Definition
Describes two or more liquids that will not blend together to form a uniform mixture. When combined, immiscible liquids separate into distinct layers, with the denser liquid settling below the lighter one.
Plain English
Two liquids that refuse to mix. No matter how much you stir or shake them, they eventually separate back into layers.
Context Anchor
Seen in discussions of fuel contamination, aircraft fluids, and chemical compatibility.
Derivation
From the Latin 'miscere,' meaning 'to mix,' with the prefix 'im-' meaning 'not.' So 'immiscible' literally means 'not mixable.' Knowing the root makes the opposite term, 'miscible' (mixable), easy to recognize too.
Why Pilots Care
Understanding immiscibility helps pilots recognize why water contamination in fuel settles and can be detected by sumping the tanks.
Analogy
Think of oil and vinegar in a salad dressing. Shake the bottle and they appear mixed, but leave it sitting and they separate cleanly into layers. That is immiscibility.
Intuition Check
Immiscible does not mean “dangerous by itself.” It means “will not evenly mix with another liquid.”
Example Sentence 1
Because water is immiscible with avgas, it sinks to the bottom of the tank where it can be drained from the fuel sump.
Example Sentence 2
Because aviation gasoline and water are immiscible, pilots sump the fuel tanks to check for separated water.