Objective:

    Understand how hard water decreases the efficiency of soap.

Vocabulary:

    Magnesium and Calcium - chemicals that make water hard.

Materials:

    Epsom Salts
    Measuring Spoons
    A Cup
    Bar of Soap

Instructions:

   1.  Mix 1 tablespoon of Epsom salts in 250 mL (1 cup) of warm tap water.  Look

         on the label; it has magnesium sulfate - one of the many culprits (along with

         calcium carbonate) in hard water.

   2.  Now, grab a bar of soap and wash your hands.  Be sure to make a good

         lather with lots of bubbles.  Then, rinse your hands, - get all of the soap off

         of them.

   3.  Ask a partner to pour some of the Epsom salts and water solution into you

         hands - now try making lather with the bar of soap like you did before -  Just

         a little magnesium sulfate can make a big difference.

   4.  Try mixing 1/2 tablespoon of Epsom salts with 1 cup of water, and see if you

         can make lather.

   5.  What is the minimum amount of Epsom salts that you can mix with a cup of

         water and still not make lather?

How does it work?

Water that has a hard time making lather when it mixes with soap is called hard.  Hard water contains a lot of calcium and/or magnesium that is picked up when the water flows over or through limestone or some other rock that has a lot of calcium or magnesium.

Soap contains long, chain like molecules that have one end that interacts with water and another end that interacts with dirt and oil to remove them from you skin and clothes.  When soap interacts with water, the end that interacts with dirt and oil spreads out, moving away from the water.  That spreading is the bubbles that you see.

But if the water contains calcium or magnesium, the soap molecules interact with the calcium and the magnesium before they interact with the water, resulting in scum. Soap scum is a soap molecule clumping together.  As a result, the soap does not spread out like it does when it interacts with water. So, we can't make bubbles with hard water solutions.

Hard water is bad in your home because is combines with soap to make a scum or film that you can see as a bathtub ring or white crust on the shower head. Soap scum also sticks to clothes making them look dirty.

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