Water is often used as a standard for measurements due to its properties. Which of the following standards is incorrect?

Water is commonly used as a standard measurement because it has unique properties that make it suitable for various scientific and engineering applications. The kilogram, however, has historically not been defined in relation to water, despite water's significance in measurements. Instead, the kilogram was originally based on a physical artifact known as the kilogram prototype, a platinum-iridium cylinder kept in France. The standard now has been redefined in terms of physical constants, specifically referencing the Planck constant, rather than in terms of water.

In contrast, the Celsius scale is based on the freezing and boiling points of water, making it a direct measurement related to water's physical characteristics. The calorie is defined as the amount of energy needed to raise the temperature of one gram of water by one degree Celsius, clearly linking it to water as a reference standard. Specific gravity, which is the ratio of the density of a substance to that of water, likewise relies on water as a comparative standard for measuring the density of other liquids or solids.

Therefore, while the Celsius scale, calorie, and specific gravity all have direct associations with water, the kilogram does not share this direct relationship, establishing it as the incorrect standard in this context.