Definition
A mixture made up of two or more substances that do not blend uniformly, so the individual components remain physically distinct and can be identified or separated. The composition varies from one part of the mixture to another.
Plain English
A mixture where the parts don't fully blend together. You can still see or separate the different ingredients, and different scoops of it won't be the same.
Context Anchor
Seen in piston-engine discussions about fuel-air mixing, combustion, and engine roughness.
Derivation
From the Greek 'heteros' meaning 'different' and 'genos' meaning 'kind' — literally 'of different kinds.' That captures the idea: the mixture contains different kinds of material that stay distinguishable from one another.
Why Pilots Care
Water suspended in fuel, dirt in oil, or ice crystals in fuel are heterogeneous mixtures. Recognizing that the components are not uniformly distributed explains why a fuel sample drawn from a sump can look very different from fuel higher in the tank — and why sumping matters.
Analogy
Think of a bowl of cereal in milk. The flakes and milk are mixed together but stay separate — you can see them, and the spoonful you take depends on what you scoop.
Grounding Statement
Picture fuel and air entering an engine cylinder unevenly, with some areas richer in fuel and other areas leaner in fuel.
Intuition Check
A mixture does not always mean an even blend. Heterogeneous means the parts are mixed together, but not evenly throughout.
Example Sentence 1
Water contamination in avgas is a heterogeneous mixture, which is why water settles to the bottom of the tank where it can be drained from the sump.
Example Sentence 2
A fuel sample taken during preflight showed a heterogeneous mixture, indicating possible contamination.