Chemical and Physical Changes

This experiment provides hands-on experience with chemical and physical changes. Safety goggles must be worn!

 

Materials

Water

Vinegar

Hydrogen Peroxide (3% solution)

Epsom salt

Baking soda Alka-Seltzer® tablets

Potato Powdered laundry detergent

Empty film containers

Ice

Boiling water

Beakers or clear plastic cups

 

What To Do

Chemical Changes

Put on safety goggles! Pour baking soda into a clear plastic cup so that the bottom of the cup is just covered. Slowly add vinegar to the cup to fill it about halfway and observe the formation of carbon dioxide gas, an indication that a chemical change has taken place. Place a piece (about the size of large eraser) of raw potato in a clear plastic cup. Add enough hydrogen peroxide to cover the piece of potato and observe the formation of oxygen gas as the enzymes from the food break the peroxide into water and oxygen gas. The gas formation is an indication that a chemical reaction is taking place. The kids can also feel the outside of the container – it will be cold to touch. This is another indication of a chemical reaction. Place an Alka-Seltzer® tablet in a cup of water and observe the release of carbon dioxide gas. Then, combine half of a tablet with some water in a film container, cap it tightly, step back at least 5 feet, and wait for the lid to pop off. The carbon dioxide being formed builds up pressure inside the sealed film container until the lid can no longer hold it. It will make a loud noise and the lid will pop off, shooting into the air. Avoid setting the canister directly below light fixtures, because the lid may be able to break light bulbs. Please be careful! Combine half of a cup of warm water with about a teaspoon of powdered laundry detergent and stir to dissolve. Make small amount of Epsom salt solution by dissolving 1 part magnesium sulfate in two parts water. Add a drop of food coloring to the Epsom salt solution, and then use an eyedropper to add a few drops of the colored solution to the laundry detergent solution. As a clear indication of a chemical reaction, a solid will immediately form and settle to the bottom of the cup. These solutions may already be prepared and labeled for use. If so, fill the cup about one fourth of the way with the detergent solution and pour in a few milliliters of the Epsom salt solution to observe the formation of the solid.

Physical Changes

Dissolve Epsom salt in water until no more will dissolve to make a saturated solution. Paint a few drops of this solution on a piece of dark paper to watch the salt crystals reappear as the water evaporates. The change in form, from solid to part of a liquid solution, back to solid, is purely physical. Observe ice melting and water boiling to learn about phase changes as physical changes.

 

Questions

1. Why is it important to wear safety goggles?

2. What is a chemical change? What is a physical change?

3. How can the two types of changes be distinguished? What are the signs of a chemical change? What are the signs of a physical change?

4. What are some examples of chemical changes?

5. What are some examples of physical changes?

 

Summary

Chemical changes are chemical reactions; they transform substances into different substances. Chemical changes are indicated by a number of signs, including, but not limited to, the formation of gas, the formation of solid, a change in temperature, and evolution of light. Physical changes do not form new substances and do not change the chemical nature of the substances involved. Physical changes are those that change only the physical properties of a substance without changing the chemical properties.

 

Sources

"The Best of Wonder Science." Ed. Jay Whitney, Delmar Publishers, 1997.

"Solids, Liquids and Gases." The Ontario Science Centre, Kids Can Press, 1998.

© S. Olesik, WOW Project, Ohio State University, 2002.