Measurements
Chapter 4
Accuracy and Precision

Accuracy = how close a single
measurement comes to the actual or true
value of the quantity measured.


Ex. If you need 10 grams of salt, and the
amount that you scale registered is 10 grams,
then the measurement is considered accurate.
Precision = how close SEVERAL
measurements are to the same value.

Ex. To attain precision, you need to make
more than one measurement and compare the
results
The Metric System

Metric System = A common
language used all around the world
for measurement.



Beverages are sold by the liter
Food labels indicate the mass in grams
Your car has both miles and kilometers
Metric Units



In the metric system, the unit for measuring
length is the meter (m)
Gram (g) to measure mass
Liter (L) to measure volume
Metric Prefixes

The metric system is based on the
decimal system = numbers are
expressed in units of ten.

Prefixes are used to indicate what multiple or
fraction of the base unit is used in a given
situation.


These prefixes can be combined with any unit of
measure. Ex, the prefix kilo means 1000. Just as
1000 meters is equal to 1 kilometer, 1000 grams
equals 1 kilogram and 1000 liters equals 1
kiloliter.
The prefixes mili, centi and deci describe
fractions of a meter.

For example, a centimeter is one one-hundredth
(0.01) of a meter.
Metric Prefixes
Prefix
Meaning
Numerical
Value
Unit
Example
Relationship
to Unit
Kilo (k)
Thousand
1000
Kilometer
(km)
1 km = 1000
Deci (d)
Centi ©
Mili (m)
Tenth
Hundredth
Thousandth
0.1
0.01
0.001
Decimeter
(dm)
Centimeter
(cm)
Millimeter
(mm)
10 dm = 1 m
100cm = 1m
1000mm = 1m
Comparison of Metric and
English System
(Comparison Chart)
Metric
Unit
Symbol
Measurement Visualizations
English
Equivalent
Gram
Kilogram
g
kg
A little heavier than a paper clip
Almost nine sticks of butter
0.035 oz
2.2 lb
Meter
Kilometer
Decimeter
Centimeter
millimeter
m
km
dm
cm
mm
From the floor to about the doorknob
About two-thirds of a mile
About the diameter of an average grapefruit
Length of a staple
Width of a dime
1.09 yards
0.62 miles
3.94 inches
0.39 inches
0.039 inches
Liter
militer
L
mL
Just over a quart of milk
A little less than ¼ teaspoon
1.057 quarts
0.03 fluid oz
Estimating

As you use the scales on laboratory equipment,
make it a habit to estimate to one decimal place
past the unit of measure marked on the scale.
 Ex. suppose you’re using a 100-ml graduated
cylinder marked to whole milliliters, and the
bottom of the meniscus is between 24 and 25
mL. You would report this as 24.3 mL or 24.
8 mL, depending on how far above 24 mL the
volume is. Anyone who reads the data table
will know that the last digit is just an
estimate, but it will tell them whether the
volume was closer to 24 mL or 25 mL.
Recording Final Zeros

What is the bottom of the meniscus is
exactly on the 24-mL line? You need to
write 24.0 ML, you make it clear that the
volume is exactly 24 mL.


By recording the final zero, you report your data
more precisely.
The rule of recording data also applies to using
an electronic balance.
Measuring in Metric

Measuring Length
Use a metric ruler or meter stick
 Millimeters are generally the smallest
division
 Ex. there are 10mm in 1 cm
 Each mm line, represents a tenth of a
centimeter

Measuring Length
Measuring Volume
Questions
1.
2.
3.
4.
5.
6.
7.
8.
Convert
Convert
Convert
Convert
Convert
Convert
Convert
Convert
5 inches to centimeters.
4.5 ft. to meters.
45 miles to kilometers.
1 ½ gal. to liters.
4.7 liters to qt.
110 lbs to kilograms.
6oz. to grams.
1.2 acres to hectares.
The Celsius Temperature Scale

Fahrenheit scale was invented in the early
1700’s by a German physicist, Gabriel
Fahrenheit.


Anders Celsius, Swedish astronomer
developed it in 1742.


He used mercury instead of alcohol for making
the thermometer more accurate.
The scale was formerly called the centigrade
scale, which means divided into 100 parts.
When measuring temperature, be sure
the thermometer bulb is completely
immersed in the container, but not
touching the sides or bottom.
Note that 100
degrees separate
freezing and boiling
points on the
Celsius scale. One
the Fahrenheit
scale, however the
difference is 180
degrees. Therefore
a Celsius degree
represents almost
twice the
temperature
changes as a
Fahrenheit degree.
Ch. 4 Questions
1.
2.
3.
4.
5.
Can you get precise results when using
a faulty thermometer?
How are units of measurement related
to the base unit in the metric system?
What would the measurement “6 dL” tell
you about a substance
Should you expect to pay more for a qt.
of milk or a liter?
How many 115-g servings of fish can
you get from a 0.75-kg package of fish
fillets?
Ch. 4 Questions Continued
6.
7.
8.
9.
If a graduated cylinder is marked in
whole milliliters, why wouldn’t you
report a volume as 25.55mL?
What is the significance of the zero in
the measurement 15.60 g?
Generally, how do you choose a
measuring device for a given task?
Why might you have trouble accurately
measuring the temperature of a solid
with an English laboratory thermometer?