Physical and Chemical Changes with Goop - Oobleck

Learning Objectives

In this activity, students will experience physical and chemical changes as cornstarch is mixed with water and iodine. Through their observations, students will note the differences between these processes.

Materials

• Basin
• Water
• Cornstarch
• Food colouring
• Iodine (can be obtained from larger drug stores, should be pretty inexpensive)
• Plastic Gloves (optional, but this activity does get messy)

Motivation

This activity is an excellent way to start your unit on physical and chemical changes. Nothing that occurs in this activity is very complicated, and all of it is fun and dramatic.

The first thing to note is that the Oobleck is actually a very special kind of fluid. 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). 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. 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.

When the cornstarch is mixed with water, you are causing a physical change, even though it may not be obvious. There is no new substance produced, it is simply a property of cornstarch that it arranges itself in this way when mixed with water. In order to discuss the differences between physical and chemical changes, you can cause a dramatic reaction by adding iodine to the mixture. First the cornstarch mixture will likely turn bright yellow, and there will be interesting movement of the liquid. Finally, when the reaction is over, it will settle into a dark purple. The details of this reaction are complicated, but the important observations to be made are the colour changes. This is a common indication that a chemical reaction has occurred rather than a physical change. Common indications of chemical reactions are colour change, bubbles, the formation of a new substance or the emission of a gas. The two step process of creating the cornstarch-water mixture followed by the addition of iodine illustrates the differences between chemical and physical changes.

Procedure

Part 1

Mix twice as much cornstarch in with water. Knead with hands to create the Oobleck. Careful, this will be messy. Experiment with it, and have the students make notes about the properties. Was this a physical or a chemical change? Feel free to add some food colour, but the reaction when we add iodine is more dramatic if the Oobleck is a light colour (yellow and plain white are very impressive).

Part 2

When the students are done playing with the Oobleck and have come to a consensus on whether it is a liquid or a solid, it is time for the next step. Add a small tablespoon of iodine to the goop. Have the students take notes on what they observe. This reaction is quite impressive, so the students may want to do see it more than once.

Note

Clean up for this activity is very easy, even though it can get messy. You can simply pour all of the contents down the drain; all the chemicals used are harmless. You may want to flush the drain well with water to prevent clogging.

Investigating Questions

• What are indicators of a chemical change?
• What are indicators of a physical change?
• Did adding water to cornstarch give a chemical or physical change?
• Did adding iodine to Oobleck give a chemical or physical change