May 22, 2013
1. The water in test tube No. 1 is a negative control. No reaction is expected to occur because water is not a carbohydrate. Reaction by a negative control is evidence of contamination.
2. The color changes expected for positive test results are Benedict’s test: orange to brick red Tollen’s test: metallic silver Starch iodine test: blue-black
3. Identify each of the samples in Part 1 as monosaccharide, disaccharide or polysaccharide.
Water = not a saccharide Glucose = monosaccharide Fructose = monosaccharide Galactose = monosaccharide Sucrose = disaccharide Lactose = disaccharide Starch = polysaccharide Honey = primarily monosaccharides with some disaccharide Saccharine = not a saccharide Nutrasweet = not a saccharide
Part 1: Benedict’s Test
Place ten drops of 1% solution of carbohydrate and ten drops of water in individual test tubes. Add 2 mL of Benedict’s reactant and place tubs in hot water bath of 200 mL water in a 400 mL beaker for t0 minutes. Record observations.
BENEDICT’S TEST TUBE | SOLUTION | OBSERVATION | 1 | Water | Clear light blue | 2 | Glucose | Opaque brick red | 3 | Fructose | Opaque orange-red | 4 | Galactose | Opaque brick red | 5 | Sucrose | Clear light blue | 6 | Lactose | Clear dark blue w/ thin red-brown layer on top | 7 | Starch | Cloudy orangish-yellow | 8 | Honey | Opaque brick red | 9 | Saccharin | Clear light blue | 10 | Nutrasweet | Clear light blue |
Saccharides that show a positive result in this test do so because there is an anomeric carbon available in the cyclic form of the molecule which allows it to open into a straight-chain form containing an aldehyde group. The aldehyde in the straight-chain reacts with the copper ions in the reagent to produce the color change.
Positive Results. Glucose, galactose and honey all contain aldehyde