Science project

Testing Water Hardness

Find out what will happen as you add more Epsom salts to water.


  • Distilled water
  • Epsom salt
  • Labeling tape
  • 5 Cups of the same size
  • Teaspoon
  • Hand soap
  • Water hardness testing strips (can be found at places like Home Depot, Lowes)
  • Notebook and pen/pencil


  1. Fill each of the five cups with the same amount of distilled water.
  2. Label your cups. Your first cup will get no salt—this is the control. The last cup will get four teaspoons of salt.
  3. Skipping the first cup, fill your cups with 1, 2, 3 and 4 teaspoons of salt, respectively. Be sure to label correctly.
  4. Stir each cup until all the salt is dissolved.
  5. Put a small amount of hand soap on your hands and wet them with one of the water samples. Record your observations on how easy it is to produce bubbles. Which water samples are the hardest based on this test?
  6. Use the strips in the hardness testing kit to rank the water samples from hardest to softest. Does this match up with your bubble test?


The water with the highest amount of dissolved salts will be the hardest.


Epsom salt is a common name for magnesium sulfate. When mixed with water, it separates into Mg2+, a component of water hardness, and sulfate anion, SO4-2. The more salt you put in the water, the higher its dissolved magnesium concentration will be, causing the water to be harder.

The harder the water, the harder it will be to produce bubbles. Cup number 5 will be the most difficult to turn into a later on your hands, and may even leave your hands feeling sticky and dirty from the soap scum.

Going Further

Compare your results to water samples from your house tap water or filtered water.

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