Water Chemistry 101

You don’t need a chemistry degree to read a water report or understand what’s actually wrong with your water. You need about four ideas. This page is those four ideas, in plain language, with no math you didn’t ask for.

Three kinds of trouble

Almost everything that can go wrong with water falls into one of three categories, and the category determines what can fix it. This is the same logic the Filtration Spectrum is built around.

Dissolved — something broken down into ions or molecules small enough to be part of the water itself, not suspended in it. Salt in seawater is dissolved; so is nitrate, fluoride, and most heavy metals. You can’t filter dissolved things out with a strainer, no matter how fine — they’re not particles, they’re part of the solution. Removing them takes a mechanism built for it: reverse osmosis, distillation, or ion exchange, depending on what it is.

Suspended — solid particles floating in the water rather than dissolved into it: silt, rust flakes, sand. These are physically caught by a filter fine enough to block them — a sediment filter, mechanically straining water as it passes through.

Living — bacteria, viruses, and protozoa. Too small to strain out reliably at a household scale, and the fix isn’t removal so much as disinfection: UV light, or a filter fine enough to physically exclude organisms (ultrafiltration).

Most real fixes are more than one stage precisely because most water problems aren’t purely one category — sediment ahead of everything else, because particles clog and shield whatever comes next.

pH — not a health number, but not nothing

pH measures how acidic or basic water is, on a scale of 0 to 14. Seven is neutral; lower is more acidic, higher is more basic. The EPA’s recommended range for drinking water is 6.5 to 8.5 — but that’s a secondary standard, meaning it’s about corrosion and taste, not a direct health threshold on its own.

Here’s why it still matters: water on the acidic end is more corrosive, and corrosive water dissolves metal out of pipes and solder — which is one of the ways lead ends up in tap water even when the source itself is clean. Low pH is a real thing to know about your water, just not because the acidity itself is dangerous to drink.

pH also decides whether some contaminants can be removed at all — for two different reasons.

Precipitation. Dissolved iron and manganese are hard to filter out directly — they’re small enough to pass straight through most filters while they’re still dissolved. Raising pH speeds up their reaction with oxygen, converting them from dissolved ions into solid particles a sediment filter can actually catch. That’s the whole reason oxidation treatment often adjusts pH first: get the chemistry right, and an ordinary sediment cartridge does the rest.

Chemistry that changes what a contaminant even is. Arsenic shows up in water in two different chemical forms, and pH — along with oxidation — determines which one you’re dealing with. One common form carries no charge at typical water pH and slides past ion-exchange resin and most adsorptive media untouched. The oxidized form is negatively charged and readily removable by that same equipment. Arsenic treatment often starts by adjusting pH and oxidation state — not because the water needs to look any different, but because the arsenic needs to become a chemically different, removable thing first.

Hardness — an appliance problem, not a health problem

Hardness is dissolved calcium and magnesium — naturally occurring minerals, not contaminants in any health sense. They’re harmless to drink. What they do is deposit scale in pipes, water heaters, and appliances, fade soap’s ability to lather, and leave spots on dishes and glassware.

Hardness is usually measured in grains per gallon (gpg) or as an equivalent mg/L of calcium carbonate. Most people start noticing real effects — scale buildup, soap not lathering — somewhere around 7 to 10 grains per gallon. Below that, it’s rarely worth doing anything about. See Ion Exchange for what actually treats it.

TDS — a number that tells you almost nothing

TDS stands for Total Dissolved Solids — literally everything dissolved in the water added together into one number, usually measured with a cheap conductivity meter rather than a real lab test. It’s the most commonly misunderstood number in water treatment, because a single number can’t tell you whether what’s dissolved is a problem.

High TDS could mean the water is loaded with harmless minerals — calcium, magnesium, potassium — or it could mean something worth worrying about is mixed in with them. Low TDS doesn’t guarantee safety either; some genuinely dangerous contaminants show up in trace amounts too small to move the TDS number at all. A TDS meter is a decent sanity check for whether an RO membrane is still working, and a bad way to decide whether your water is actually safe. That takes a real test, not a $10 pen meter. Start at Test Your Water.

Reading a water report without losing your mind

Whether it’s your city’s Consumer Confidence Report or a private lab panel, a few terms do most of the work:

mg/L and ppm — for water, these are effectively the same thing (milligrams per liter and parts per million), just two different ways of writing the same concentration. You’ll see both used interchangeably.

ppb — parts per billion, a thousand times smaller than ppm. Used for things that matter at trace levels, like PFAS.

MCL (Maximum Contaminant Level) — the legally enforceable limit for a public water system. If a public system is under its MCL, it’s meeting the law. It says nothing about a private well, which isn’t covered by any MCL enforcement at all.

MCLG (Maximum Contaminant Level Goal) — the health-based target regulators would set with zero cost or feasibility constraints. It’s aspirational, not enforceable, and for a few contaminants — lead is the best-known example — the MCLG is zero, meaning no amount is considered fully without risk, even though the enforceable MCL has to be a real, achievable number above that.

Secondary standards — non-enforceable guidelines for taste, odor, and appearance. Not a health limit; a housekeeping one.

The most common misreading of a report is treating “under the MCL” as “risk-free” and “over it” as “emergency.” Neither is quite right — the honest read is closer to “under the MCL means legally compliant,” and how much margin that leaves depends entirely on which contaminant you’re looking at.

None of this replaces actually knowing what’s in your water. Test Your Water is where that starts; the Glossary has definitions for anything more specific this page didn’t cover.