You will need access to a refrigerator and microwave for this activity.

We suggest students work in groups of four, which works well because there are four experimental set-ups. Homeschool students can work on their own, but will need all four experimental set-ups so that they can make comparisons.

You will need several small plastic bottles for this activity (4 bottles for every group of four students, plus a few extras to have on-hand). We encourage you to use recycled bottles when possible.

Prepare the Water Bottles

You’ll need to start preparing for class the night before by refrigerating the cold bottles overnight. You can prepare the hot bottles in a microwave an hour before class.

First, if you are using recycled bottles, fill all the bottles with tap water. Then divide your bottles evenly. Half of these will be your cold water bottles and half will be your hot water bottles.

To prepare the cold water bottles:

• Open each bottle, drip in a few drops of blue food coloring, and close them back up.
• Place them in the refrigerator for at least a few hours to cool them down.
• If you’re in a rush, you can chill them faster in the freezer. You don’t want them to freeze, just be really cold.

To prepare the hot water bottles:

• Open each bottle and drip in a few drops of red food coloring.
• Leave the bottles open. (This is important! If you don’t, they may burst.)
• In our microwave, we heat 6 to 8 bottles at a time. We heat on high for one minute, check the temperature, then repeat until the water is hot. You will have to determine the best power setting, time, and number of bottles for your microwave. We suggest erring on the side of less power and time, to avoid overheating the bottles.
• When the water is hot, carefully remove the bottles (you may want to wear oven mitts!) and screw the tops back on.

Store hot and cold bottles in separate coolers until class. In our experience, they’ll stay warm or cool for at least an hour in a cooler.

Separate Materials for Easy Distribution

You may want to separate supplies for easier classroom distribution. Each group of four students needs the following: