The Soap Station
Oil & Water
1. Pour ~35mL of oil into a 250mL beaker and then pour ~100mL water into the same beaker. Ask the students what they see. The water and oil form two separate layers.
2. The concept of density can be brought up here: which is denser - the oil or water? How do we know? The water is in the bottom layer and therefore more dense than oil. You can mention that it is good that oil is less dense than water because it makes it easier to clean up oil spills. How is oil on the surface of lakes or oceans harmful to plants and animals? Ingestion of toxic levels of oil can lead to death. Oil can clog a bird's feather, making it impossible for the bird to fly, and so heavy that the bird may sink. Feathers also lose insulating ability.
3. Have a student stir the beaker and ask the students if the oil and water will mix - they don't. Why? Introduce the concept of polar and non-polar and explain that non-polar things like to be with non-polar things and polar things like to be with polar things. Show molecular models. Using a molecular model of H2O, tell the students that oxygen attracts electrons more strongly then hydrogen. Since electrons have a negative charge and they spend more time near oxygen, the water molecule has a negative end and a positive which is why it is called polar. Using butane, show students that in oils, the hydrocarbon tail is non-polar because hydrogen and carbon have a similar attraction for electrons.
4. Draw a table on the board similar to the one below:
|Polar or Non-Polar
Tell the students that water is polar and fill that part of the table in. Ask the students: if we know water is polar, what is oil? Oil did not mix with the water and is non-polar. Fill in the chart.
a. Have a student add food coloring and choose the color: where does the color go? It mixes with water, so this means that it is polar. Add to chart.
b. Add the "mystery substance" and stir - what happens? Both the oil and water mixed with the mystery substance. Is the mystery substance polar or non-polar? What might the substance be? Have students smell the substance and try to guess. The mystery substance is soap and soap is both polar and non-polar. Add to chart.
c. Show molecular model of a soap molecule - explain that soap has a polar head and a non-polar body. The water attaches to the head and the oil attaches to the body.
Food Color Explosion
1. Put a plastic plate on a stool and have students stand around it. Pour just enough milk onto the plate to cover the bottom.
2. Tell the students that the milk is whole milk with lots of fat. Is the fat polar or non-polar? Is fat more like oil or water? It is non-polar. What will happen when food coloring is added to the milk? Food coloring is polar so it won't mix with the globules of fats.
3. Have students each carefully add drops of the four different colors of food coloring onto the plate x don't squirt a lot at once. Cover most of the surface of the milk with food coloring. Make sure students do not touch their pipette to the milk or move the plate.
When finished, add a couple drops of dishwashing liquid and watch what happens! Why do the food colors swirl in the milk when mixed with soap?
4. As soon as dishwashing soap is added, the drops of food coloring and the milk rapidly mix together. The soap is made of molecules that have two ends: a fat-friendly end and a water-friendly end. One end of soap attracts the fat in milk (the non-polar end), and the other end attracts the water in food coloring (the polar end). The food coloring mixes with the milk because the dishwashing liquid acts as a bridge between the two.
5. Now we know soap works because its molecule has a split personality! Its tail loves fat, but its head is wild for water. Soap is non-polar and polar!
6. Discard the food coloring & milk and oil & water into the sink.
Application: How does washing our clothes in soapy water take grease out of our clothing? Can we wash grease out of our clothing with just water and no soap? Why not?