A suspension is the type of mixture that separates upon standing. When you stir sand into water, mix flour into a liquid, or shake a bottle of Italian salad dressing, the particles spread throughout but eventually sink or float once you stop mixing. This settling happens because the dispersed particles in a suspension are large enough for gravity to pull them out of the surrounding liquid or gas.
What Makes a Suspension Different
Chemistry recognizes three broad categories of mixtures: solutions, colloids, and suspensions. The key difference between them is particle size, and that single factor determines whether a mixture stays uniform or separates when left alone.
- Solutions contain particles smaller than 1 nanometer (individual atoms, ions, or molecules). Salt dissolved in water is a solution. The particles are so small they remain evenly distributed indefinitely and cannot be filtered out.
- Colloids contain particles between 1 and 1,000 nanometers. Milk, fog, and smoke are colloids. These particles are small enough to stay dispersed without settling, yet large enough to scatter a beam of light, a phenomenon called the Tyndall effect.
- Suspensions contain particles larger than 1,000 nanometers. Muddy water, sand stirred into a glass, and many liquid medications are suspensions. Gravity wins out over the particles’ ability to stay mixed, so they settle to the bottom (or float to the top) over time.
Both colloids and suspensions are classified as heterogeneous mixtures, meaning the substances in them are not uniformly distributed at the molecular level. But only suspensions visibly separate when left undisturbed. That visible settling is the defining trait people are usually asking about.
Why Suspension Particles Settle
The physics behind settling is straightforward. Every particle in a liquid experiences a downward pull from gravity and an upward drag from the surrounding fluid. For very small particles, like those in a solution or colloid, the fluid’s resistance is strong enough relative to the particle’s weight that the particle stays put. For the larger, heavier particles in a suspension, gravity overcomes that resistance and the particle drifts downward until it reaches the bottom.
The speed at which a particle settles depends on a few variables: the particle’s size, the density difference between the particle and the liquid, and the thickness (viscosity) of the liquid. A large, dense particle in thin water settles quickly. A smaller particle in a thick, syrupy liquid settles slowly. This relationship, described by Stokes’ law, explains why fine clay in water can take hours to fully settle while coarse sand drops in seconds. Settling speed increases with the square of the particle’s diameter, so doubling the particle size makes it settle roughly four times faster.
Everyday Examples of Suspensions
You encounter suspensions more often than you might realize. Muddy water after a rainstorm is a classic example: soil particles stay mixed while the water is turbulent, then gradually settle into a layer of sediment at the bottom of a puddle or glass. Flour stirred into water behaves the same way. Italian dressing separates into an oil layer and a vinegar layer with herbs sinking to the bottom, which is why the bottle says to shake before pouring.
Orange juice with pulp, paint that needs stirring, and chalk dust in water are all suspensions. In each case, the particles are large enough to see (or at least large enough to make the liquid look cloudy), and given enough time, they’ll settle out. You can also filter suspension particles out using ordinary filter paper or a fine mesh, something you cannot do with solutions or colloids.
Suspensions in Medicine
Many liquid medications are formulated as suspensions because the active ingredient doesn’t dissolve in water. This is why pharmacy labels often include “shake well before use.” If you skip that step, the medication that settled to the bottom of the bottle will be far more concentrated than what remains at the top. A report to the Institute for Safe Medication Practices documented a case where a child’s seizure medication, a carbamazepine oral suspension, varied wildly in concentration from bottle to bottle because it hadn’t been adequately shaken during preparation. Some bottles were three times more concentrated than expected, while others were significantly weaker. The practical lesson: if your liquid medication says to shake it, that instruction exists because it’s a suspension and the dose you measure depends on the particles being evenly distributed.
Suspensions in Industry and the Body
Wastewater treatment plants rely on the same settling principle to clean water. In primary sedimentation tanks, wastewater sits relatively still so that solid particles, and even tiny microplastics, sink to the bottom or float to the surface under gravity alone. This passive separation removes a significant fraction of contaminants before any chemical treatment begins.
Your own blood is a biological suspension. Red blood cells are suspended in plasma, and when a blood sample is placed in a tall, thin tube and left undisturbed, the cells gradually sink. The speed at which they fall is called the erythrocyte sedimentation rate, or ESR, and it’s a common lab test. In healthy blood, cells settle relatively slowly. During infections, autoimmune flare-ups, or certain cancers, proteins in the plasma cause red blood cells to clump together into heavier aggregates that sink faster. A high ESR doesn’t point to a specific disease, but it signals that something is driving inflammation in the body.
How to Tell if a Mixture Is a Suspension
If you’re trying to identify whether a mixture is a suspension, colloid, or solution, three quick tests help. First, let it sit. If particles visibly settle after minutes or hours, it’s a suspension. Second, try filtering it through paper or cloth. Suspension particles get trapped; colloid and solution particles pass right through. Third, shine a flashlight through it. A colloid will scatter the beam into a visible path (the Tyndall effect). A suspension may scatter light too, but if there are enough particles, the mixture will simply look opaque rather than translucent. A true solution won’t scatter the beam at all.
These three checks, settling, filterability, and light behavior, reliably distinguish the three mixture types and confirm whether what you’re looking at is a suspension.