Hard Water Experiment

Learn about the difference between hard and soft water by mixing some of your own in this hard water experiment.

Hard water just means water that has minerals in it.  The high mineral count is usually comprised of calcium, magnesium, and other minerals that are dissolved in the water.  You can’t usually see whether or not you have hard water just by looking at it, but you’ll probably be able to tell other ways.  One way people quickly notice is that they tend to get “ring around the tub” and other mineral deposits in their sinks and bath tubs.  You can also tell if you have hard water by how it lathers.  Water with a lot of minerals won’t bubble up as much when soap is added as soft water will.  You can easily see the difference by comparing plain water with hard water that you mix yourself in this hard water experiment.

Hard Water Experiment

You’ll need:

  • 2 plastic cups
  • a permanent marker
  • 2 straws
  • water (works best if this is soft water that you begin with – through a water softener or distilled)
  • plaster of Paris
  • dish soap

Begin by labeling one cup “plain” and the other “hard” and then put water in each of the two cups.  Stir a spoonful of plaster of Paris (calcium sulfate) into the cup labeled hard.  It’s not entirely soluble, but that’s okay.  It’s fine for some to sink down to the bottom of the cup.  By adding calcium to the water you are creating your own hard water to test.


We’re going to observe how soap has difficulty lathering in hard water.  Add a few drops of dish soap into each cup and stir it in.  My daughter is handling the soft water throughout the experiment while my son is handing the hard water cup.


 Now put a straw into each cup and begin gently blowing air into the water.  Both will bubble up somewhat, but the soft water will produce more bubbles.


You can’t exactly tell from the pictures, but Tyler is trying to get his bubbles going much harder than Elizabeth is.  He worked twice as long and twice as hard to get his bubbles to form.  It’s killing him that his sister’s bubbles are more successful!


The plain, soft water clearly suds more successfully.


 After the final conclusion of the experiment the kids decided to keep blowing into the soft water.  It just kept bubbling and sudsing!


It’s interesting though – most people prefer soft water for cleaning, but hard water for drinking.  In the laundry, when doing dishes, when cleaning the bathroom, and even when cleaning ourselves, soft water is best.  It works well with soap and doesn’t leave a nasty residue.  However, most people prefer the taste of hard water.  Try to drink distilled water.  You probably won’t like it.  It’s the minerals within water that make them taste good to us.  A lot of people have favorite brands of bottled water for this reason.  You might think, “It’s just water.  It’s all the same.”  But that’s actually not true.  It’s not the actual H2O flavor that we like in the bottled water; it’s the minerals within it.  Try doing a blind taste test of several brands of bottled water and see which you prefer.

Record the results of your experiment and your taste test, and write about what you’ve learned about water in your science notebook.


  1. Hi Karen!

    Thanks for sharing this experiment. I’ve noticed a huge difference in the amount of soap I need in the washer after getting a softener installed.

    I noticed you mentioned that many people prefer hard water for drinking. Here in OKC the hardness of the tap water makes it unbearable to drink!

    I much prefer the taste of soft water from the tap, but then again I also run my drinking water through my Brita before it hits my lips so perhaps that improves the taste of the soft water.

    Thanks for taking the time to read my comment 😉

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