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Activities
Materials:
* Lab aprons and safety glasses/goggles should be worn. Procedure:Divide students into 5 groups. Assign each group one of the following "recipes": Team PVA Sodium borate solution 1 100 mL 2 mL 1. Add PVA to paper cup. 2. Stir in food coloring if desired. (just a few drops) 3. Add sodium borate solution - stirring quickly making sure to scrape the sides and bottom using the wooden craft stick. 4. Pour into a plastic baggy and knead. 5. Take slime out of baggy and "investigate" properties. 6. Have groups compare properties of the different slimes: bouncing stretching slowly stretching quickly letting it "pour" from one hand to another etc..... Have the students record the appearance and characteristics of the PVA at each step. This method of doing the slime lab allows the students to work as teams and then causes the teams to interact to make comparisons. The students sometimes struggle to find the words to describe the slime in step #6 - encourage them to devise some sort of chart or graphic organizer to record their results/conclusions. Notes:By having different groups use different amounts of sodium borate, students should be able to more easily determine the effects of crosslinking on the properties of the polymer. The more sodium borate used, the more crosslinking that occurs. Polymer chains are formed by covalent bonds which are strong bonds. In making slime, individual polymer chains are "hooked" together by weak hydrogen bonds. It is evident that this crosslinking is weak because of the ease with which the slime pulls apart. It is sort of like "tying" together strong strands of string with limp strands of cooked spaghetti. Even though this crosslinking is weak, it does alter the properties of the polymer. Have the students leave some of the slime stretched out on a counter overnight. The water will evaporate and the PVA will become a dry, brittle film that is mostly transparent. Students like to stretch or roll the slime into long ropes. It becomes a contest as to which slime (which amount of crosslinking) is best for this. An observation that the students can make is how cool the slime feels. As the slime is stretched it has more surface area exposed to air and the evaporation rate of the water is increased. What they are feeling is the effects of evaporative cooling. The PVA solution can be made using a stirrer hotplate. But it is much faster and easier to make it using a microwave oven. Do not let the solution boil whichever method you use. Boiling will alter the properties. You can use a glass bottle with a plastic screw cap lid. Add 96 mL of tap water and 4 grams of PVA powder. Set the cap on the bottle but don't tighten. Microwave for 2 - 3 minutes and then tighten the cap and shake. Loosen the cap and microwave an additional minute at a time if needed. The prepared solution should be clear and somewhat viscous. It stores well in the refrigerator until needed. A beaker may be used to make the PVA in larger amounts. Cover the beaker with microwaveable plastic wrap and stir between heating segments. The PVA solution is sticky. Have the students try to figure out a way to measure the required amount of PVA without putting it into a graduated cylinder. This is a quick, simple opportunity for them to problem solve. I have my students measure out 100 mL of water in a graduated cylinder into a cup and then mark the fluid level. Pour out the water. Add PVA to the mark. Some students will usually figure this out for themselves. It really saves on clean-up. Author: Tug Sezen |
