A solution is mixed together at a molecular level, while a suspension is mixed at a larger level. The easiest way to test this is by letting a solution or suspension sit for many days. A solution (like water and salt) will not have any separation. The salt will stay mixed. A suspension (like powdered chalk in water) will separate out after some time. An emulsion is like a suspension -- it will separate out -- but the things being mixed are two liquids (like salad dressing).

A solution is a homogeneous mixture of two components. This means that wherever you look at the solution, it appears the same throughout the entire container. This is different than a suspension, which is a heterogeneous mixture of two components that will separate if left long enough.

For example, if you dissolve salt in water, it will make a solution, and you can't separate the two, no matter how long you leave it. In contrast, if you mix mud and water, the mixture will look cloudy, but if you let it sit long enough, the mud will eventually settle and you'll have clear water on top, and mud sitting on the bottom.

In a solution, the dissolved substance is interacting chemically with the solvent and dissociated into its component molecules or ions. In a suspension, the particles are not dissolved and may still be part of crystals or other larger structures than just the molecules.

In a solution, the solute molecules (the solid or liquid in solution) are surrounded by solvent molecules (the liquid in which you are dissolving the solute). In this case, the solute is evenly distributed in the solvent. In a suspension, particles or crystals of the solute remain – this is generally because the interaction of the solvent or solute with itself is much stronger than the interaction between the solute and solvent. Suspensions are generally unstable and the particles will grow bigger over time. Solutions, though, are stable over time – unless there’s a drastic change in temperature or there’s not enough solvent to dissolve the solid.

A great example of solutions and suspensions is an oil and vinegar salad dressing. Vinegar is a solution of acetic acid dissolved in water – notice that you can’t see any particles in the liquid. When you add oil to vinegar and shake hard, you’ll see small pockets of oil in the vinegar and vice versa, all mixed together in a suspension. If you wait a while, the two liquids will separate into layers. Now try adding some salt and shaking – which layer will it dissolve in? In the water layer, the salt dissolves to form a solution. In the oil layer, the grains of salt remain suspended and not dissolving.

Most salad dressing that you buy is stabilized with an emulsifier (also called a surfactant) than allows the suspension to last longer. Emulsifiers are molecules that have a side that likes water (hydrophilic) and a side that likes oil (hydrophobic). When mixed in salad dressing, they coat the surface of the oil pockets and keep them from reforming into an oil layer after you shake the dressing. Some examples of emulsifiers in your kitchen are the mayonnaise (the egg proteins) and mustard. Add a little of either of these to your oil and vinegar mixture before and you’ll notice that the layers take much longer to reform than before.