Hydrocarbon series trend: As the number of carbon and hydrogen atoms increases in a hydrocarbon molecule (heavier, longer chains), how does the density of petroleum products generally change at the same temperature?

Introduction / Context:
Product density is a fundamental property used for custody transfer, blending, and process design. Across homologous hydrocarbon series, molecular structure strongly influences density, boiling point, and viscosity. This question checks the standard qualitative trend with increasing carbon number at fixed conditions.

Given Data / Assumptions:

• Comparison at the same temperature and pressure.
• Normal trend across straight-chain or mildly branched hydrocarbons.
• No unusual additives or dissolved gases that skew density.

Concept / Approach:
As molecular size (and average molecular weight) increases, intermolecular attractions rise and packing efficiency changes, leading to higher liquid density for series members at a given temperature. While branching can lower density relative to straight chains at the same carbon number, the primary effect of increasing carbon number itself is a density increase for petroleum cuts taken as mixtures centered at higher boiling ranges.

Step-by-Step Solution:

Verification / Alternative check:
Crude assay tables show monotonic rises in density (or API gravity decreases) as cut end points move to higher boiling ranges dominated by larger molecules.

Why Other Options Are Wrong:

Common Pitfalls:
Over-weighting branching effects and forgetting the overarching influence of carbon number on bulk petroleum product density.

Final Answer:
increases