The Chemistry of Water

H₂O. The one chemical formulation we all can name. Two hydrogen atoms for every oxygen atom. Water. One of the most abundant compounds on Earth. Essential to life. NASA’s holy grail.

What more is there to say about water?

If you’ve recently stood in the bottled water aisle with words such as purified, distilled, spring, mineral, hard, soft, ionized, deionized swirling around you, you know water is not simple.

It is REALLY hard to find informed and unbiased information about water. Water is a massive industry. Between bottled water and water purifiers, U.S. consumers spent $23.39 billion in 2019. Many people who discuss different types of water and their benefits and disadvantages are selling something. Some of them said some pretty off-the-mark, though scientific-sounding, stuff.

First, let me bust some common water myths likely at the core of your understanding of water.

MYTH #1: Pure H₂O is what’s best for our bodies.

REALITY: While theoretically water is H₂O, in practice we don’t encounter water containing only hydrogen and oxygen. This is not a problem. Pure H₂O is quite a reactive compound, due to its polarity or charge. It is really great at dissolving things from a spoonful of salt to the rock of the Grand Canyon. It specifically hungers after other charged particles, or ions, and readily grabs them out of the air, ground, or surfaces it touches. Some major ions found in water include bicarbonate, sulfate, chloride, calcium, magnesium, potassium, and sodium. Only under very controlled laboratory circumstances is water ever only H₂O. As soon as it touches air, it ionizes. If somehow you were to bathe with or drink this deionized water, the water would grab ions out of your skin, teeth, and other tissues, depleting them of essential minerals.

MYTH #2: Water has a neutral pH of 7.

REALITY: Only pure H₂O, the kind I just said you’ll likely never encounter, has a neutral pH of 7. The water you experience coming out of your tap or bottle or filter will likely have a slightly acidic or slightly alkaline pH. Whatever ions the water has picked up, plus anything else dissolved in this most universal of solvents, changes the pH. I know I’m contradicting the one truth you felt was inviolate from your 8^th grade science class, and I do apologize for the disillusionment. I only recently learned this myself. Tap water wanders from 6.5 to 8. Reverse osmosis produces water between 5 and 7, which is acidic. Water marketed as Alkaline usually hovers around 8-9 (right about the pH of Dr. Bronner’s soaps). This is rarely an issue, but if something you’re doing is very pH dependent, keep this in mind.

Other Watery Words

Bottled water – Without more information, all we can know for sure is that it’s water in a bottle. Is it filtered? Spring? Purified? Mineral? Are there any additives such as salt, antimicrobials, or fluoride? Read the label for more info. Please keep in mind that single-use bottled water is an insane and unnecessary luxury that provides only the slightest momentary convenience. Plan ahead with a refillable bottle.

Purified water – On its own, this word is vague regarding how the water is purified, but the FDA regulates this term and it applies to water that is safe to drink. There are many methods of purification such as distillation or reverse osmosis. “Purified” also makes no statement about the source, so if you’re looking for something fancier than tap water, keep looking.

Filtered water – Another vague term that doesn’t communicate where the water came from or how it was filtered. It’s not a regulated term.

Distilled water – A purification method where water is evaporated and recondensed. The water’s evaporation leaves certain impurities such as salts, minerals, and particulates, behind but other volatile organic compounds may evaporate with the water. The source of the original water impacts its final cleanliness. Spring water is best.

Spring water – As per the FDA, “Derived from an underground formation from which water flows naturally to the surface, this water must be collected only at the spring or through a borehole that taps the underground formation feeding the spring.”

Mineral water – A drinking water which the FDA specifies must come from an underground source and naturally contain at least 250 parts per million total dissolved solids.

Alkaline water – Water with a pH over 8.

Reverse osmosis – A filtration method where water passes through an exceedingly fine membrane that takes out contaminants down to 0.0001 microns. Considered the best filtration on the home level. Generates a lot of wastewater per gallon of filtered water.

Hard water – Water that contains a high concentration of certain minerals, usually calcium and magnesium, picked up from running through the ground. Hard water is slightly alkaline.

