Kids can test a variety of ways to clean coins and learn about what works best for making them shinier. They will need to start with older, dirty coins that are approximately the same color. They can either test different cleaning solutions or different amounts of time in the solution before comparing the shininess of the coins. Substances and solutions to consider include water, lemon juice, vinegar, soda, lemonade and orange juice.
One of the interesting properties of water is its surface tension. Do an experiment with coins and a cup of water to learn about surface tension and displacement. Fill the cup of water to almost the top and have kids guess how many coins they can fit in without spilling any water. Have them test their guesses by gently placing coins in, one at a time. The water will rise above the edge of the cup and take a few more coins before it finally spills over. Vary the experiment by using the same amount of water in several cups counting how many of each type of coin it takes. Another variation is to test the surface tension of other liquids, such as milk and vinegar.
A similar project to a coin cleaning experiment is one that uses coins to test the acidity of different types of soda. The acid in the soda cleans off a tarnished coin, so more acidic sodas will clean coins more quickly. Students should set up cups with some of their favorite sodas and put equally tarnished coins in each cup. Observe the coins every day to see how clean they have gotten in each type of soda.
The ideal gas law says that a gas's volume increases as its temperature increases. Demonstrate this law by using a coin and an empty glass bottle with an opening smaller than the coin. Submerge both the coin and the bottle in an ice bath for about five minutes to cool down. Kids should drain any water in the bottle and place the coin over the bottle's opening to create a seal. Have them set the bottle on a table and place their hands around the bottle while watching the coin. After a little while, the coin should start moving because the air in the bottle heats up and increases in volume, so it needs to escape.