Fog is not a suspension. It is a colloid, specifically a type of colloid called an aerosol, where tiny liquid water droplets are dispersed in air. The distinction matters because suspensions and colloids behave very differently, and fog’s properties line up squarely with the colloidal category.
Why Fog Is a Colloid, Not a Suspension
The difference between a colloid and a suspension comes down to particle size and stability. Suspension particles are large, typically bigger than 1,000 nanometers (1 micrometer). They’re visible to the naked eye, can be filtered out, and will eventually settle to the bottom if left undisturbed. Think of sand stirred into water: give it a minute and the sand sinks.
Colloid particles are smaller, generally in the range of 2 to 500 nanometers. They don’t settle out on their own and can’t be removed by ordinary filtration. Fog fits this behavioral profile. Its water droplets are small enough that air resistance overwhelms gravity, so they remain suspended in the atmosphere rather than quickly falling to the ground. If you’ve ever walked through fog, you’ve noticed it doesn’t “rain down” on you. It just hangs there.
That said, fog droplets are actually somewhat larger than the textbook nanometer range for colloids. Measurements of real fog show most droplets fall between 5 and 30 micrometers in diameter, which pushes into territory that might technically overlap with suspension-sized particles. This is why the IUPAC (the international body that standardizes chemical terminology) deliberately broadened its definition of aerosols beyond strict colloidal size limits. The key factor isn’t particle size alone but settling behavior: fog droplets have such a small settling velocity that they exhibit real stability in air, behaving like a colloid rather than a suspension.
How Fog Forms
Fog forms when water vapor in the air condenses into liquid droplets. This doesn’t happen spontaneously. Water molecules need a surface to condense onto, and in the atmosphere those surfaces are microscopic particles of dust, salt, smoke, or clay floating in the air, known as condensation nuclei. Millions of these particles exist in every cubic meter of air.
When air cools to its dew point (the temperature at which it can no longer hold all its moisture as vapor), water molecules latch onto these condensation nuclei and form droplets with a radius of about 20 micrometers or less. The resulting collection of droplets is fog when it forms near the ground, or a cloud when it forms at higher altitudes. The physics are identical.
Why Fog Droplets Don’t Fall
One of the defining traits of a suspension is that its particles settle out over time. Stir flour into water and it will eventually sink. Fog doesn’t do this, at least not quickly, and the reason is drag. At the tiny scale of fog droplets, air resistance is enormous relative to the droplet’s weight. The physics follows Stokes’ law: as a droplet begins to fall under gravity, the upward drag force from the surrounding air quickly matches the downward pull of gravity, and the droplet reaches a terminal velocity that is extremely slow.
For a 10-micrometer fog droplet, that settling speed is on the order of fractions of a centimeter per second. Even gentle upward air currents are enough to keep droplets aloft indefinitely. This is the same reason clouds persist at altitude for hours or days. In a true suspension, particles are large and heavy enough that gravity wins decisively, and the mixture separates.
The Tyndall Effect: Proof in a Headlight Beam
You’ve seen this without knowing its name. When you drive through fog and your headlights create visible beams cutting through the air, that’s the Tyndall effect, a hallmark of colloids. The water droplets in fog are large enough to scatter light in all directions, making the beam itself visible. In a true solution (like perfectly clean air), light passes straight through and you wouldn’t see the beam at all. In a suspension, the particles are large enough to block light rather than scatter it, creating shadows rather than a glow.
This scattering behavior is one of the classic tests for identifying a colloid. Smoke, milk, and fog all exhibit it. It’s also why fog reduces visibility so effectively: incoming light from headlights, streetlamps, or the sun gets scattered in every direction before it reaches your eyes, turning the world into a bright, featureless white.
Where Fog Fits Among Mixture Types
Chemistry divides mixtures into three broad categories based on particle size and behavior:
- Solutions have particles smaller than 2 nanometers (individual molecules or ions). They’re completely transparent, don’t scatter light, and never separate. Saltwater is a solution.
- Colloids have particles roughly 2 to 500 nanometers. They scatter light, appear translucent or opaque, can’t be filtered with ordinary paper, and don’t separate on standing. Fog, smoke, milk, and butter are all colloids.
- Suspensions have particles larger than about 1,000 nanometers. They’re visibly cloudy, can be filtered, and will separate if left alone. Muddy water and sand in a jar are suspensions.
Within the colloid category, fog is specifically classified as an aerosol: a dispersion of liquid particles in a gas. Other aerosol examples include mist and spray from a can. Smoke is also an aerosol, but with solid particles dispersed in gas rather than liquid ones. The IUPAC defines an aerosol as any sol where the dispersed phase (solid, liquid, or both) exists within a continuous gas phase, typically air. Fog checks every box.