A heterogeneous mixture is simply any mixture that is not uniform in composition - it's a non-uniform mixture of smaller constituent parts. By contrast, a mixture that is uniform in composition is a homogeneous mixture. For the purposes of this discussion, "not uniform" means anything that clearly has different parts visible to the naked eye. You can find many examples of heterogeneous mixtures in solid, liquid and gaseous form throughout nature.
Heterogeneous mixtures are defined by what phase of matter they most resemble. Most heterogeneous mixtures contain multiple parts in multiple phases, but they usually behave on the whole as if they belong to one particular phase of matter:
The whole world is a solid heterogeneous mixture! Solid heterogeneous mixtures can contain liquid or gaseous components, but as a whole, they act as solids.
Mixed nuts at a party are a type of heterogeneous mixture that can be separated. Simply sort the pile into separate piles for each type of nut, and you will have broken down a heterogeneous mixture into its component parts.
Rocks in the sand at the beach are a heterogeneous mixture. Natural processes have mixed up sand, stones, shells and even living things, scattering them across the beach as a mixture.
A salad with lettuce, cheese, seeds, tomatoes, broccoli, and other vegetables is an example of a heterogeneous mixture.
When a mixture contains multiple distinct components, but the whole mixture acts like a liquid, that is a liquid heterogeneous mixture. Here are some examples:
A bottle of balsamic vinaigrette salad dressing is a mixture that is heterogeneous, consisting primarily of oil and vinegar. You can (and should!) shake up a vinaigrette to make the mixture appear and taste more combined, but it will always separate into its component parts when left alone.
A bowl of oatmeal with raisins is a heterogeneous mixture. The raisins may be solid, but turn the bowl over and you will see the mixture as a whole most definitely acts like a liquid.
Mud puddles are a heterogeneous mixture. Dirt, leaves and all sorts of other runoff mixes with rainwater and pools into a mixture where the component parts can be clearly seen.
Some heterogeneous mixtures are primarily gaseous. Gaseous mixtures may contain liquids or even solids, but as a whole, they act as a gas.
Mist and fog can be considered heterogeneous mixtures, as tiny droplets of water hang visibly in the air.
Perfume and cologne are gaseous heterogeneous mixtures, with tiny droplets of scent-carrying chemicals drifting like a gas onto the wearer's skin.
Smog is a heterogeneous mixture of various particles and pollutants suspended in the air. The dirty particles that make up the smog can be removed from the air and breathed into the lungs, making smog quite a problematic heterogeneous mixture.
While almost all heterogeneous mixtures contain some substances in different phases, many are defined by the presence of things in distinct, different phases of matter.
The classic chemistry class example of a multiphase mixture is a bowl of cereal. Here you have a heterogeneous mixture of solid cereal in liquid milk.
Carbonated water (or carbonated anything) is a multiphase heterogenous mixture, with gaseous carbon dioxide (CO2) bubbling through liquid water (H2O).
Lava from a volcano is a multiphase mixture, with some solid chunks of rock surrounded by hot, molten stone.
Homogeneous and heterogeneous mixtures are just the simplest, most straightforward way mixtures are defined in science. Things get more interesting as they get more specific.
Suspensions: Solid particles that float in a liquid or gas
Solutions: Homogeneous mixtures in which one substance, the solute, dissolves perfectly into another, the solvent
Colloids: Tiny unmixable droplets float in a different substance
All these mixtures are a part of daily life. Examples can be found in your kitchen, your car and your own human body. Understanding what mixtures are and how science defines them is an important part of learning how the world works. When you're prepared to perform some scientific experiments of your own, be sure to follow the scientific method. It's the official process for testing hypotheses!