SI Base Units — Definition & Overview

Measurements are an important part of scientific research. Whether it's pure or applied science, you always need to record your data in a way that lets other scientists understand your results and lets them replicate your experiments. This is why scientists always use SI, or metric, units. SI stands for International System of Units, originally in French.

The metric system gives us units like centimeters and kilograms. These units are the combinations of the prefixes, like "centi" and "kilo", and the base units, like "meters" and "grams." The prefixes indicate a conversion using scientific notation. For example, a centimeter is $1\times10^{-2}m$. The metric system provides base units for all of the measurements used in chemistry. We can even use the metric system for tiny particles like atoms, protons, neutrons, and electrons!

The most common metric base units are pictured in the table above with their abbreviations. Each unit measures a specific quantity such as length, mass, or volume. The most common units used in everyday measurement are the meter for length, the gram for mass, and the liter for volume. In chemistry, we make use of meters, grams, and liters along with Celsius and Kelvin for temperature, joules for energy, hertz for frequency, the pascal for gas pressure (1 atmosphere= 101,325 Pascals), and the mole for atomic quantities among many others. SI units can also be used to define important numbers, like Planck's constant or the Boltzmann constant.

Some units in the metric system are directly related by a proportion. For example, the liter is directly related to the meter. This stems from the fact that volume can be measured by length times width times height and by displacement of liquids. In this way 1 Liter (L) = 1 cubic decimeter ($dm^{3}$). Grams and liters are also related to the mass of water at a certain volume. That's the density! 1 Liter (L) of water at 4 degrees Celsius= 1000 grams (g). We can convert between different proportions using dimensional analysis.

The International System of Units (SI) Introduction

(Video) The Metric System and SI Units

by Mr. Causey

This video by Mr. Causey is a great introduction to the standard systems of measurements that are used in various fields of chemistry, and in all areas of science in general. The standard systems help to keep our quantitative data accurate and precise. We'll also use it when we study significant figures, uncertainty, and types of error.

First, he describes the metric system, which is based on powers of ten. Next, he talks about the seven SI base units, which can be used to find derived units. Then, he talks about the CGS system, which uses smaller units that are more lab friendly.

Finally, Mr. Causey discusses the most common prefixes and units that you will see in a chemistry lab.