Soft water – Water with a low concentration of calcium and magnesium ions, though it may contain an abundance of other minerals. Naturally occurring soft water comes from lakes and rainwater. Water can be artificially softened. See discussion below. Soft water is slightly acidic.

And the rest – There are soooo many other marketed adjectives for water that I’m not going list out here. Read the back labels. If anything has been added to the water to give it some magical power, the label will tell you.

Hard vs. Soft Water

When I was in the throes of teenage acne, I found that when I travelled, my skin cleared up. The only difference I could find was that we had a water softener at home and places we travelled to generally did not. So when I was home and had a breakout, I would go outside and wash my face in the backyard spigot to avoid the softened water. 

Years later I learned that my brother Mike, who battles psoriasis, also would use the outdoor water to help alleviate his psoriasis during a flare-up. Both of us found that when washing in softened water, soap never seemed to rinse fully off of our skin. Even after a thorough rinse, I could rub my hands together and re-lather them. There was still soap left. It took me over two decades to learn why.

My individual experience is no adequate scientific study, and there are myriad skin types out there. However, my situation illustrates the point that discussing hard and soft water is not as simple as one being bad and the other good. There are benefits and drawbacks to both, and it comes down to personal preference.

As mentioned earlier, it is the presence or absence of calcium and magnesium ions, primarily, that makes water hard or soft. Hard water has them. Soft water doesn’t. Where it gets tricky is whether the soft water is naturally without these ions – as with rain or lake water – or has gone through a water softening system to remove them. It is the later type of soft water I will discuss here.

The USGS even has a handy table of relative water hardness and a map to indicate where in the US has naturally harder or softer water. You can also test for hard water by adding a few drops of a true soap (not a detergent) into a glass of water as shown in my video here.

How an Ion-Exchanging Water Softener Works

There are several types of water softening systems, but the most common one to have on a whole house is an ion exchanger, or salt-based system. There are a variety of other types of softeners that are less common or only are for one point of use. Because the salt-based ion exchanger is by far the norm, I’m going to dive deeper into that one.

In these systems, incoming hard water passes over a sticky resin coated in positively charged sodium or potassium ions that displace the calcium and magnesium ions in the hard water. Regularly, these water softeners need to be recharged with a fresh batch of sodium chloride or potassium chloride pellets.

Because of this mineral swap-out, it would be incorrect to say that softened water has fewer minerals than hard water. It has just as many minerals as hard water; they’re just different minerals. The more calcium and magnesium ions in the initial hard water, the more sodium and potassium in the resulting soft water.

A potential inadvertent impact of the addition of sodium and chloride to the water is that the wastewater from your home can have a negative impact on waterway habitats into which wastewater treatment plants empty. Sodium and chloride are not cleaned out of water during treatment. Additionally, if wastewater is used in greywater systems that irrigate landscaping, the heightened levels may adversely affect plants.

Why does hard water leave my skin and hair feeling tacky, like there’s some stickiness to rinse off?

This is not a matter of minerals being left on your skin. This is a matter of pH. The minerals in hard water usually give it a slightly alkaline pH, which is causing this tackiness. (Remember above that I mentioned that water as we experience it never has a neutral pH. It is always slightly acidic or alkaline.)

Alkalinity causes the cuticles on our hair strands as well as the snags from any roughened skin to raise up creating a texture somewhat like Velcro. This can even be itchy. In reverse, acidity causes these to lie flat, creating smoother hair and skin. This is why hair products have an acidic pH. If you have hard water and this bothers you, you could rinse your hair with something acidic like apple cider vinegar. Because I wash my hair with soap, which is alkaline, I do this anyway. All this to say, there’s no residue on your skin from the hard water. Also, your skin’s secretions are acidic and will soon after your bathing, rebalance the pH.

Why does soft water leave my skin feeling a bit slick?

It’s the reverse answer to the above question. The slight acidity of soft water smooths skin and hair slightly. If you’re not used to it, this can feel slippery at first.

But my skin feels REALLY slippery, like there’s still soap on it.

This may be because there is still soap on it. There are two reasons for this.

