Chemistry of Water: Essential Properties, Reactions, and Uses

Chemistry of Water - hands washing in water

H₂O. The chemical formula everyone knows: two hydrogen atoms bonded to one oxygen atom. Water is one of the most abundant and essential compounds on Earth and plays a central role in life, industry, and science.

What more is there to say about water?

Although the formula H₂O seems straightforward, the water you encounter daily—tap, bottled, filtered, or spring—is rarely just pure H₂O. Labels like purified, distilled, spring, mineral, hard, soft, ionized, and deionized describe important differences. The water market is large and sometimes confusing, and it can be hard to find clear, unbiased explanations. Many claims about water come from companies selling products, so separating marketing from science can be challenging.

Before diving into comparisons like hard versus soft water, let’s clear up some common misconceptions.

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

REALITY: Truly pure water—only H₂O—hardly exists outside controlled laboratory settings. Water is a polar molecule and an excellent solvent, so it quickly dissolves ions and other substances it contacts from the air, soil, pipes, or containers. Common ions found in natural water include bicarbonate, sulfate, chloride, calcium, magnesium, potassium, and sodium. Deionized water will actively absorb ions from whatever it touches; bathing in or drinking completely deionized water could strip minerals from tissues. In everyday life, moderate mineral content in water is normal and generally harmless.

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

REALITY: Only chemically pure water has a neutral pH of 7. Most water you use—tap, bottled, filtered, or spring—contains dissolved substances that shift pH slightly acidic or alkaline. Tap water commonly ranges from about 6.5 to 8. Reverse osmosis tends to produce slightly acidic water (around pH 5–7), while marketed alkaline waters often measure around pH 8–9. For most uses this variation is not problematic, but pH can matter for specific applications like certain cleaning, cooking, or cosmetic needs.

Other Watery Words

Bottled water – Simply water sold in bottles. It might be purified, spring, mineral, or treated; check the label for specifics. Single-use bottled water creates significant waste; using a refillable bottle is usually the better choice.

Purified water – A regulated term meaning the water is safe to drink, but it doesn’t specify the purification method or source. Methods include distillation and reverse osmosis, among others.

Filtered water – A vague term that indicates some filtration occurred but doesn’t define the source or the filtration method.

Distilled water – Made by evaporating water and condensing the vapor. Many dissolved solids are left behind, but some volatile organics can carry over. The original source water influences the final quality.

Spring water – Water that flows naturally to the surface from an underground formation and is collected at the spring or via a borehole tapping the formation.

Mineral water – Water from an underground source that naturally contains at least 250 parts per million total dissolved solids.

Alkaline water – Water with a pH above about 8.

Reverse osmosis – A filtration method using a very fine membrane to remove contaminants down to approximately 0.0001 microns. It is one of the most effective home filtration technologies but produces wastewater.

Hard water – Water high in minerals such as calcium and magnesium, typically picked up as water moves through mineral-rich ground. Hard water tends to be slightly alkaline.

Soft water – Water low in calcium and magnesium ions. Naturally soft water comes from rain or surface sources; water can also be artificially softened by treatment systems. Soft water tends to be slightly acidic.

The rest – Many other adjectives are used to market water. If something has been added to confer specific benefits, it should be listed on the label.

Hard vs. Soft Water

Hard and soft water behave differently when used for washing, cooking, and on household fixtures. I personally noticed effects as a teenager: at home, where water was softened by our system, my skin tended to break out, while when traveling to places with unsoftened water my skin improved. My brother found relief from psoriasis when using outdoor unsoftened water. These experiences highlight that reactions are individual and that neither hard nor soft water is universally better.

Hardness or softness is primarily determined by calcium and magnesium content. Naturally soft water lacks those ions, while artificially softened water has had those ions replaced—most commonly via an ion-exchange system—with sodium or potassium ions. Because of this mineral exchange, softened water has minerals too; they are just different ones.

How an Ion-Exchanging Water Softener Works

Most whole-house softeners use ion-exchange resin coated with sodium or potassium ions. As hard water passes through the resin, calcium and magnesium are captured and displaced by sodium or potassium. Periodically the resin is regenerated with salt (sodium chloride or potassium chloride) pellets.

One consequence of this process is that household wastewater can become higher in sodium and chloride, which wastewater treatment plants do not typically remove. Elevated chloride and sodium in effluent can harm downstream waterways and can affect plants if reclaimed greywater is used for irrigation.

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

This sensation usually relates to pH. Hard water tends to be slightly alkaline; alkalinity raises the hair cuticle and roughens the skin surface, creating a velcro-like texture that can feel sticky or itchy. Acidic rinses smooth the cuticle, which is why many hair products are formulated acidic. If hard water bothers you, a mildly acidic rinse—such as diluted vinegar—can help. Your skin’s normal acidic secretions will also restore balance after washing.

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

Soft water tends to be slightly acidic, which smooths hair and skin surfaces and can feel slippery compared with the textured feel left by hard water. That smoothness is normal and not a sign of residue.

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

That lingering feeling can be due to excess soap or an interaction between soap and artificially softened water. With hard water you often use more soap because some is lost reacting with calcium and magnesium. In soft water you need less soap; if you use the same amount you will have more soap left on your skin. Additionally, in salt-based softened water, the sodium or potassium ions in the water and in some soaps can repel each other. Since skin carries a slight negative charge, soap can cling to the skin, making rinsing feel less effective. Using less soap, or rinsing well and possibly finishing with a mild acidic rinse, can address this.

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

Soap scum forms when calcium and magnesium react with soap to create insoluble compounds often called lime soap. Unlike synthetic detergents, true soaps react with hardness minerals and produce this residue, which does not dissolve in water. Soap scum can be removed with acidic cleaners like diluted vinegar, with mechanical scrubbing using baking soda, or with household scrubs designed for the purpose.

Is this soap scum on me, too?

We generally don’t observe soap scum building up visibly on skin because towels absorb residues and skin oils and perspiration mix and are removed through toweling. Drying surfaces promptly likewise reduces soap scum formation on fixtures.

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

That white crust is calcium carbonate scale—dissolved calcium that precipitates as water evaporates. Scale forms on faucets, glassware, and appliances and can reduce the efficiency of kettles and water heaters over time. Acidic cleaners or running vinegar through appliances dissolve scale; many dishwashers use acidic rinse agents for the same reason.

Pros and Cons Summarized

Benefits of soft water

Soap lathers better and you need less for cleaning
Less soap scum on bathroom surfaces
Reduced scale on fixtures, glassware, and appliances
Skin and hair often feel smoother and less itchy

Drawbacks to artificially soft water

Soap can feel harder to rinse from skin and hair unless you use less
Higher sodium levels may matter for sodium-sensitive individuals
Increased sodium and chloride in wastewater can harm downstream waterways and affect plants if reused as greywater
May impart a slightly salty taste
Can be corrosive to older steel pipes if pH is very low
Requires ongoing maintenance and salt or potassium input

Benefits of hard water

Soap rinses off more readily in some cases
May have a preferred taste for some people
Less corrosive to certain plumbing materials

Drawbacks to hard water

Soap lathers less effectively and more soap is required
Produces soap scum on surfaces
Causes scale buildup on fixtures and appliances
Can be more drying to skin
May leave deposits in older plumbing

Hard versus soft water is not a simple good-versus-bad choice. Each has advantages and disadvantages depending on personal preference, household needs, health considerations, and local water chemistry. Consider what matters to you—skin and hair feel, appliance longevity, environmental impacts, and taste—before deciding whether to treat your water.