A glass of water can look perfectly clear.
No particles.
No cloudiness.
No unusual colour.
And yet its chemistry can be completely different from another glass that looks exactly the same.
The reason is simple:
Not everything in water is visible.
Some substances exist as suspended particles. Others are dissolved as electrically charged ions or individual molecules. Once a substance is dissolved, the human eye usually tells us very little about whether it is present.
This is why appearance alone is not a reliable measure of drinking-water composition. The CDC explicitly notes that harmful chemicals and microorganisms often do not change the appearance, taste or smell of water.
Understanding the difference between particles and dissolved substances is also one of the foundations of modern water treatment.
Because a technology designed to trap particles is not automatically designed to remove molecules.
Clear Water Is a Visual Description, Not a Chemical Analysis
When we describe water as clear, we are mainly describing the way light travels through it.
Cloudiness, or turbidity, is caused primarily by suspended and colloidal material that scatters light. WHO defines turbidity as a condition caused by suspended or colloidal matter that results in light scattering and absorption.
Examples include:
- clay
- silt
- rust particles
- sediment
- organic particles
- some microorganisms
- particulate material from pipes
If enough of these particles are present, water can appear cloudy.
But dissolved substances behave differently.
They are dispersed throughout the water at a scale where they generally do not produce visible particles.
Removing cloudiness therefore does not automatically remove dissolved chemicals.
And perfectly transparent water does not automatically mean that nothing unwanted is dissolved in it.
Three Different Things Can Be Present in Water
A useful way to understand water is to separate its constituents into three broad categories:
1. Suspended particles
These are pieces of material physically present in the water.
Examples can include:
sand,
rust,
silt,
larger microplastics,
and other suspended solids.
They may settle over time or be physically retained by suitable particle filtration.
2. Dissolved ions
Ions are atoms or groups of atoms carrying an electrical charge.
Common naturally occurring examples include:
calcium²⁺
magnesium²⁺
sodium⁺
chloride⁻
bicarbonate⁻
nitrate⁻
The major dissolved solids in natural water commonly include calcium, magnesium, sodium, potassium, bicarbonate, chloride and sulfate ions.
3. Dissolved molecules
Other substances can be present as dissolved molecules or molecular species.
These can include various natural organic compounds as well as synthetic chemicals.
Depending on their chemical properties and pH, some substances can also occur partly as neutral molecules and partly as charged species.
The important distinction is not simply whether something is “small”.
It is whether the substance is suspended as a separate particle or dissolved into the water phase.
What Does “Dissolved” Actually Mean?
In everyday language, dissolved means that a substance has mixed into the water so completely that we no longer see separate particles.
Think of table salt.
Place salt crystals into water and stir.
Initially, the crystals are visible.
After they dissolve, the water may look exactly as clear as before.
The salt has not disappeared.
Its components are now present in solution as sodium and chloride ions.
In water analysis, “dissolved” is also an operational laboratory definition. A commonly used convention classifies material passing through a 0.45 micrometre filter as dissolved, while material retained is considered particulate or suspended.
This is worth emphasising because:
“Dissolved” does not mean “nothing is there.”
It means the substance is present in another physical form.
Minerals Are the Most Familiar Example
The minerals discussed in water hardness provide an easy illustration.
Hard water can contain relatively high concentrations of dissolved calcium and magnesium.
Yet a glass of cold hard water can look completely transparent.
The calcium becomes obvious only when conditions change.
When water is heated, calcium carbonate can precipitate and eventually become visible as limescale.
The mineral was already present before the white deposit appeared.
It simply existed in dissolved form.
This gives us an important principle:
Visibility can change even when the underlying substance was present all along.
Nitrate Can Be Present Without Changing the Appearance
Nitrate is another good example of a dissolved ion.
It can enter groundwater through natural nitrogen cycling and human activities, particularly agricultural nutrient inputs.
Once present in water, nitrate occurs as a dissolved ion.
You cannot determine its concentration by holding a glass against the light.
Germany therefore sets a drinking-water parameter value of 50 mg/L for nitrate and monitors it analytically. The latest German reporting data show that exceedances in large drinking-water supply systems are rare.
The important point here is not that German tap water generally contains problematic nitrate levels.
It is that:
a regulated chemical parameter can be completely invisible.
Only analysis provides the concentration.
Metals Can Be Dissolved Too
The word “metal” often creates an image of solid particles floating in water.
That is not necessarily how metals occur in drinking water.
Metal atoms can enter solution as ions.
One important example is lead.
