SI units in chemistry refer to an international measurement system that’s been agreed upon by scientists around the world. The seven base SI units measure:
- Length
- Time
- Amount of substance
- Electric current
- Temperature
- Mass
- Luminous intensity
Taking precise and accurate measurements is crucial in industrial, manufacturing (especially chemical manufacturing!), and academic settings. Not only does it help us to determine the composition and molecular structure of unknown substances, but accurate measurements also ensure instruments are properly calibrated.
Using the International System of Units makes it easier to record and share key measurements with others worldwide. Read on to learn more about SI units in chemistry, why they’re used, and the different types.
In this post:
What are SI units?
The SI units include both the seven base units and various derived units. They’re part of the International System of Units, which is the modern metric system. Virtually all countries use SI units in trade and commerce. Car manufacturers and car parts suppliers, for example, must agree on the exact dimensions of the components or else nothing would fit.
Some countries use the SI system along with other measurement systems like the British Imperial System. Ironically, in the UK, the SI system is the preferred system of measurement. Meanwhile, in the US, the imperial system is still commonly used because many Americans are more accustomed to it.
The SI system is the most widely adopted system of measurement mainly because of the convenience it brings. It’s more systematic and rational compared to other awkward systems of measurement. As the system is based around the number ten, it’s easy to convert from one unit into another.
For example, to convert a metre into a kilometre and vice-versa, you simply need to multiply or divide by a factor of one thousand. Hence, one kilometre is equivalent to 1,000 metres. Compare this to the clunky imperial system, in which one mile is equivalent to 5,280 feet, and the advantages are obvious.
Why do scientists use the international system of units?
Scientists need to use the International System of Units to facilitate research and collaboration. Without a consistent form of measurement it would be virtually impossible to reproduce and verify experiments.
As a scientific pursuit, any significant research work in chemistry has to be peer-reviewed. This process involves checking the measurements, data, and conclusions to ensure they’re valid and reliable. This simply wouldn’t be achievable (or at least it would be very confusing) if there was no standard system of measurements.
A standardised framework like the International System of Units allows chemists and other scientists to collaborate internationally and share results in a precise and accurate manner. It also minimises the probability of errors and helps to overcome language barriers.
International system of units
The International System of Units, or SI, is the modern metric system that’s been agreed upon by the members of the General Conference on Weights and Measures.
It’s maintained through rigorous scientific standardisation of measurement based on physical constants and well-established laws. For example, one second is defined as the specific cycles of radiation produced by the caesium-133 atom between two levels.
Almost all countries in the world, with a few exceptions, use the SI units of measure as their standard units. The system is coherent and convenient to use. As we explain below, it has seven base units – metre, second, mole, ampere, kelvin, kilogram, and candela.
What are the 7 SI units?
The seven SI units are called the base units because other units are derived from them. They’re either combined with other units or converted into smaller or bigger units of themselves. 
The seven SI base units measure length, time, amount of substance, electric current, temperature, mass, and luminous intensity.
1. Length
Metre is the standard SI unit for length. It can easily be converted to larger units like kilometres or smaller ones like centimetres by using a factor of ten. The metre is almost equal to a yard but it’s scientifically defined based on the speed of light in a vacuum. It is the distance travelled by light in a vacuum in 1/299,792,458th of a second.
2. Time
The SI unit of time is the second. It’s based on the ancient time-keeping and number system of dividing a day into 24 hours, 60 minutes per hour, and 60 seconds per minute. The modern scientific definition of the second is the 9,192,631,770 cycles of radiation between two energy levels in a cesium-133 atom.
3. Amount of substance
The amount of substance can be measured in different ways, such as mass, volume, density, and divisible units. In chemistry, all of these are used but the standard for predicting and calculating chemical reactions is the mole. A mole of a substance is equivalent to the total molar mass of the elements of a substance.
4. Electric current
Electricity can be produced through spontaneous oxidation-reduction reactions of two different electrodes (anode and cathode) in a galvanic cell. Any two electrodes with different electric potentials can produce electricity. The current is the actual amount of electrons that flow through a circuit. Measured in amperes, it’s the equivalent of 6.241509074×1018 electrons worth of charge passing a point in one second. 
5. Temperature
Various units of temperature are used in chemistry, including the Celsius and Fahrenheit scales. However, the more scientific SI unit of temperature is the Kelvin scale. On the Kelvin scale, zero is absolute zero, which is equivalent to −273.15 °C. It’s the theoretical temperature wherein the vibrational motions of atoms and molecules cease.
6. Mass
Although the gram is more commonly used in chemistry laboratory experiments for the sake of convenience, the kilogram is the base SI unit. The scientific definition of the kilogram is quite tricky to understand. It’s determined by three fundamental physical constants, which are the cesium atom transition frequency, the speed of light, and the Planck constant.
7. Luminous intensity
Atoms emit specific wavelengths of light depending on how their electrons are excited to jump to the next energy level and back. Elements can be identified in this manner and it’s also how scientists determine the composition of stars.
In the SI system of units, the luminosity of an object that emits light is measured in candela. This is the measure of luminous power per unit solid angle. As the name implies, it’s roughly based on the luminosity of a standard candle. It’s sometimes referred to as candle power.
Summary
As a scientific discipline, chemistry requires accurate and precise measurements. These measurements must also be standardised to facilitate peer review and international communication. The seven base SI units in chemistry measure length, time, amount of substance, electric current, temperature, mass, and luminous intensity.
