Using Soap to Test for Hard Water

A hand holding a glass with hard water deposits - Using Castile soap to test for hard water. Test for Hard Water

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 hard water 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.

Remedies if you have hard water

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.

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.

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!

Further reading

This use and many more are in my book, Soap & Soul: A Practical Guide to Minding Your Home, Your Body, and Your Spirit with Dr. Bronner’s Magic Soaps, available now in hardback on or at your favorite bookseller, and as an eBook and audiobook (read by me!) from wherever you download or listen.  

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Rachel says:

If I accidentally used hard water instead of distilled in my castile foaming hand soap, is it still as effective?

Lisa Bronner says:

Hi Rachel- As you’ve likely noticed, the solution will be cloudy, however it is still an effective cleaner.

Rachel says:

Wow thank you so much! Good to know I dont have to dump it and start over 🙂

Your products are magic. Appreciate all you do!

Luc says:

I made a simple liquid soap that consists of only 2% lauryl glucoside to which is added vinegar to bring the whole slightly acidic. It keeps the sink free of scum and the hands feel much nicer than even soft ordinary soap. Any cationic soap would have calcium stearate or palmitate sticking to the skin. This is difficult to rinse off. The feeling when rinsing with the lauryl glucoside soap is the same as rinsing off ordinary soap in super soft water.

Marcelino says:

What dilutions do I use to make a floor cleaner that takes the smell off of the floor, and how many gallons per solution is a reasonable amount to use please and thank you 💯🙏😁

Lisa Bronner says:

Hi Marcelino- I have a full article on mopping, which you can find here, If you use a mopping bucket, use 2 ½ Tbsp. (37.5 mL) Pure-Castile Soap OR 1 tsp. (5 mL) Sal Suds in 1 gallon of water. My article also has dilutions for more or less water, depending on how much flooring you’re cleaning. Of the two, Sal Suds is slightly more clean rinsing and more heavy duty, but the Castile Soap works excellently as well.

Trevor Carlton says:

This is actually effective, I gave this a try while waiting for the water hardness test result from our local water testing center and it definitely gave the reaction it’s supposed to give for hard water and the results indicated that we have mineral-rich water coming out of our tap. Knowing all the negative impacts that hard water brings, we bought a salt-based water softener system right away, there’s also a salt-free option and you can check this site for the differences between the two if you’re not sure which one you should get.

christine says:

Dear Lisa
I wonder if you can help . I’m new to all this and feel determined to make this natural , planet friendly approach work in the house. I’m struggling to make Castile soap work for washing dishes though – especially greasy dishes . I live in London where the water is very hard . What are your top tips ? Is it down to ratios or should I be adding something else ? Much thanks

Lisa Bronner says:

Hi Christine- For very hard water, I recommend the Dr. Bronner’s Sal Suds Biodegradable Cleaner for dishes and household cleaning. This was designed specifically to be completely clean rinsing even in the hardest of water.

Debbie P says:

I want to thank you for your wonderful products. They are amazing. Our family appreciates your family honoring time-tested traditions and sticking with them. We are all a lot better off for it. There is always going to be the sourpus that complains on here, but they will complain about anything sadly. You are doing a great job. If I can convert my husband to shampooing with the castile soap, anything is possible!!!

Lisa Bronner says:

Hi Debbie- Thank you for your kind words! It’s great to hear your family enjoys our products.

marlene says:

I have hard water, I live in Canada.
The problem is that I also have iron in the water, not a lot but I still have it.
I used the castile soap to do the dishes as I could not, for the longest time, locate the sal suds.
It did a fairly good job, the castile, but I used the sal suds for the dishes and it’s a disaster. Everything feels less clean. I hand wash only as I do not have a dishwasher machine. If there is any greasy food, the sal suds break down fast in the water and do not clean as well; unless I put in a lot of the sal suds, like 4 or more tablespoons.

Lisa Bronner says:

Hi Marlene- Yes, it turns out that water with particularly hard iron content may react with Sal Suds. As you’ve found, the Castile soap works better in that situation.

Marilyn says:

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.

S says:

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.

Andi Zara says:

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.

Testing for Water Hardness says:

[…] Check out my 2018 update of this video, with far better sound quality: Using Soap to Test for Hard Water. […]

Kathy says:

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

Lisa Bronner says:

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.

Andrea Whittington says:

I find that adding some baking soda to the dishwater seems to help.

Sal Suds or Castile Soap - Which One Should You Use? says:

[…] is a true soap, it reacts with the minerals contained in hard water. (Here’s my test to find out if you have hard water.)  The more dissolved minerals there are, the “harder” the water.  The reaction of soap with […]

About Lisa Bronner

My grandfather was Dr. Bronner, my family makes soap, and I share ways to use it plus tips on greener living.

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