Collecting Evidence
Introduction to Measurement
Chemistry and Physics of Forensics
Ms. Grobsky
Types of Observations and
Measurements
• We make QUALITATIVE
observations of reactions
– Some examples include:
» Changes in color
» Physical state
• We also make QUANTITATIVE
MEASUREMENTS, which involve
numbers
Stating a Measurement
• In every measurement there is a….
Number followed by a
 Unit from a measuring device
The number should also be as precise as the measurement!
Standards of Measurement
• When we measure, we use a measuring tool to compare
some dimension of an object to a standard
For example, at one time the standard
for length was the king’s foot. What
are some problems with this standard?
STANDARD UNITS OF
MEASUREMENT
• Standard units of measurement are
called SI units which is based on the
metric system
Length
Meter, m
Mass
Kilogram, kg
Volume
Liter, L
Time
Seconds, s
Temperature
Celsius degrees, ˚C
Kelvin, K
NOTE – Mass is NOT Weight
• Mass is the
amount of matter
in grams
– Measured with a
BALANCE
• Weight is the force
exerted by mass,
only when present
with gravity
– Measured in pounds
with a SCALE
Can you hear
me now?
Some Tools for Measurement
Which tool(s)
would you use to
measure:
A. Temperature
B. Volume
C. Time
D. Weight
Learning Check
Match
L) length
M) mass
V) volume
M A.
____
A bag of tomatoes is 4.6 kg.
L B.
____
A person is 2.0 m tall.
M C.
____
A medication contains 0.50 g Aspirin.
V
____ D. A bottle contains 1.5 L of water.
Metric Prefixes
• Sometimes, the base SI unit is not always convenient so
instead, we use different prefixes!
• Kilo- means 1000 of that unit
– 1 kilometer (km) = 1000 meters (m)
• Centi- means 1/100 of that unit
– 1 meter (m) = 100 centimeters (cm)
– 1 dollar = 100 cents
• Milli- means 1/1000 of that unit
– 1 Liter (L) = 1000 milliliters (mL)
Metric Prefixes
Metric Prefixes in Action
Learning Check
1. 1000 m = 1
___
a) mm b) km c) dm
2.
0.001 g = 1 ___
a) mg
b) kg c) dg
3.
0.001 L = 1 ___
a) mL
b) cL c) dL
4.
0.01 m = 1 ___
a) mm b) cm c) dm
Learning Check
Select the unit you would use to measure
1. Your height
a) millimeters
b) meters
c) kilometers
2. Your mass
a) milligrams
b) grams
c) kilograms
3. The distance between two cities
a) millimeters
b) meters
c) kilometers
4. The width of an artery
a) millimeters
b) meters
c) kilometers
Parts of a Measurement
• In addition to a number and a unit, all measurements
also have some uncertainty associated with them
• All measurements contain some uncertainty
because…
– Errors occur
– The numbers reported in a measurement are limited by the
measuring tool
• Uncertainty is measured with:
• Accuracy
• How close to the true value
• Precision
• How close to each other
Uncertainty and Measurements
Three targets
with three
arrows each to
shoot.
How do they
compare?
Both
accurate and
precise
Precise but
not
accurate
Neither
accurate nor
precise
Taking Uncertainty into Account when
Measuring
• When making a measurement, you
must include the known digits
PLUS one estimated digit
• Note - We can only estimate one
place beyond what we are sure of
• What does that mean?!
Let’s Try It!
. l2. . . . I . . . . I3 . . . .I . . . . I4. .
First digit (known)
=2
cm
2.?? cm
Second digit (known) = 0.7
2.7? cm
Third digit (estimated) between 0.05- 0.07
Length reported
=
2.75 cm
or
2.74 cm
or
2.76 cm
Is one answer better than the other? NO!
Known + Estimated Digits
In 2.76 cm…
• Known digits 2 and 7 are 100% certain
• The third digit 6 is estimated
(uncertain)
• In the reported length, all three digits
(2.76 cm) are significant including the
estimated one
Learning Check
. l8. . . . I . . . . I9. . . .I . . . . I10. .
cm
What is the length of the line?
1) 9.6 cm
2) 9.62 cm
3) 9.63 cm
How does your answer compare with your
neighbor’s answer? Why or why not?
Another Example
. l3. . . . I . . . . I4 . . . . I . . . . I5. .
cm
What is the length of the line?
First digit
5.?? cm
Second digit
5.0? cm
Last (estimated) digit is
5.00 cm
Always estimate ONE place past the smallest mark!
USING MEASUREMENTS FOR
CALCULATIONS…YAY!
DENSITY - An important
and useful physical property
Density 
Mercury
mass (g)
volume (cm3)
Platinum
Aluminum
13.6 g/cm3
21.5 g/cm3
2.7 g/cm3
DENSITY
• Density is an
INTENSIVE property of
matter
– Means it does NOT
depend on quantity of
matter
– Another example is
temperature
• This is different than an
EXTENSIVE property
of matter
– Depends on quantity of
matter
– Examples include mass
and volume
Styrofoam
Brick
How to Calculate Density from Measurements
mass
(g)
Density 
volume (cm3)
1. Measure the length, width, height of an object and calculate
volume in cubic centimeters
V=lxwxh
OR
2. Measure the volume of a liquid using a graduated cylinder
3. Measure the mass of the object using a balance
4. Calculate the density by dividing mass and volume
Learning Check
Osmium is a very dense metal. What is its
density in g/cm3 if 50.0 g of the metal
occupies a volume of 2.22cm3?
1) 2.25 g/cm3
2) 22.5 g/cm3
3) 111 g/cm3
Solution
• Placing the mass and volume of the osmium
metal into the density setup, we obtain:
D = mass = 50.00 g =
volume 2.22 cm3
= 22.522522 g/cm3 = 22.5 g/cm3
Measuring the Volume of a Weird Object
by Displacement
A solid displaces a matching volume of water
when the solid is placed in water
33 mL
25 mL
Learning Check
What is the density (g/cm3) of 48 g of a metal
if the metal raises the level of water in a
graduated cylinder from 25 mL to 33 mL?
1) 0.2 g/ cm3
2) 6 g/m3 3) 252 g/cm3
33 mL
25 mL