See 2 video demonstrations of diffusion in action. The first video shows a drop of food coloring diffusing through water. It is a video that is time lapsed at 1 frame/15 seconds. The second video shows a Ziploc bag of iodine placed into a beaker of starch water. It is a video that is time lapsed at 1 frame/30 seconds. The iodine diffuses out of the bag and into the starch water. Where the iodine is diffusing the starch water turns a dark blue.
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Signs of a chemical reaction include color change, gas production, temperature change, precipitation, and other changes in properties …including density, taste, texture, smell, melting point, boiling point, etc. This demonstration will really make them think about it. Place a large plastic container on the counter. Invert a large jar or beaker into the container. Set a 3/4 full liter of fresh cola on top. Carefully but quickly pour a cupful of sugar into the soda and stand back. The cola will burst out of the bottle. Most students will assume it is a gas production sense they can see all of the foam. Have the students taste the left over soda. It is very sweet and very flat, indicating the soda is still there and the carbon dioxide left. It is a physical change and not a chemical change. The sugar pushes out all of the carbon dioxide gas.