SI Units (Simplified): Study with Video Lessons, Practice Problems & Examples

The International System of Units (SI) comprises six base units: mass (kilogram, kg), length (meter, m), time (second, s), temperature (kelvin, K), amount of substance (mole, mol), and volume (cubic meter, m³). While the SI unit for volume is cubic meters, liters (L) are more commonly used in practice. Understanding these units is essential for accurate measurements in scientific calculations, particularly in chemistry, where concepts like moles and their applications are frequently encountered.

SI Units (Simplified) Concept 1

We're going to say here that the international system of units, SI, is related to the metric system, and it comes with 6 base units. Now, when it comes to these 6 base units, let's take a look. We have our physical quantity, and each of these physical quantities is attached to an SI base unit. This SI base unit name has a symbol. So, when it comes to mass, the SI base name is kilogram, and its symbol is kg. Okay? So, the SI base unit for mass is kilograms. Next, we have length. Length would be meters, which is abbreviated as m. Time would be seconds, which is abbreviated as just s. Temperature. The SI unit for temperature is kelvin, which is abbreviated as K, capital K. The amount of a substance, so you need to familiarize yourself with this particular term in terms of chemistry. The amount of the substance is the mole and its symbol is mol. So, we were just to remove the e. Now, when you're doing calculations with moles later on in chemistry, you'll see them use either the full name or the symbol name. K? So just remember they're the same thing. Now for volume here, you'll see that there are 2 spots. That's because the top one is the SI base unit that we use, but you'll find time and time again that you'll not see the SI base unit used for volume, instead, you'll see the metric one. So the SI base unit for volume is going to be our cubic meters, which is m3. And here, the metric counterpart is liters. You'll see liters more often than you will see cubic meters. And the symbol for liters is just a capital L. So here we have our 6 base units, as well as 1 metric unit. In terms of volume, the metric unit of liters is oftentimes used more than the SI unit of meters cubed.

SI Units (Simplified) Example 1

Which of the following values is given with SI units? So here we have 1.25×104 minutes, 6.82×10-3 milligrams, 25.6 degrees Celsius, and 101.3 seconds. Alright. So minutes is a unit of time. We know the SI unit for time is seconds, not minutes. Next, milligrams is our units for mass, but we know kilograms is the SI unit for mass. Celsius, units for temperature, but we know temperature should be in Kelvin when we're dealing with SI units. Here, out of all our options, option d is the correct choice. Seconds is an SI base unit.

Which of the following represents volume in SI units? 

• A. 2.71×10^–1 cm³
• B. 207.2 km²
• C. 5.81×10⁵ m³
• D. 582.28 mm

Convert 12 minutes into acceptable SI units.

• A. 720 s
• B. 0.20 s
• C. 3.33×10^–3 s
• D. 4.32×10⁴ s

Here’s what students ask on this topic:

What are the six base units of the International System of Units (SI)?

The International System of Units (SI) comprises six base units, each corresponding to a fundamental physical quantity. These are:

• Mass: kilogram (kg)
• Length: meter (m)
• Time: second (s)
• Temperature: kelvin (K)
• Amount of substance: mole (mol)
• Volume: cubic meter (m³)

These units are essential for accurate measurements in scientific calculations, particularly in fields like chemistry and physics.

Why is the liter more commonly used than the cubic meter for volume measurements?

While the SI base unit for volume is the cubic meter (m³), the liter (L) is more commonly used in practice because it is more convenient for everyday measurements. One liter is equivalent to 0.001 cubic meters (1 L = 0.001 m³), making it a more practical unit for measuring smaller volumes, such as liquids in laboratory settings or daily life. The liter's smaller size and ease of use make it the preferred unit for most volume measurements outside of scientific research.

What is the SI unit for temperature and its symbol?

The SI unit for temperature is the kelvin, abbreviated as K. Unlike other temperature scales such as Celsius or Fahrenheit, the kelvin scale starts at absolute zero, the theoretical point where all molecular motion ceases. This makes it particularly useful in scientific contexts where precise temperature measurements are required. For example, 0 K is absolute zero, and 273.15 K is the freezing point of water.

How is the amount of substance measured in the SI system?

In the SI system, the amount of substance is measured in moles, abbreviated as mol. One mole is defined as the amount of substance that contains as many elementary entities (such as atoms, molecules, or ions) as there are atoms in 12 grams of carbon-12. This number is known as Avogadro's number, approximately 6.022 × 10²³ entities per mole. The mole is a fundamental unit in chemistry for quantifying the amount of a substance involved in chemical reactions.

What is the SI base unit for mass and its symbol?

The SI base unit for mass is the kilogram, abbreviated as kg. The kilogram is defined by the mass of the International Prototype of the Kilogram, a platinum-iridium alloy cylinder stored in France. It is the only SI base unit still defined by a physical object, although efforts are ongoing to redefine it based on fundamental constants. The kilogram is widely used in scientific, industrial, and everyday contexts for measuring mass.