Definition
A substance formed when two or more chemical elements combine in fixed proportions and are held together by chemical bonds, producing a new substance with properties different from those of the original elements.
Plain English
Two or more basic substances joined together to make a new substance that behaves differently from the parts that went into it.
Context Anchor
Seen in discussions of aviation fuels, oils, battery fluid, cleaning products, fire extinguishers, and aircraft materials.
Derivation
From the Latin 'componere,' meaning 'to put together.' A compound is literally things put together — but in chemistry, the parts are bonded so tightly they form something genuinely new, not just mixed.
Why Pilots Care
Many aircraft fluids and materials are specific compounds (like 100LL avgas additives or hydraulic fluid). Mixing or substituting compounds incorrectly can cause corrosion, system failures, or fire hazards.
Analogy
Table salt is a common chemical compound. It is made from two basic substances joined together, but it behaves as one new material.
Intuition Check
Do not read “compound” here as just “a mix.” A chemical compound is joined in a fixed way; a mixture is materials placed together without becoming one new substance.
Example Sentence 1
Water is a chemical compound made of hydrogen and oxygen, and small amounts of it in fuel can cause serious engine problems.
Example Sentence 2
Aviation hydraulic fluids use a specific chemical compound to maintain stable viscosity.