Specific Gravity Definition Explained

The specific gravity of a liquid quantifies its density relative to a reference substance, typically water, under specific conditions.

Formal Definition

Specific gravity (SG) is defined as the ratio of the density of a liquid at its actual conditions to the density of pure water at a standard reference temperature and pressure.

According to the provided definition, the reference conditions for water are:

• Temperature: $4^\circ C$
• Pressure: $101\ kN/m^3$ (This pressure value is unusual for defining water density reference, standard is usually 1 atm or 100 kPa, but adhering to the provided text.)

Formula

The specific gravity ($SG$) can be represented by the formula:

$SG = \frac{\rho_{liquid}}{\rho_{water}}$

Where:

• $\rho_{liquid}$ = Density of the liquid at actual conditions.
• $\rho_{water}$ = Density of pure water at reference conditions ($4^\circ C$ and $101\ kN/m^3$).

Analysis of Options

• Option A correctly states the definition using the specified reference conditions for water ($4^\circ C$ and $101\ kN/m^3$).
• Option B uses an incorrect reference temperature ($25^\circ C$) for water.
• Option C reverses the ratio, defining it as water density over liquid density.
• Option D omits the crucial reference conditions for water density.

Therefore, the first option accurately represents the specific gravity of liquids based on standard physical definitions, using the specific reference conditions mentioned.