Experiment with it, and have the students make notes about the properties. Mix twice as much cornstarch in with water. The two step process of creating the cornstarch-water mixture followed by the addition of iodine illustrates the differences between chemical and physical changes. Common indications of chemical reactions are colour change, bubbles, the formation of a new substance or the emission of a gas. This is a common indication that a chemical reaction has occurred rather than a physical change. The details of this reaction are complicated, but the important observations to be made are the colour changes. Finally, when the reaction is over, it will settle into a dark purple. First the cornstarch mixture will likely turn bright yellow, and there will be interesting movement of the liquid. In order to discuss the differences between physical and chemical changes, you can cause a dramatic reaction by adding iodine to the mixture. There is no new substance produced, it is simply a property of cornstarch that it arranges itself in this way when mixed with water. When the cornstarch is mixed with water, you are causing a physical change, even though it may not be obvious. When the students start to play with the Oobleck, they will likely be unsure whether it is a solid or a liquid, which is something they probably have not experienced before. As you will have discussed, a gas spreads out and takes up whatever volume it is contained in, a solid maintains a rigid volume and shape, and a liquid takes the shape of the container it is in, but maintains its volume. It is a liquid if left at room temperature, but it begins to solidify when pressure is applied (like when you squeeze it in your hands). The first thing to note is that the Oobleck is actually a very special kind of fluid. Nothing that occurs in this activity is very complicated, and all of it is fun and dramatic. This activity is an excellent way to start your unit on physical and chemical changes. Plastic Gloves (optional, but this activity does get messy).Iodine (can be obtained from larger drug stores, should be pretty inexpensive).Through their observations, students will note the differences between these processes. In this activity, students will experience physical and chemical changes as cornstarch is mixed with water and iodine.
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