Crude oil is best described as which of the following?

• A. A mixture of hydrocarbons
• B. A pure chemical compound
• C. A solution of water and oil
• D. A mixture of metals

Answer: (A)

Crude oil is a mixture of hydrocarbons. It’s made up of many different carbon–hydrogen molecules present together in varying amounts, with a range of chain lengths and ring structures. That’s why crude oil isn’t a single substance with a fixed formula. Its properties come from the blend of numerous hydrocarbons, which is why refining it uses fractional distillation to separate it into different fuel fractions, each with its own boiling point. A pure chemical compound would have a fixed composition and definite properties, which crude oil does not. It isn’t a solution of water and oil, since oil and water don’t form a single homogeneous solution. And it’s not a mixture of metals, because it contains organic carbon–hydrogen compounds, not metallic elements.