Which term describes a substance formed when two or more elements are chemically combined in fixed proportions?

• A. Element
• B. Mixture
• C. Colloid
• D. Compound

Answer: (D)

When two or more elements chemically combine in fixed proportions, they form a compound, a pure substance with its own unique properties. The atoms join by chemical bonds in a definite ratio, so the resulting substance has a specific chemical formula (like water being H2O). This fixed composition distinguishes compounds from elements, which are made of a single type of atom, and from mixtures, where substances are combined without a fixed ratio and can often be separated by physical means. A colloid is a special kind of mixture with finely dispersed particles and also doesn’t have a fixed composition. So the term for a substance formed when elements combine in fixed proportions is a compound.