SI Base Units/Derived units

System International Base Units

Unit 2

Measurement

• Chemistry involves collecting and

analyzing data.

• Data collected by chemists is metric.
• Scientists use standards-exact quantities that people use for comparison

International System of Units

• Metric System-late 1700s
• 1960 accepted worldwide
• Easy because it is based on multiples of 10

Base Unit

• Unit in a system of measurement that is

based on a physical object or event

• Prefixes may be added to the unit to

change its magnitude or size of the unit.
– Ex: centi- centimeter 1/100 of a meter

​There are 7 base units listed in your textbook. We use 5 in chemistry.

Length

• Distance between two points
• Meter-based on the distance the light

travels through a vacuum in 1/299 792 458
of a second

• m

Time

• Interval between two events
• Second-based on the time that a frequency of radiation is given off by a cesium atom

• s

Mass

• Amount of matter in an object
• Kilogram-based on a physical object of a

platinum iridium cylinder kept in a
museum in Sévres, France.

• Kg

• Note: in lab we use the gram because we

use smaller amounts of substances

Temperature

• The measure of kinetic energy of particles
• 273 K water freezes(0 degrees celsius)
• 373 K water boils (100 degrees celsius)
• 0 K absolute zero (no molecular movement
-273 degrees celsius)

• Kelvin is the SI Base Unit
• Note: Scientists also use Celsius in labs

it is easy to produce, but can easily be converted
to Kelvin. We will use celsius, as well.

Mole

• Amount of particles in a specific mass of a

substance

• Mole- 6.02 x 10²³ particles(atoms,

molecules or formula units)

• mol

What is a derived unit?

​a derived unit is a unit that combines base units and requires a formula to calculate(It is derived from a calculation)

• Volume is the amount of space an object

occupies.

• Regular shaped solid objects such as a

cube can be calculated by the formula
v=lwh

Derived Units used in

Chemistry:

Irregular shaped objects can be
measured by water displacement.
ml or cm³
1ml = 1 cm³

Liquids can be measured in a graduated
cylinder ml
Units of volume: m³ L ml cm³

• Density: the ratio of mass to volume of an object

• All elements have densities that do not change

• Density = mass
• volume
• Units: g/cm³ g/ml Kg/m³