your instructor if you are not sure if you have a positive result. • Iodine tests for starch. Iodine starts out red/brown, but when added to a solution where starch is present, it will turn dark blue or black. If no starch is present, the iodine will remain reddish-brown. • Benedict’s solution tests for the presence of sugar. It is blue and when added to a solution it will immediately turn the solution blue. After heating, though, if the solution contains glucose it will change to orange/red (looks like pumpkin). A light green or yellow color indicates a very slight amount of sugar, which probably reflects a little bit of leakage from your bag, and is not a positive test. Materials: • 6 test tubes in a test tube rack • Test tube holder • Ruler • Marking pencil Procedure: (Read the entire section before you start.) 1. Make sure you have 6 clean test tubes! Wash them yourself to make sure and rinse several times with distilled water. Dirty test tubes will result in a false positive in several of these tests. Label two of the test tubes “Salt”, two “Sugar”, and two “Starch”. Now label one set of 3 (1 Salt, 1 Sugar, and 1 Starch) with a small letter “c” or the word “control” to denote the control tubes. 2. Using the ruler, place a line on each tube 1cm from the bottom of the tube. Fill the control set of test tubes to the 1cm line with stock solution . Fill the other tubes with water from the distilled water bath you saved from above. Do not add stock solution to the second set of test tubes, only water from the beaker that held the tubing bag. Be careful not to confuse which tubes are which. Go to table 1 and fill in the column marked “initial color of indicator solution”. 3. To the test tubes labeled “Salt” and “Salt control,” add one dropper full of silver nitrate. ( AVOID GETTING THIS ON YOUR SKIN ) Swirl the tubes gently, then observe the color of the solutions. Record your results in Table 1. 4. To the test tubes labeled “Starch” and “Starch control,” add one dropper full of iodine solution. Swirl the tubes gently and observe the color of the solutions. Record your results in Table 1.
6 5. To the test tubes labeled “Sugar” and “Sugar control,” add one dropper full of Benedict’s solution. Swirl the tubes gently to mix the contents. Carefully lower these two test tubes into the boiling water bath and boil for 5min. Use the test tube holder to remove the tubes from the boiling water and place in the test tube rack. Observe the color of the solutions. Record your results in Table 1. Table 1. Results of the Chemical tests To show presence of Tube Indicator solution Initial color of indicator solution Final Color in tube Result (+ or -) Salt Control Silver Nitrate Salt Experimental Test Silver Nitrate Starch Control Iodine Starch Experimental Test Iodine Sugar Control Benedict’s Sugar Experimental test Benedict’s
You've reached the end of your free preview.
Want to read all 8 pages?