The German Environment Agency explains that lead can dissolve from unsuitable plumbing materials into drinking water. This is why lead exposure from old plumbing is assessed through water analysis rather than simply by looking for metallic particles.
Again:
the water may remain visually clear.
The contaminant is present in the dissolved phase.
This is also why plumbing materials can influence water quality on the final metres between the building connection and the tap.
Organic Trace Contaminants Can Be Invisible at Extremely Low Concentrations
The same principle applies to many organic substances.
Modern analytical laboratories can detect certain chemicals at concentrations measured in:
micrograms per litre — µg/L
nanograms per litre — ng/L
and in specialised applications even lower.
The German Environment Agency notes that modern analytical methods can detect an increasing number of environmental substances at nano- and even picogram-per-litre levels.
At these concentrations, there is usually nothing a person could visually identify.
One microgram per litre equals:
0.000001 grams in one litre of water.
For a nanogram, another factor of one thousand separates us from that.
Modern water analysis is therefore examining a chemical world that exists far below human vision.
PFAS Are Another Example
Many PFAS can occur as dissolved chemical species in water.
They do not need to form visible particles in order to be present.
This is precisely why PFAS treatment is not simply a question of installing a finer sediment screen.
The U.S. EPA identifies technologies such as activated-carbon adsorption, ion exchange and high-pressure membrane processes among established approaches used for the reduction of different PFAS in drinking water, with effectiveness depending on the specific compounds and treatment conditions.
The water can look equally clear before and after treatment.
The difference may exist almost entirely at the molecular level.
Clear Water and Clean Water Are Not the Same Measurement
This distinction is easy to miss because our senses are naturally powerful tools.
If water contains visible rust, we notice it.
If it smells strongly, we notice it.
If it is brown or cloudy, we notice it.
But chemical water quality cannot be evaluated only through sight, taste and smell.
The CDC specifically advises that appearance, taste and smell are not always reliable indicators of water safety because some harmful chemicals and germs cause no obvious sensory change.
The reverse is also true.
Water can have an unusual taste or smell without necessarily representing a health hazard.
Klar2O has already discussed this with substances such as geosmin and MIB: very small concentrations can create noticeable earthy or musty odours even when the issue is primarily aesthetic rather than an indication of toxic contamination.
So we have two very different situations:
Invisible does not mean absent.
And:
noticeable does not automatically mean dangerous.
Turbidity Measures Particles — Not Everything Dissolved in Water
Turbidity is valuable.
It can indicate suspended material and is particularly important in drinking-water treatment because excessive particles can interfere with processes such as disinfection.
WHO notes that turbidity reflects suspended or colloidal material and that clear water can still contain pathogens or other hazards.
But low turbidity should not be interpreted as a universal chemical test.
Water can have extremely low turbidity while still containing significant amounts of dissolved minerals.
It could also contain a dissolved contaminant that needs specific laboratory analysis to detect.
Turbidity answers: “How much light-scattering material is present?”
It does not answer:
“Which dissolved chemicals are present?”
TDS Does Not Answer That Question Either
Another number commonly used to describe water is TDS — Total Dissolved Solids.
WHO defines TDS as the combined concentration of dissolved inorganic salts and small amounts of organic material. Major contributors commonly include calcium, magnesium, sodium, potassium, bicarbonate, chloride and sulfate.
A TDS reading can therefore tell us something about the overall amount of dissolved material.
But it usually does not identify which individual substances make up that total.
Consider two hypothetical glasses of water.
Both measure:
300 mg/L TDS.
In one, most of the dissolved solids may consist of calcium and bicarbonate.
In another, the ionic composition could be quite different.
The same total number does not mean the same chemistry.
And a trace contaminant measured in micrograms or nanograms per litre may contribute almost nothing noticeable to the overall TDS value.
TDS is a bulk measurement.
Contaminant analysis is substance-specific.
Conductivity Has the Same Limitation
Electrical conductivity is another useful water-quality parameter.
Dissolved ions allow water to conduct electricity, so conductivity can provide information about the overall ionic content of a sample.
If ionic concentrations change substantially, conductivity may change too.
But conductivity does not identify each individual ion.
A conductivity meter cannot tell you:
“this signal comes from nitrate”
or
“this part comes from calcium”
or
“lead is present at this concentration.”
That requires targeted analytical methods.
This is why simple meters can be useful for process monitoring but cannot replace comprehensive laboratory analysis.
Why Particle Filters Cannot Simply Remove Every Dissolved Substance
Now the physical distinction becomes especially important.
Imagine pouring water containing grains of sand through a sufficiently fine screen.
The holes are smaller than the grains.
