The composition of petroleum varies because it is a

Analysis

The question asks why the composition of petroleum (crude oil) is not constant but varies depending on its source. We need to identify the chemical nature of petroleum that accounts for this variability.

Concept Explanation

Petroleum is a complex substance formed from the remains of ancient marine organisms buried under layers of sediment for millions of years.

• Chemical Nature: Petroleum is not a single chemical compound with a fixed molecular formula. Instead, it is a mixture of various hydrocarbons (alkanes, cycloalkanes, and aromatic hydrocarbons) along with small amounts of impurities like sulfur, nitrogen, and oxygen compounds.
• Why it varies: Because it is a mixture, the proportions of these different hydrocarbons and impurities depend entirely on the geological conditions, temperature, pressure, and the specific organic matter present in the location where the oil was formed. Since there is no fixed chemical ratio (unlike a pure compound like water, $H_2O$), the composition changes from one oil field to another.

Step-by-Step Solution

1. Evaluate Option A (Hydrocarbon): While petroleum is composed of hydrocarbons, being a hydrocarbon does not explain why the composition varies. Many pure hydrocarbons (like methane) have a fixed composition.
2. Evaluate Option B (Liquid): Being a liquid is a physical state, not a chemical property that dictates composition.
3. Evaluate Option C (Mixture): By definition, a mixture consists of two or more substances that are not chemically bonded. The components of a mixture can be present in any proportion. Therefore, the fact that petroleum is a mixture is the direct reason why its composition is inconsistent.
4. Evaluate Option D (Natural resource): This describes the origin and economic classification of petroleum, not its chemical composition.

Conclusion

Because petroleum is a mixture of various organic compounds, its composition is not fixed and varies depending on the geological location.

Exam Tip: Whenever you see a question about "composition varying" or "components being separated by physical means" (like fractional distillation), the answer is almost always that the substance is a mixture.