Which Brand of Soap Releases the Most Water Quickly?

Talk It Over

Soap is partly water. In a microwave oven, water heats quickly and makes steam. What might happen to soap in the microwave? How can you use the microwave to find out how quickly a soap gives up water?

Get

• 2 or more bars of soap, different brands
• 2 or more microwave-safe plates (not paper or plastic)
• Kitchen scale
• Hot pad
• Access to a microwave oven

Go

1. Weigh a plate on the kitchen scale. Put a bar of soap on the plate and weigh again.



2. Repeat step 1 for each brand of bar soap you want to test.
3. One at a time, place the bars of soap in the microwave oven. Heat on high power for 1 minute. Using the hot pad, remove the plate and soap from the oven.
4. Cool completely.
5. Weigh each plate along with its soap again.
6. In order to compare bars of soap that did not start out weighing the same, you'll need to calculate percent change. Here's how:
   1. Calculate the weight of the bar of soap before and after microwaving. To get the weight of the bar of soap:

   (weight of soap + plate) – (weight of plate) = weight of soap

   2. Calculate the change (the amount of water that left the soap):

   weight of soap after – weight of soap before = change in weight

   3. Percent change equals

   (change in weight ÷ weight of soap before) × 100 = percent change

Stay Safe

Do not touch the soap after it has been in the microwave. It gets very hot. Also, do not microwave for more than 1 minute. Some soaps get hot enough to crack even microwave-safe plates.

Go Easy

Ask an adult to help you with the microwaving and the calculations. Talk about what percent change means and how your numbers compare.

Go Far

This experiment works because microwaves heat water molecules. They make the molecules move faster. As the water molecules heat, they change their physical state: from liquid water into gaseous water vapor. The vapor expands and leaves the soap, moving into the air. You can use this idea and the "Go" procedure to test other materials for their water content. Try comparing meats, vegetables, or different types of cheese. Make sure whatever you test is safe to put in the microwave.

Show Your Results

Put weights and calculations in a data table like this:

Make a bar graph that compares the percent change of different brands. Put the brands on the horizontal axis. Put percent change on the vertical. Display your soaps as part of your project. State a conclusion about which brand of soap loses water most quickly.

Tips and Tricks

• A triple-beam balance borrowed from your school's science lab will give you more accurate weights than a kitchen scale will.
• The change in the weight of the soap is the amount of water lost. Expand your experiment by measuring the amount of water lost in 15-second intervals of microwaving. Is it always the same, or does the soap lose more water early in the heating process than later?

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