WHAT YOU NEED:
Clear jars (same size)
Food coloring (primary colors work best)
Hot and cold water
Card slightly larger than the mouth of the jars (thicker than just a piece of paper)
WHAT YOU DO:
PRO TIP: Practice this one first; it’s not easy to get this right the first time. Since we are working with hot water (danger of burning) and food coloring (potential to stain), do this the first time without involving the kids and without food coloring. Practice assembling it and moving all the pieces. Once you master that, then bring the kids in and add the food coloring. You will also want to lay down paper towels (not your good bathroom towels because you WILL spill food coloring).
Fill one jar with hot water and a few drops of red food coloring and the other with cold water and a few drops of blue food coloring.
The jars need to be full, all the way up to the lip of the jar. This is critical to success of the experiment, so it’s best to do this in a sink or right next to a sink. That also makes cleanup SIGNIFICANTLY easier.
Leave the red jar of hot water on the table. Tightly hold the card against the mouth of the blue cold water jar, flip it upside down, and stack it on top of the red, warm jar with the card in between. (PRO TIP: it helps to run your fingers around the card where it connects to the jar to make a better seal).
Stop here and observe what is happening. After observing, carefully (but as quickly as possible) remove the card and observe what happens now.
OTHER WAYS TO DO THIS:
We did this experiment with one jar of hot water and one jar of cold water. You can also try it with two cold jars, two hot jars, one cold and one room temperature, or one warm and one room temperature to see what changes. You can also put the hot water on the top to see what changes (this is my favorite way to do this experiment after you’ve done the version I explained above).
WHAT’S HAPPENING:
First, think about the differences between hot and cold water, and remember air behaves the same way water does in this experiment so I use those words interchangeably. Molecules in hot water move faster than cold because they are more energetic. That also makes it less dense. The hot water molecules need more room to move around than the same amount of cold water molecules. Since our jars are the same size, that means we can fit more cold water molecules in the jar than we can fit hot water molecules making the molecules in the cold water jar more densely packed together. When we put the cold water on top it will sink like a rock as we remove the card because it is more dense than the hot water. Since the hot water is less dense, it also wants to rise as the cold sinks and forces it up.
DISCUSSION IDEAS:
Thinking about the different ways hot and cold air behave, how do you think this influences weather patterns?
