SI Units and Making �Precise Measurements
Vocabulary:
“The Pantheon”, architect: Apollodorus of Damascus, 125 CE
Measurement
A lot of science is qualitative (observations that do not involve measurement.)
Examples:
Classifying organisms
Interpreting rock strata
Understanding the Skeletal System
Measurement
However, a very important aspect of science is that much of it is quantitative (observations that involve measurements)
Examples:
Determining the amount of CO2 in the atmosphere
Measuring the age of the earth
Calculating the amount of energy produced by uranium
Standardization
In order for scientists to communicate quantitative measurements, there must be standardized units (units that are agreed upon between scientists.)
Imperial (or English or Standard) System
Imperial units are the units we use most of the time here in the US.
Length: the distance from one point in space to another
Imperial (or English or Standard) System
Imperial units are the units we use most of the time here in the US.
Mass: the amount of matter (atoms) in a substance
Imperial (or English or Standard) System
Imperial units are the units we use most of the time here in the US.
Volume: the amount of space a substance occupies
Imperial (or English or Standard) System
To convert between these units require conversion factors. For instance:
1 foot = _____ inches
1 gallon = _____ quarts
1 min = _____sec
1 mile = _____ feet
12
4
60
5280
Imperial (or English or Standard) System
More conversion factors (and this is just for volume!)
Imperial (or English or Standard) System
Even though the units are standardized, the complicated and inconsistent nature of the Imperial System is why only 3 countries out of 192 in the world use it.
Countries that use the Imperial system:
SI (International System)
All 189 other countries use the more consistent and logical SI units. They were adopted in 1960 by the General Conference of Weights and Measures and are founded on some base units:
length: meter (m)
mass: kilogram (kg)
time: seconds (s)
and some others we won’t cover right now
What are some other SI units besides the base units?
length:
mass:
volume:
Notice that each type of measurement can have the same prefixes: kilo, centi, milli, etc.
SI (International System)
m, km, cm, mm, etc.
kg, g, cg, mg, etc.
L, mL, kL, cL, etc.
That’s what makes the SI units so appealing.
1 meter = 1000 millimeters
1 gram = 1000 milligrams
1 liter = 1000 milliliters
1 second = 1000 milliseconds
The prefixes are consistent no matter what measurement you are making and all conversions are based on factors of 10.
SI (International System)
But...
We will talk more about metric conversions at another time.
Now we’re going to turn our attention to making measurements.
Lab Time!
Making Precise Measurements
Precise measurements
What is the volume in the graduated cylinder?
Things to think about:
Precise measurements
What is the volume in the graduated cylinder?
Always estimate and measure at the bottom of the meniscus between lines!
Precise measurements
So the volume is 52.8 mL.
Significant Figures
Any digit in a measured value is called a “significant figure.”
How many significant figures are in the previous measured value?
52.8 mL
3 significant figures
Let’s try again…
What is the reading on the triple beam balance shown below?
Let’s try again…
What is the reading on the triple beam balance shown below?
Remember to estimate between the lines!
Let’s try again…
373.34 g
Let’s try again…
373.34 g
How many significant figures?
Let’s try again…
373.34 g
How many significant figures?
5
Let’s try again…
Let’s try again…
The reading is 6.75 mL
Let’s try again…
The reading is 6.75 mL
How many sig figs?
Let’s try again…
The reading is 6.75 mL
How many sig figs?
3
And again…
And again…
The reading is 2.34 cm
How many sig figs?
And again…
The reading is 2.34 cm
How many sig figs?
3
What about this one?
What about this one?
The reading is 2.20 cm
How many sig figs?
What about this one?
The reading is 2.20 cm
How many sig figs?
3
Now….
Time to practice on your own!