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The comparison standard for assessing the specific gravity of liquids is water. Specific gravity is defined as the ratio of the density of a substance to the density of a reference substance, which in the case of liquids, is usually water. This is because water has a density of 1 gram per cubic centimeter (g/cm³) at 4 degrees Celsius, making it a convenient reference point.

When a liquid has a specific gravity less than 1, it is less dense than water and will float, while a specificity greater than 1 indicates that the liquid is denser than water and will sink. This standardization against water allows for easy comparison and understanding of the density of various liquids in relation to the well-known properties of water.

The other substances mentioned, such as mercury, alcohol, and brine, are not typically used as standards for specific gravity in general testing. Mercury has a much higher density and is usually compared to in specialized contexts; alcohol has a lower density than water; while brine is a concentrated salt solution and not a universal reference. Thus, water serves as the most practical and universal comparison standard for determining specific gravity across various fluids.