If you’re used to hard water, you’re used to using a little more soap than you need for washing. That is because you have to budget for the soap that ends up dancing with the calcium and magnesium ions and then have soap left for washing. With soft water, soap doesn’t go chasing after the ions, so you’ve got lots more hanging around on your body. Use less soap with soft water.

The second reason is that soap and artificially softened water have a very slight repulsion. They not 100% hunky-dory with each other. I believe this is what sent my brother and me to the outdoor spigot. Follow me closely here.

With an ion exchanger, the resulting soft water is filled with sodium and potassium. A soap molecule at one end is also sodium or potassium, depending if the soap was made using sodium hydroxide or potassium hydroxide. In chemistry (and elsewhere), like repels like. So the sodium or potassium head of the soap repels the sodium or potassium in the softened water. Sodium and potassium are positively charged ions. Do you know what has a negative charge? Our skin. So the soap is drawn to our skin, repelling the water. The result is that the water is slightly less effective at rinsing off soap. The slight residue can irritate skin and exacerbate problem-prone skin. It can also weigh down hair.

Why does hard water produce soap scum on my bathroom fixtures?

Soap scum is a reaction between true soaps like Dr. Bronner’s Castile Soap and hard water. If you don’t have soap scum on your tub and sinks, you either have soft water or you’re using a non-soap cleanser, usually marketed as a shower gel, body wash, or some other euphemism for synthetic detergent.

Those calcium and magnesium ions (we just can’t get away from them) bump off the sodium/potassium head of the soap and forms a strange new, but completely harmless, substance called lime soap. What’s weird about this lime soap is that it is insoluble in water, so no use whatsoever in cleaning, and an aggravation to shiny bathrooms everywhere. Believe it or not, there is a positive use for lime soap: it’s commercially sold as a waterproofing agent and industrial lubricant. But scraping it off your bathroom sink is not the best way to produce it for these purposes.

Soap scum is dissolved by acids, such as 50/50 vinegar/water solution, or by mechanical means such as scouring it with baking soda or GIY Soft Scrub.

Is this soap scum on me, too?

We do not find a build-up of soap scum on our skin. One reason is that we often towel off, and the towel absorbs potential deposits. In fact, if you dried your bathroom surfaces after use, you would also notice less soap scum.

What is the hard whitish build up I notice on my faucets where no soap has been?

This is calcium carbonate, commonly called scale. It is the dissolved calcium in the hard water that dries and builds up on objects water frequently is left to air dry. It’s the same stuff as hard water spots on your glassware. It is dissolved again by acids, such as vinegar. Rinse agents in your dishwasher are acidic for this reason. This is also behind the recommendation to run vinegar through your coffee pot occasionally. This deposit can cause electric water heaters over time to become less efficient.

Pros and Cons Summarized

Benefits of soft water

• Soap produces a better lather
• Less soap is needed for cleaning
• Less soap scum on bathroom surfaces
• Less scale on fixtures, glass & dishes, and appliances
• Leaves skin and hair smoother and less itchy

Drawbacks to artificially soft water

• Soap does not rinse off skin and hair as easily
• Higher sodium content can impact sodium-sensitive people
• Higher sodium and chloride content in wastewater can pollute downstream waterways and grey water
• Can taste salty
• Can be corrosive on steel pipes (used in homes built ~1940s-1970s) if pH is below 6.5
• Requires regular input

Benefits of hard water

• Soap rinses off skin better
• Can taste better (not excessively hard water, though)
• Is not corrosive

Drawbacks to hard water

• Soap does not lather as well
• More soap is required for cleaning
• Produces soap scum on bathroom surfaces
• Deposits scale on fixtures, glasses & dishes, and appliances
• Can be more drying on skin
• May leave deposits in steel pipes (used in plumbing from 1940s-1970s)

There is no simple statement for hard and soft water. It really is a complicated topic, and the more I’ve learned about it, especially in writing this article, the more I find I do not know. You’ll have to make your own choices. Weigh what’s important to you, what bothers you, what’s possible for you. May you and your water come to an understanding.