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Science loves units. But, then again, they are a necessity. If you are going to measure something, you will need a way to tell people how much you have. Here are only a few units that we use in chemistry...


You use percentages when you need to know how much of something you have compared to the whole amount. You can figure out the percentage by dividing and then multiplying by 100. Divide the final amount by the original amount or divide the amount you have by the total. Then bring in that 100 and you get a percent. And yes, you can have percentages greater than 100%.

- "I have 20% of what I started with." That means you lost 80%.

- "Only 0.1 grams of the total one gram of material is left." It looks like only 10% is left.

- "Start with 10 grams and burn off 8 grams." There will only be two grams left. 2/10 is 20%.

Grams, Kilograms, Milligrams

These are measures of mass. A gram is the base measurement. The prefix in the word tells you if you have more or less than a gram. A Kilogram is 1000 times the mass of one gram. A milligram is one one-thousandth the mass of a gram.

- "This candy bar has a mass of only a few grams."

- "A man might weight almost one hundred kilograms."

Liters, Milliliters

These are measures of volume. They also use the same prefixes as grams.

- "I just bought a 2 liter bottle of soda."

- "Just add three milliliters of solution into the flask."

Celsius, Fahrenheit, and Kelvin

These are different measurements scientists use for temperatures.

- "It's freezing outside." That could mean it is 0oC, 32oF, or 273K (just Kelvins, no degrees).

Atomic Mass Units

Atomic mass units are the way scientists measure the mass of different atoms. In atomic mass units, one proton or neutron is counted as about one amu.

- "Carbon-12 has a mass of 12 amu."

- "Carbon-14 has a mass of 14 amu because it is an isotope with two extra neutrons."


A calorie is a unit of energy. You will hear a lot about kilocalories in chemistry and food science.

- "A calorie is the amount of energy needed to raise the temperature of 1 gram of water 1 degree Celsius."


A joule is a standard unit of energy. Chemistry and physics use many types of measurements so you will find both calories and Joules. Calories were used before Joules were used as standard units.

- "4.18 Joules is the same as 1 Calorie."

Seconds, Minutes, and Hours

As you already know, these are measures of time. You can use these measurements to study rates of reactions.

- "There are 60 seconds in a minute and 60 minutes in an hour."

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Useful Reference Materials (SI Units):
Encyclopædia Britannica (International System of Units):

- Biology4Kids: Scientific Method
- Geography4Kids: Science Instruments
- Physics4Kids: Temperature Scales

- National Institute of Standards and Technology

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