Many pentoses and hexoses can form ring structures. Phloroglucinol is a reagent of the Tollens' test for pentoses. They tend to grow best with pentoses as carbon sources. Furfural may be obtained by the acid catalyzed dehydration of 5-carbon sugars (pentoses), particularly xylose. Furfural from pentoses gives a blue or green color. Twenty hexoses and nine pentoses, including xylulose, were considered to be "rare sugars". A bright pink color on the paper is positive for the presence of pentoses. Most pentoses and some hexoses are all substrates for D-xylose isomerase. Hydrochloric acid dehydrates pentoses (sugars containing five carbon atoms) to produce furfural. These include carbohydrates (pentoses, hexoses and oligosaccharides), sugar alcohols and amino acids. The chemical formula of all pentoses is , and their molecular weight is 150.13 g/mol. Unlike hexoses such as glucose, pentoses are difficult to ferment. In the cell, pentoses have a higher metabolic stability than hexoses. Pentoses are then dehydrated to furfural, while hexoses are dehydrated to 5-hydroxymethylfurfural. Pentoses give a strong reaction, and hexoses give a much weaker reaction.