Using Soap to Test for Hard Water (Video)

Full disclosure: I’m revisiting a topic I covered seven years ago, back when we filmed with our phones, no microphone, in one continuous take.  Even with its terrible sound quality, that video has been viewed more than 190,000 times.  I thought it was time to spruce it up.

Hard water is defined by the calcium and magnesium minerals it contains.  The higher the mineral content, the “harder” the water.

This test for hard water was something I used to see my dad do in our kitchen.  At the time I had no idea what he was doing, other than squirting soap into a drinking glass, which seemed weird.  Now I know he was testing to see if our water softener was working.

A true soap can tell you if you have hard water.  This is because true soap molecules react with the calcium and magnesium ions in hard water and form a whitish precipitate.  This is what we call soap scum on bathroom surfaces.  Or, if you make up a batch of my All Purpose Castile Spray using hard water, you’ll see a cloudy layer that settles to the bottom.  This mineral fallout is not toxic but does lessen the cleaning ability of the spray.  Not a huge deal, but it can be avoided by using some form of purified water.

The Test:

Fill a clear glass with tap water.

Squirt in some true soap, such as Dr. Bronner’s Pure Castile Liquid Soap.  (A detergent wash/shower gel/cleanser won’t work here.)

If the soap turns cloudy as it enters the water, you have hard water.  If the soap swirls around but stays pretty much clear, you don’t.

That’s the test.  It’s not fancy.  It’s a simple binary test and does not give you any details such as how hard, but at least you know if you have it or you don’t.

It also makes for a quick little science experiment to do with kids to study the chemistry of water.  Or, if you know you have hard water, you can use this test to see if your favorite cleanser is, in fact, a true soap. It all depends on what is the known constant, and what is the variable… (I might have been the mom who made copies of my son’s Algebra homework for my own enjoyment.  And we might have raced to see who got done first.  What do you do for fun?)

This test isn’t even included as one of the “18-in-1” uses for the Castile Soap.  Consider it a bonus.


If you have hard water, there are a couple ways you’ll see the effects.

Laundry: Hard water doesn’t rinse the soap off as well.

Remedy: Add ½ to 1 cup of vinegar to your rinse cycle.

Shiny bathroom surfaces: Hard water leaves soap scum (ring around the tub).

Remedy: Wipe surfaces dry, and clean once or twice a week with a 50% vinegar spray, or scour the scum away with this fun (yes, I did just use that word) GIY Soft Scrub.

Hair: Hard water can make hair stiff and a little tacky feeling.

Remedy: Use a slightly acidic rinse after you wash your hair with soap.  I use a 50% apple cider vinegar solution, or a couple capfuls of the Dr. Bronner’s Hair Rinse diluted in a cup of water.

Despite these negatives, I don’t see hard water as a wholly bad thing.  I have some thoughts brewing about why hard water, though problematic for shiny bathrooms, actually can benefit our skin and overall health.  I’ll share all that once I get it all thought through.

For fun, you can check out my original video from my early days on testing for hard water here. We’ve come a long way since then!

8 thoughts on “Using Soap to Test for Hard Water (Video)

  1. If washing dishes by hand, I have found that rinsing them with cool water instead of hot prevents a lot of the spotting from hard water.

  2. after moving to the pacific northwest from CA, i see a tremendous benefit in not having hard water. my hair, skin and nails are all healthier and feel better. you don’t have to use as much soap or hair products. everything tastes better, too, especially coffee and soup. i’m not a fan of hard water but I do miss CA so i’ll have to find a way to adjust once i return.

  3. Clean hard water is a healthy drink because it provides ca and mg,essential minerals.Use soft water to clean,but drink hard water for health.

  4. Lisa,
    What is the best way to remove hard water spots from glassware and plastic cups?

    • Hi Kathy – A bath of vinegar and water will remove the spots. Start with one cup of vinegar in a bowl of water and add more vinegar if needed. Stubborn spots may require a rag and some elbow grease. Hand drying dishes before the spots can accumulate will help keep them at bay in the future.

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