compound

A compound forms when two or more elements combine chemically in a fixed ratio, creating a brand-new substance with properties completely different from the elements it came from.

• Water is the clearest example — hydrogen is a flammable gas and oxygen supports burning, yet together, in a fixed 2:1 ratio, they form water, which puts fires out.
• That fixed ratio is what separates a compound from a mixture: sodium and chlorine always combine as sodium chloride in a strict 1:1 ratio, never any other proportion.
• Unlike a mixture, a compound's elements can't be pulled apart by physical tricks like evaporation or filtering — only a chemical reaction can break them apart again, the way heating sugar releases the carbon and water locked inside it.
• Because the properties genuinely change, a compound can look nothing like its ingredients — iron and sulfur, both solids on their own, combine into iron sulfide, a completely different-looking, non-magnetic substance.