Reading specific gravity in fuels: Specific gravity (or density) of a petroleum product primarily indicates which aspect of the hydrocarbon mixture?

Introduction:
Specific gravity (or density) is a key bulk property for fuels and lubricants. While it correlates with many behaviours, the question asks what it primarily indicates about composition.

Given Data / Assumptions:

• We consider typical liquid petroleum streams.
• We are not changing temperature in comparisons.

Concept / Approach:
Denser petroleum liquids tend to be richer in aromatic/naphthenic structures, while lighter ones are more paraffinic. Thus, specific gravity gives a broad clue to hydrocarbon type distribution (aromaticity vs paraffinic content). It does not precisely reveal sulphur content or refine history and is not a direct predictor of atomisation across all nozzle/temperature scenarios.

Step-by-Step Solution:

Verification / Alternative check:
Refinery assays show aromatic-rich streams exhibit higher density and often lower hydrogen-to-carbon ratio, consistent with this interpretation.

Why Other Options Are Wrong:

• Degree of refinement: Needs distillation/contaminant data, not just density.
• Ease of atomisation: Depends on viscosity, surface tension, temperature, nozzle design.
• Exact sulphur content: Requires compositional analysis; density is not a sulphur meter.
• Flash point: Empirical and compositional; cannot be read off density alone.

Common Pitfalls:
Treating density as a single-property predictor for all handling/combustion behaviours.

Final Answer:
Hydrocarbon family character (more aromatic vs more paraffinic)