Re: What is in tap water?

The question is a little confusing, because Benedict's reagent can only determine if a monosaccharide is present. It does not react with disaccharides or polysaccharides.

Benedict's solution works via the reaction of copper ion with the aldehyde or ketone portion of a reducing sugar. The classical experiment is to add benedict's reagent to each of a solution of monosaccharide, disaccharide, and polysaccharide. Only the monosaccharide will form the red color that indicates a positive test. The other two will remain blue.

If the tap water remained blue with benedict's solution, it did not indicate that a polysaccharide was present; it meant only that a monosaccharide was not.

To determine if a polysaccharide is present, you need a second reagent: iodine. This is the second part of the classical experiment. If a solution does not turn red with Benedict's solution, then a second portion of the solution is treated with iodine. If it turns dark purple or black, the iodine has reacted with a polysaccharide (also known as a starch). If it does not, then a polysaccharide is not present.

Thus:

Benedict's=positive, iodine=negative ---> monosaccharide

Benedict's=negative, iodine=negative ---> disaccharide

Benedict's=negative, iodine-positive ---> polysaccharide.

Now, if your tap water gave a positive test with iodine, then there is either something seriously contaminating your tap water, or there were traces of starch in your test tube to begin with. That is not as remote a possibility as you would think. Many packing peanuts today are made from starch, and if your test tubes were not tightly wrapped when they were shipped or stored, they may have gotten some of the starch inside.