Science involves taking measurements, analyzing data, and sharing it with others. Whether you're an engineer, chemist, biologist, physicist, or medical doctor, you need a consistent way to communicate measurements like mass, temperature, time, amount, and distance. To make sure everyone can understand each other, scientists all over the world use a common system of units called SI units.

In this article, we'll dive into the world of SI units in chemistry. First, we'll explain what base units and derived units are. Then, we'll focus on some of the most important SI units, including pressure, mass, volume, and temperature. So, get ready to learn about the language that scientists use to communicate their measurements! And if you're studying chemistry or any other scientific field, understanding SI units will be crucial to your success.

SI units definition for chemistry

Nowadays, the most widely used system of units is the International System of Units or SI. The term SI stands for Systeme International d'Unites, which is French for International System of Units. So, when we talk about SI units, we're referring to the units of measurement used in this system.

Base units

There are 7 base units in the SI system. Each of these shows a different physical quantity. The seven SI base units, which are comprised of length, time, amount of substance, electric current, temperature, luminous intensity, and mass, are listed in Table 1 below:

Quantity UnitSymbol
Lengthmeterm
Timeseconds
Masskilogramkg
Electric current ampere A
Temperature KelvinK
Amount of a substancemolemol
Luminous intensity candelacd

The unit candela (cd) comes from the Italian word for candle and is referring to the “candlepower” which was used in the past when candles were the main means of illumination for people.

Derived units

Apart from the seven base units, there are other quantities that are mathematically derived from the seven base units. These derived units are called derived units and are expressed in terms of the base units. Some common examples of derived units are shown in Table 2 below:

Quantity Unit Symbol
Area Square meter m2
Volume Cubic meter m3
Density Kg per cubic meter kg m-3

So, when working with derived units, you can always trace them back to the base units from which they were derived. This makes it easier to understand the relationships between different units of measurement.

In addition to derived units, there are also special symbols assigned to certain specific quantities commonly used in chemistry. These symbols are used as SI units to simplify the representation of the units. Table 3 below shows some of the most important of these quantities and their special symbols, along with a breakdown of their explanations into their SI units:

Quantity Unit Explanation
Force N Newton= kgms-2
Pressure Pa Pascal = Nm-2
Energy J Joule= Nm
Electric potential V Volt= J/C
Electric charge C Coulomb = A*s
Power W Watt = J/s

As you progress through your studies in chemistry, you will become more familiar with these special symbols and their corresponding SI units.

SI units of pressure in chemistry

Atmospheric pressure is commonly measured using an instrument called a barometer. The derived unit of pressure is the Pascal, named after Blaise Pascal who was a French mathematician and physicist. One Pascal (symbol Pa) is equivalent to one Newton per square meter, as shown in the table above. This makes sense when one considers that Pressure is defined as the amount of force applied over a certain area divided by the area size. So, why is it important to be familiar with this? Sometimes, certain measurements are taken in other units, which were or are more common, for example Celsius for temperature measurements or mmHg for pressure. When applying those measurements to calculations it will be necessary to convert those measurements into their SI units. Here’s a simple example below:

On a particular day, the atmospheric pressure was measured to be 780 mmHg. Calculate the pressure in Pascals. Since standard atmospheric pressure is 760 mmHg which is equal to 101.3 Pa, then in order to convert 780 mmHg to Pa, all you need to do is the following:= 103.96 Pa, which can be rounded up to 104 Pa.

SI unit for mass

Knowing how to convert between different units of mass, such as grams, milligrams, and kilograms, is essential in chemistry calculations. It is important to remember that the SI unit for mass is the kilogram, and that the prefix kilo means 1000 or 10^3. Therefore, 1 kg is equal to 1000 grams or 10^6 milligrams.

As an example, let's say you need to convert the mass of a 220 mg Paracetamol tablet to grams. To do this, you would need to divide 220 by 1000 or multiply it by 10^-3. This gives you a mass of 0.22 grams.

If you were asked to convert the same mass to kilograms, you could either convert milligrams to grams by multiplying by 10^-3 and then convert grams to kilograms by multiplying again by 10^-3, or you could convert milligrams directly to kilograms by multiplying by 10^-6. In either case, the answer you obtain is 2.2 x 10^-4 kg.

Therefore, understanding unit conversions is crucial in chemistry calculations, and being familiar with the prefixes used in the SI system can make these conversions easier and quicker.

SI unit for volume

Understanding the relationship between different units of volume and temperature is important in chemistry calculations. The SI unit for volume is the cubic meter (m3), which can be easily interconverted with the commonly used unit, the litre (L). 1 m3 is equal to 1000 L, 1 L is equal to 1000 mL or 1000 cm3, and 1 mL is equal to 1 cm3. To convert between units, divide by 1000 to convert to a larger unit, and multiply by 1000 to convert to a smaller unit.

The SI unit for temperature is the Kelvin (K), which is related to the Celsius (oC) scale. One degree Celsius is an interval of 1 Kelvin, and 0oC is equal to 273.15 K. To convert from Celsius to Kelvin, simply add 273 to the Celsius temperature. It is important to take note of which units are required for the final answer in any chemistry problem and to carry out any necessary unit conversions.

SI units chemistry

What are the SI units in chemistry?

SI units refers to an international system of units which has been agreed upon and is used by all scientists around the world. There are seven base SI units. These are meter (m), kilogram (kg), second (s), ampere (A), Kelvin (K), mole (mol) and candela (cd).

What are derived units?

Derived units are other quantities which are related to and mathematically derived from the seven basic units.

What are some examples of derived units?

Some common derived units are square meter (m2), cubic meter (m3) and kilogram per cubic meter (kg m-3).

What is the SI unit for mass?

The SI unit for mass is the kilogram, symbol kg.

What is the SI unit for length?

The SI unit for length is the meter, symbol m.

What is the SI unit for volume?

The SI unit for volume is the cubic meter, m3.

What is the SI unit for temperature?

The SI unit for temperature is Kelvin, symbol K.

What is the SI unit for pressure?

The SI unit for pressure is Pascal, symbol Pa.

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