Units and Unit Conversions

SI units

Scientists around the world use the same base units, called SI units.

Prefixes

A prefix in front of a unit makes it bigger or smaller by a power of ten.

The conversion rule

Volume

Volume units trip up many students. Learn these links.

$$1\text{ L}=1000\text{ mL}1\text{ mL}=1{\text{ cm}}^{3}1\text{ L}=1{\text{ dm}}^3=1000{\text{ cm}}^3$$

Temperature

Chemists use two scales. The kelvin scale starts at absolute zero, the coldest possible temperature, where particles have the least possible energy.

$$T(\text{K})=T{(}^{\circ }\text{C})+273T{(}^{\circ }\text{C})=T(\text{K})-273$$

A change of 1 °C is the same size as a change of 1 K. There are no negative kelvin temperatures, and we write "300 K", not "300 °K".

Scientific notation

Chemists deal with very big and very small numbers. Scientific notation writes them as a number between 1 and 10 times a power of ten.

$$602000000000000000000000=6.02\times 10^{23}$$ $$0.0000000001\text{ m}=1\times 10^{-10}\text{ m}$$

• Moving the decimal point left makes a positive power (big number).
• Moving it right makes a negative power (small number).

Quick summary

• SI units: kg, m, s, K, mol. Chemists often use g, mL, L and °C too.
• kilo = × 1000, centi = ÷ 100, milli = ÷ 1000.
• Big unit to small unit: multiply. Small to big: divide.
• 1 L = 1000 mL and 1 mL = 1 cm³.
• K = °C + 273.
• Scientific notation: a number from 1 to 10 times a power of ten.

Quiz