The sand remains behind.
This is mechanical filtration.
Many sediment filters work according to this basic principle.
Particles larger than the effective openings or retention threshold are physically captured.
But what happens when the contaminant is dissolved?
There is no visible grain to strain out in the same way.
The treatment mechanism has to change.
This is why water-treatment technologies include more than increasingly fine screens.
Different Contaminants Require Different Mechanisms
Modern treatment systems may use several fundamentally different mechanisms.
Mechanical filtration
Best understood as physical separation.
It can target suspended particles depending on their size and the filter structure.
Examples include sediment, particulate rust and selected microplastic size ranges.
Adsorption
Adsorption works differently.
Rather than simply trapping a particle because it is larger than a pore, a dissolved substance can interact with and accumulate on the surface of a treatment medium.
The EPA describes granular activated carbon as a highly porous adsorption material with a very large internal surface area, commonly used for selected organic compounds, taste- and odour-producing substances and other contaminants.
Here, surface chemistry matters, not only pore size.
Ion exchange
Ion-exchange materials are designed to interact with charged substances.
Depending on the resin chemistry, positively or negatively charged ions can be exchanged with other ions bound to the treatment medium.
The EPA lists both cation and anion exchange among established drinking-water treatment technologies.
Water softening is one familiar example of ion exchange.
Other specialised ion-exchange systems can target different charged contaminants.
Membrane separation
Nanofiltration and reverse osmosis use semi-permeable membranes and pressure to separate water from various dissolved substances.
The EPA notes that reverse osmosis can reduce many dissolved solids, inorganic constituents and synthetic organic compounds, while nanofiltration can be useful for hardness and selected organic compounds.
Each approach works differently.
That is the key.
Smaller Does Not Automatically Mean Harder — Chemistry Matters Too
It is tempting to describe water treatment entirely as a size problem:
large particle = easy to filter
small molecule = difficult to filter.
But that is incomplete.
For adsorption and ion exchange, chemical properties can be more important than physical size.
A treatment surface may strongly bind one dissolved substance and interact only weakly with another.
PFAS provide a useful example.
EPA research shows that activated carbon performance can vary between different PFAS molecules. Longer-chain compounds such as PFOA and PFOS generally adsorb more effectively than some shorter-chain PFAS under comparable conditions.
Factors such as these can matter:
- molecular structure
- electrical charge
- polarity
- solubility
- pH
- competing substances
- contact time
- temperature
- treatment-media chemistry
So the question is not only:
“How small is the contaminant?”
It is also:
“How does that contaminant behave chemically in this water?”
The Water Around the Contaminant Matters
A treatment technology does not encounter an isolated target molecule.
It encounters an entire water matrix.
That water may contain:
calcium,
magnesium,
natural organic matter,
bicarbonate,
chloride,
sulfate,
other contaminants,
and many additional dissolved constituents.
Those substances can influence treatment performance.
For adsorption systems, for example, other organic material can compete for available surface sites. EPA research on PFAS treatment specifically notes that performance depends on factors including the target PFAS, flow rate, temperature and the type and concentration of organic matter and other constituents in the water.
This is another reason why water-treatment performance cannot be understood from pore size alone.
The contaminant matters.
The medium matters.
The surrounding water matters.
This Is the Principle Behind Selective Filtration
Klar2O has previously discussed the concept of selective filtration: different water-quality problems require different treatment mechanisms because particles, metals, PFAS, pesticides and other substances do not behave identically.
The distinction between particles and dissolved substances explains why.
A sediment filter can be excellent at removing suspended solids and still have little relevance for dissolved nitrate.
An ion exchanger can target certain dissolved ions while allowing suspended sediment to pass unless another treatment stage is present.
An adsorbent may bind selected molecules but not every inorganic salt.
A membrane may reject a broad range of dissolved constituents while creating a concentrate stream that has to be managed.
There is no contradiction here.
The technologies simply perform different jobs.
Does Clear Tap Water in Germany Mean There Is a Problem?
No.
The fact that substances can be invisible should not be interpreted to mean that clear German tap water is generally contaminated.
Germany’s latest national data for larger drinking-water supplies show that more than 99% of measurements complied with requirements for almost all monitored microbiological and chemical parameters during the 2020–2022 reporting period.
Drinking-water quality is controlled through regulatory monitoring, treatment, source protection and operational management.
The point is different:
Visual clarity cannot tell you which substances are present or at what concentration.
That information comes from measurement.
When Does Water Analysis Become Useful?
Laboratory analysis becomes particularly valuable when there is a specific question.
