Science involves taking measurements, looking at this data, and sharing this data with others. Whether you are an engineer, chemist, biologist, physicist, or medical doctor, you need a consistent way to communicate measurements such as mass, temperature, time, amount, and distance, amongst others. You need to be understood by all scientists all over the world. This is why a common system of units was needed and developed. It basically allows scientists from all over the world to communicate measurements using this common “language”.
- This article is about the SI units in chemistry.
- We'll first look at the definition and explanation of the base units and derived units.
- We'll then focus on some of the most important SI units, covering the SI units for pressure, mass, volume, and temperature.
SI units definition for chemistry
Although various systems of units have been used over the years, nowadays the most commonly used one is the International System of Units. The abbreviation SI comes from the French term Systeme International d’Unites. So, this is why we refer to them as SI Units.
Base units
There are 7 base units in the SI system. Each of these shows a different physical quantity.
A base unit is a fundamental unit in the SI system that is based on an established standard and which can be used to derive other units.
These are shown in Table 1 below:
Quantity | Unit | Symbol |
Length | meter | m |
Time | second | s |
Mass | kilogram | kg |
Electric current | ampere | A |
Temperature | Kelvin | K |
Amount of a substance | mole | mol |
Luminous intensity | candela | cd |
Table 1: SI base quantities and units
The unit candela (cd) comes from the Italian word for candle. This 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 these seven basic units, there are other quantities which are related to and mathematically derived from the seven basic units. This is why we refer to them as derived units.
A derived unit is a unit of measurement derived from the seven base units of the SI system.
Some common examples 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 |
Table 2: Derived quantities and their SI units
So, it is clearly visible that derived units are expressed in terms of base units. This means that you can work out the relationship of a derived unit using the base units.
For certain specific quantities which are commonly used in chemistry, special symbols have been assigned to them. These are there to simplify the symbols which represent the units. In this case, we use these special symbols as SI units. You shall become very familiar with these throughout your chemistry studies. The most important of these are shown in Table 3 below:
Quantity | Unit | Explanation |
Force | N | Newton= kg*m*s-2 |
Pressure | Pa | Pascal = N*m-2 |
Energy | J | Joule= N*m |
Electric potential | V | Volt= J/C |
Electric charge | C | Coulomb = A*s |
Power | W | Watt = J/s |
Table 3: Common quantities and their special symbols. Breakdown of explanations into their 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 780mmHg. Calculate the pressure in Pascals.
Since standard atmospheric pressure is 760mmHg which is equal to 101.3Pa, then in order to convert 780 mmHg to Pa, all you need to do is the following:
$$780mmHg\cdot \frac{101.3Pa}{760mmHg}=103.96Pa$$Which can be rounded up to 104 Pa.
SI unit for mass
The SI unit for mass is the kilogram (symbol kg). An interesting point about the kilogram is that it is the only one amongst the SI base units whose name and symbol include a prefix. The prefix kilo means 1000 or 103, meaning that 1 kg is 1 x 103 grams. 1 milligram is 1 x 10-3 grams, meaning that it is 1 x 10-6 kg.
Why do you need to know this? This is important to know since it will be necessary to convert units such as grams or milligrams into kilograms or vice-versa in chemistry calculations.
Let's have a look at a practical example of this. Let's say you are asked to convert the mass of a 220 mg Paracetamol tablet into grams. You will need to use the conversion factor given above for your calculation. So, in this case, you would need to divide 220 by 1000 or alternatively multiply 220 by 10-3:
220mg = ?g
$$\frac{220mg}{1000}$$
or
$$220mg\cdot 10^{-3}=0.22g$$
You will get the same answer in both cases i.e. 0.22 grams. Simple, right?
Now, let's try a more complex conversion. In this case, you are being asked to convert 220mg to kg. There are two ways in which you can do this. You could either first convert milligrams to grams by multiplying by 10-3 and then convert grams to kilograms by multiplying again by 10-3.
$$220mg\cdot 10^{-3}=0.22g$$
$$0.22g\cdot 10^{-3}=2.2\cdot 10^{-4}kg$$
Alternatively, you could convert mg to kg directly by multiplying the amount in mg by 10-6. This would give you your answer in kg directly. In both cases, the answer you obtain is 2.2 x 10-4 kg.
$$220mg\cdot 10^{-6}=2.2\cdot 10^{-4}kg$$
SI unit for volume
The SI unit for volume is the derived unit cubic meter (m3). This is related to the commonly used unit litre (L). The two can be easily interconverted using the following relationship:
1 m3 = 1000 L
Since in chemistry we usually work with volumes that are smaller than 1000 litres, it is useful to know that 1 L = 1000 cm3 and 1 L = 1000 mL.
Once again, we usually work with smaller volumes than this when performing experiments in the chemistry lab. This is why we commonly use a smaller unit of volume which is the millilitre, symbol mL. The use of the capital L is not a mistake but standard practice and the correct way to write the unit.
1 mL = 1 cm3
So, basically 1 L = 1000 mL = 1000 cm3
Once again, the conversion factor is 1000. So, you need to divide your volume by 1000 to convert it to the larger unit, let's say from mL to L. And you need to multiply your volume by 1000 in order to convert it from the larger unit to the smaller one, for example liters to millilitres.
SI unit for temperature
The SI unit for temperature is the Kelvin, represented by the symbol K. If you remember, this is also one of the seven base SI units. It is very useful to know the relationship between the Kelvin and degrees Celsius (oC) since we tend to be more familiar with this unit of measurement.
1 degree Celsius is an interval of 1 K. Specifically, 0oC = 273.15 K
So, basically, all you have to do to convert temperature in degrees Celsius to Kelvin is to add (not multiply!) 273 to it.
For example, you need to work out a chemistry problem where you are given the temperature in oC but are asked to do the calculation and give your answer in K. This means that you first need to convert your temperature from degrees Celsius to Kelvin. If, for example, the temperature given is 220oC, you just need to do the following:
$$273 + 22 = 295 K$$
It is very important to take note of which units you are asked to give your answer in and not to forget this conversion step!
SI units chemistry - Key takeaways
- SI units refers to an international system of units.
- There are seven base SI units. These are meter (m), kilogram (kg), second (s), ampere (A), Kelvin (K), mole (mol) and candela (cd).
- Apart from these base units, there are derived units. These are other quantities which are related to and mathematically derived from the seven basic unit.
- For certain specific quantities which are commonly used in chemistry, special symbols have been assigned to them, such as the symbol Pa for pressure.
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