A suspension is a mixture where relatively large particles are dispersed throughout a liquid or gas but don’t dissolve. Unlike a solution (salt fully dissolved in water, for example), the particles in a suspension are big enough to see, make the mixture look cloudy, and will eventually settle to the bottom if left undisturbed. Muddy water, paint, and blood are all everyday suspensions.
What Makes a Suspension Different
The key to understanding suspensions is particle size. Mixtures fall into three categories based on how big the dispersed particles are:
- Solutions: particles smaller than about 1 nanometer. These are atoms, ions, or individual molecules fully dissolved. Saltwater and white wine are solutions. They’re completely transparent and won’t separate over time.
- Colloids: particles roughly 1 to 1,000 nanometers. Think milk, fog, or ink. The particles are small enough to stay evenly distributed and won’t settle out, but they’re large enough to scatter light (which is why milk looks white and opaque rather than clear).
- Suspensions: particles larger than 1,000 nanometers (1 micrometer). These are big enough to be filtered out with paper, and they settle under gravity when left alone. Muddy water, hot cocoa, and paint all qualify.
That settling behavior is the most practical difference. Leave a glass of muddy water on a counter and the dirt sinks to the bottom. Leave a glass of saltwater and nothing changes. A suspension is always temporary unless you keep stirring or shaking it.
How Suspensions Interact With Light
Suspensions are cloudy or fully opaque because their large particles block and scatter light in all directions. This is related to something called the Tyndall effect, which is easiest to see in colloids: shine a flashlight through fog and the beam becomes visible because the tiny water droplets scatter the light. In a true solution like clean water, the beam passes straight through without scattering.
Suspensions scatter light even more dramatically than colloids because their particles are larger. That’s why a jar of freshly mixed paint is completely opaque rather than just slightly hazy.
Suspensions in Everyday Life
You encounter suspensions more often than you might think. Paint is one of the most familiar examples: pigment particles are suspended in a liquid base, and you stir or shake the can before use because those particles settle during storage. Orange juice with pulp works the same way, with the fiber particles gradually sinking.
Blood is a biological suspension. About 50% of human blood consists of red blood cells, each roughly 8 micrometers in diameter and 2 micrometers thick. Those cells are suspended in plasma, a liquid that’s mostly water and dissolved proteins. If you put a tube of blood in a centrifuge (or simply let it sit), the heavier red cells separate and sink to the bottom, leaving the pale yellow plasma on top. This is exactly the settling behavior that defines a suspension.
Suspensions in Medicine
Many liquid medications are formulated as suspensions because the active drug doesn’t dissolve well in water. Common examples include certain antibiotics, antacids, anti-seizure medications, and antifungal treatments used in infants. The drug exists as tiny solid particles floating in a flavored liquid base.
This creates a real practical issue: if the medication settles between doses, the liquid at the top is weaker than intended and the liquid at the bottom is more concentrated. Research on this problem found that when people don’t shake a suspension properly before measuring a dose, the amount of active drug they actually take can range from 64% to 122% of what the label says. That’s a huge swing. At the low end you’re getting a subtherapeutic dose, and at the high end you could be getting more than prescribed.
Getting an accurate dose from a suspension requires three steps: shaking the bottle thoroughly, measuring the right volume with an appropriate device (like an oral syringe rather than a kitchen spoon), and taking the dose promptly before particles settle again. Pharmacists consider the “shake well” label on these bottles genuinely important, not just a suggestion. Studies show that even brief verbal instructions from a pharmacist about proper shaking technique significantly improve dosing accuracy at home.
How to Tell if Something Is a Suspension
Three quick tests distinguish a suspension from a solution or colloid. First, check whether the mixture looks cloudy or opaque. Solutions are clear, colloids range from translucent to opaque, and suspensions are almost always visibly cloudy. Second, let it sit. If particles settle to the bottom over minutes or hours, it’s a suspension. Colloids and solutions stay uniform indefinitely. Third, try filtering it through paper. Suspension particles are large enough to be caught by a simple filter. Solution and colloid particles pass right through.
In practice, the boundaries between these categories aren’t always sharp. Some mixtures contain particles across a range of sizes and don’t fit neatly into one box. But for most purposes, the settling test is the simplest: if it separates when you stop stirring, you’re looking at a suspension.