For example:
Is lead entering the water from an old installation?
What is the nitrate concentration?
How hard is the water?
Are selected PFAS present?
What is the mineral composition?
Has a treatment system reduced the target substance?
Each question requires the correct analytical method.
A general “water test” is therefore not one universal measurement.
Modern water laboratories select methods according to the substances being investigated.
The German Environment Agency also emphasises that drinking water can contain naturally occurring and anthropogenic substances and that evaluation depends on both which substance is present and at what concentration.
Concentration Is Just as Important as Presence
Modern analytical instruments are extraordinarily sensitive.
Detecting a substance does not automatically mean that it is present at a concentration representing a health concern.
This distinction is essential.
Detection answers:
Is the substance measurable?
Risk assessment asks:
At what concentration is it present, how much exposure occurs, and what is known about its toxicology?
These are not the same question.
The German Environment Agency uses toxicological guideline and orientation concepts precisely because chemical evaluation requires concentration-based assessment rather than a simple present/absent interpretation.
Modern analytical capability makes it possible to detect extraordinarily small concentrations.
That is scientifically valuable.
It also makes careful interpretation more important than ever.
A Better Way to Think About Water
Instead of asking:
“Does the water look clean?”
ask a series of more precise questions.
Is there suspended material?
What is dissolved?
Which parameters are relevant to this water source or installation?
At what concentrations?
And if treatment is needed, which physical or chemical mechanism can actually address the target substance?
This leads to a simple hierarchy:
Appearance → observation
Measurement → composition
Assessment → relevance
Treatment → targeted response
Skipping the middle steps often leads to choosing the wrong technology.
Clear Water Can Contain Thousands of Dissolved Components — Most Are Not Contaminants
There is another important distinction.
Not everything dissolved in water is undesirable.
Natural drinking water contains minerals and ions.
Calcium and magnesium contribute to hardness.
Bicarbonate influences buffering capacity.
Other naturally occurring ions help define the chemical character and taste of water.
WHO’s description of total dissolved solids makes clear that dissolved water chemistry normally includes a mixture of naturally occurring salts and small amounts of organic matter.
So the objective of modern water treatment is not necessarily:
remove everything that is dissolved.
It is:
understand what is present and manage the substances that actually require treatment.
That is a much more precise approach.
The Invisible Chemistry Is Why Water Treatment Is More Than a Screen
A simple filter screen can separate objects by size.
Modern water treatment often has to do something considerably more sophisticated.
It may need to:
retain a particle,
bind a molecule to a surface,
exchange a charged ion,
separate dissolved substances through a membrane,
or combine several treatment stages.
This is why the transition from visible particles to dissolved contaminants is so important.
Once a substance becomes part of the water at the molecular or ionic level, surface chemistry and selective interactions become increasingly relevant.
Klar2O’s own Smart Surface approach is based on adsorption using biochemically coated silica beads — an example of why modern filtration research increasingly considers how contaminants interact with surfaces rather than looking only at physical pore size.
Different contaminants still require their own validated treatment strategy.
But the underlying principle remains:
water treatment is both physics and chemistry.
What You See Is Only Part of the Water
Clear water can contain dissolved minerals.
Clear water can contain dissolved salts.
Clear water can contain ions.
Clear water can contain trace organic molecules.
And clear water can still meet every applicable drinking-water requirement.
These statements are not contradictory.
They simply describe how water chemistry works.
The colour and clarity of a glass tell us something about its physical appearance.
They do not provide a chemical inventory.
At Klar2O, this distinction sits at the heart of modern water treatment:
Particles must be understood as particles.
Ions must be understood as ions.
Molecules must be understood through their chemistry.
And before deciding what needs to be removed, the first step is always the same:
find out what is actually in the water.
Sources
World Health Organization, Guidelines for Drinking-water Quality, Fourth Edition Incorporating the First, Second and Third Addenda, 2026.
World Health Organization, Water Quality and Health: Review of Turbidity.
World Health Organization, Total Dissolved Solids – Chemical Fact Sheet.
German Environment Agency, Drinking Water Quality – Toxicology.
German Environment Agency, Drinking Water Guideline Values, updated 2026.
German Environment Agency, Quality of Drinking Water from Central Supply Systems.
German Environment Agency, Distributing Drinking Water, updated 2025.
U.S. Environmental Protection Agency, Overview of Drinking Water Treatment Technologies, updated 2026.
U.S. Environmental Protection Agency, Reducing PFAS in Drinking Water with Treatment Technologies.
Centers for Disease Control and Prevention, About Choosing Home Water Filters.