The Chemistry Continues
Soap scum
Hard water is water that has minerals dissolved in it. In some areas, minerals like calcium, iron,
or magnesium are naturally dissolved in the water. These minerals can interact with soap and
create soap scum which is a precipitate.
Student activity
Question to investigate
Does hard water affect soap?
Materials
 Water
 Ivory soap
 Calcium chloride
 2 clear plastic cups
Procedure
1. Use a popsicle stick to scrape off about ½ tablespoon of Ivory
soap scrapings.
2. Add the Ivory soap flakes to about 25 mL of water. Swirl for at
least 1 minute so the soap solution is a cloudy gray color.
3. Make a calcium chloride solution by adding ½ teaspoon of
calcium chloride to 25 mL of water. Swirl until as much calcium
chloride dissolves as possible.
4. Use your spoon to remove the undissolved pieces of soap and put them on a paper
towel.
5. Slowly add about half the soap solution to the calcium chloride solution.
Expected result
White particles will form. This substance is soap scum.
Procedure
6. Try swirling the solution to see if the particles dissolve.
Expected result
The white particles do not dissolve. Soap scum can build up in bathtubs and showers if water
has too much magnesium or calcium in it.
Teacher demonstration
For this demonstration, take one cup of left over soap solution and one cup of soap scum from a
student group.
Question to investigate
Which makes better bubbles, soap or soap scum?
Materials
 Water
 Ivory soap
 Soap scum made in the student activity
 2 clear plastic cups
 2 straws
Procedure
1. Use a clean straw to gently blow into the soap
solution.
2. Use another clean straw to gently blow into the
soap scum and water.
3. Compare the amount of bubbling.
Expected results
There will be a great deal of bubbling in the soap solution, but much less in the soap scum and
water. There may be some bubbling in the soap scum and water due to some residual unreacted
soap. However, the difference in the amount of bubbling in each cup should be enough to show
that soap scum is different than the original soap and does not bubble as well.
Chemistry explanation
When calcium chloride dissolves in water, it forms the calcium ion (Ca2+) and the chloride ion
(Cl-). Soap ions have an end that is negatively charged. The calcium ion binds to the end of two
soap ions and creates a precipitate that is no longer soluble in water. This precipitate is
commonly called soap scum.