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Units provide a standard of comparison that gives meaning to numerical values. Without units, a measurement is ambiguous or meaningless. For example, reporting the Eiffel Tower's height as 300 without units leaves unclear whether it is tall or short. Units ensure measurements communicate precise information about the magnitude and nature of a property being measured.

The International System of Units defines seven base units for fundamental properties: length, mass, time, temperature, electric current, amount of substance, and luminous intensity. In chemistry, the first four are most commonly used. These standard units form the foundation for all other derived measurements in science.

SI prefixes represent fractions or multiples of base units using powers of 10. For example, the prefix kilo means one thousand, or 10³, so 1 kilometer equals 1000 meters. Similarly, centi means 10⁻², making 1 centimeter equal to 0.01 meters. This consistent approach allows easy conversion between different scales of measurement.

Mass measures the quantity of matter in an object and remains constant regardless of location. Weight measures the gravitational force exerted on that matter and varies with gravity. On the moon, an object would weigh less than on Earth due to weaker gravity, but its mass would stay the same. The SI unit of mass is the kilogram.

All three scales measure temperature but place their zeros differently. Water freezes at 273.15 K (0°C, 32°F) and boils at 373.15 K (100°C, 212°F). The Kelvin scale, called the absolute scale, has no negative temperatures; 0 K represents absolute zero. Celsius and Fahrenheit allow negative values and are commonly used in everyday contexts.

A complete measurement includes the magnitude (a number), the unit (a standard of comparison), and the uncertainty (an indication of measurement error). For example, when water boils at 100°C, the number 100 is the magnitude, °C is the unit, and any associated error represents uncertainty. Together, these elements convey precise information about the property being measured.

The four most commonly used SI base units in chemistry are the meter for length, the kilogram for mass, the kelvin for temperature, and the second for time. The meter is defined as the distance light travels in a vacuum in 1/299,792,458 of a second. These units form the foundation for measuring and reporting chemical properties and reactions.