For which liquid is the pour point numerically equal to the freezing point under standard tests, making both limits coincide?

Introduction / Context:
Pour point and freezing point are low-temperature properties used to describe handling limits of liquids. In petroleum products, these values differ because of complex mixtures and wax crystallization. For a pure substance like water, the behavior is simpler and the two limits coincide under standard equilibrium conditions.

Given Data / Assumptions:

• Pour point: lowest temperature at which oil will just flow when cooled under specific test conditions.
• Freezing point: temperature at which a liquid becomes a solid at given pressure.
• Mixtures exhibit a range of solidification behavior; pure water has a sharp phase change.

Concept / Approach:

Petroleum fractions contain multi-component waxes and asphaltenes, so pour point depends on wax network formation and can be well above any single-component freezing point. In contrast, pure water solidifies at a fixed temperature, and the inability to flow aligns with its freezing point, making pour and freezing points effectively equal.

Step-by-Step Solution:

Verification / Alternative check:

Standard test methods (ASTM D97 for pour point, water freezing at 0 degree C at 1 atm) confirm the equivalence for water but divergence for petroleum mixtures.

Why Other Options Are Wrong:

Petrol, diesel, crude: Multi-component systems with wax or light ends; pour point is an operational flow limit and does not equal a unique freezing point.

Common Pitfalls:

Assuming pour point is a thermodynamic property like freezing point; it is an empirical flow property highly formulation-dependent.

Final Answer:

Water (pure substance)