Unit 1
How do we distinguish
substances?
Module 1: Searching for Differences
Chemistry XXI
DISCUSSION
Central goal:
To identify distinctive
properties of substances
present in a system that
can be used to identify
and separate them.
Separating Plastics
Chemistry XXI
The Society of the Plastics Industry, Inc., has
developed a voluntary uniform coding system for
plastic containers which identifies containers by
material type for the convenience of sorting them.
Plastic
Identification
Code
(PIC)
Main substance from which the
plastic is made
Separating Plastics
Chemistry XXI
Each of these plastics differs in density and this
difference can be used to separate them when mixed
in the garbage. How?
In particular, the mixture of
plastics can be sorted by
flotation using liquids of
different densities in which
some plastics will sink while
others will float.
Density
Chemistry XXI
Density is an intensive property that is
commonly used as a differentiating
characteristic to sort plastics during
recycling.
Density is a measure of the mass of material
per unit volume:
r = m/V
(mass and volume are extensive
properties)
Measuring Density
To determine the density of an object
we need to measure
VOLUME
and
MASS
Chemistry XXI
What kind of tools can we use?
Measuring Mass
Measure mass
to one gram: 1 g
Home Postal scale
Cost $ 3.00
Chemistry XXI
Measure mass
to one milligram: 0.001 g
Mettler Analytical Balance
Cost $ 750-$1000
Measure mass
to one microgram: 0.000001 g
Sartorius Microbalance
Cost $ 20,000
Measuring Volume
Chemistry XXI
We have a variety of
beakers and flasks
to determine the
volume of liquids
and gases.
They can be used to
measure the volume
of solids too. How?
For geometric solids, we can use length
measurements and calculations to determine V.
Let′s Think!
Everyone measure the nail !
cm = centimeter;
m-meter
Write your answers down.
Then, compare and discuss.
Chemistry XXI
A
|-------|--------|-------|-------|-------|----------|----------|--------0
1
2
3
4
5
6
|‫|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀‬
1
2
3
4
5
6
B 0
Length of the nail using
Ruler A =
……...
cm
cm
cm
Ruler B =
cm
Let′s Think!
 Did everyone get the same numbers?
Rule:
Find range
the smallest
division
 IsThe
there
a possible
of valid results
? -About what
range?into that division.
estimate
 How
manyall
numbers
should you
write down?
Record
the measured
numbers
and
 Which ruler was
one“best”?
estimated digit.
 As a group create a rule for properly reading any
marked device.
Chemistry XXI
5.2
A
B
2
|-------|--------|-------|-------|-------|----------|----------|--------- cm
0
1
2
3
4
5
6
|‫|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀‬
0
1
2
3
4
5
6
5.15
……... cm
How
many
sig figs?
3
Vernier Caliper
6 mm
10 mm
Chemistry XXI
0.25 mm
10 + 6 + 0.25 = 16.25 mm
Vernier Caliper
What’s the
Reading?
Chemistry XXI
26.9 +/- 0.1
160.5 +/- 0.1
To measure
volume
of a solid
by displacement
you need to:
……...
|‫|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀‬
50
30 40
20
10
0
Chemistry XXI
Measuring
Volume
1) Read the volume
of the liquid
2) Carefully let the
solid sink
3) Read the new
volume
4) The increase in
volume is the volume
of the solid.
Let′s Think!
Chemistry XXI
Liquids usually don’t have a sharp edge,
but “wet” or climb up the walls of a glass container.
The curved edge is a called a “meniscus.”
We read from the bottom of the meniscus.
What is the volume
of liquid in this
graduated cylinder ?
How many sig figs?
8 ml
6 ml
‫!‪Let′s Think‬‬
‫‪10‬‬
‫‪9‬‬
‫‪8‬‬
‫‪7‬‬
‫‪6‬‬
‫‪5‬‬
‫‪4‬‬
‫‪3‬‬
‫‪……...‬‬
‫‪2‬‬
‫‪1‬‬
‫‪How many‬‬
‫?‪sig figs‬‬
‫|‬
‫‪0‬‬
‫‪|...‬׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|‬
‫‪50‬‬
‫‪30 40‬‬
‫‪20‬‬
‫‪10‬‬
‫‪0‬‬
‫|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|‬
‫‪5‬‬
‫‪4‬‬
‫‪3‬‬
‫‪2‬‬
‫‪1‬‬
‫‪0‬‬
‫‪Chemistry XXI‬‬
‫‪Read the‬‬
‫‪volume in each‬‬
‫‪graduated‬‬
‫‪cylinder‬‬
Calculating
Chemistry XXI
Many times we need to multiply and divide
measured data to calculate other
quantities, such as density.
In these cases, it is of
central importance to
keep the number of
sig figs in the calculated
number that reflects the
precision of our
measurements.
Imagine you gather data on the density of a liquid
using the four devices below
Chemistry XXI
Measure volume
Capillary
bottle
5.025 ml
Pour it in your
OR
hand
5 ml
Measure Mass
analytical
balance
4.020 g
How many sig figs?
OR
fish
scale
4.0 g
Chemistry XXI
Sig Figs
As in a chain – when MULTIPLYING
or DIVIDING the answer to
a computation can have
no more measured digits
than the SMALLEST number
of measured digits in any of the
separate measurements used.
Let′s Think!
5.025 mL +/- 0.001
Capillary bottle
DATA
5 mL +/- 1 4.021 g +/- 0.001
Hand
Analytical balance
4.0 g +/- 0.1
Fish scale
Chemistry XXI
Compute density
A. Using the analytical balance
and the capillary bottle data
B. Using the analytical
balance and the hand data
C. Using the fish scale and
capillary bottle
Densities
A
B
C
0.8002
0.8
0.80
If you want to learn more: Tro 23-27 pp.
Determining Density
Consider the following data for the mass and
volume of different samples of the same plastic:
m (g)
V (ml)
1.48
1.1
2.81
2.1
4.59
3.4
6.26
4.6
7.83
5.8
Chemistry XXI
Use this data to build a graph of mass vs.
volume using Excel.
Let′s
Think!
The slope of this graph is a direct measure of
the density of the plastic (r = m/V).
Generate the best fit line for the data and use
it to determine the density of the material.
Chemistry XXI
Mass (g)
Determining Density
9
8
7
6
5
4
3
2
1
0
5.8, 7.83
4.6, 6.26
3.4, 4.59
2.1, 2.81
1.1, 1.48
0
1
2
3
4
5
6
7
Volume (mL)
Use this data to determine the density (r = m/V)
of this plastic.
The best approach is
to find the equation
for the best fit line:
Y = mX
Chemistry XXI
M=rV
The slope of the
line should be the
density of the
material.
Let′s
Think!
Mass (g)
Mass (g)
Determining Density
99
88
77
66
55
44
33
22
11
00
5.8, 7.83
7.83
5.8,
m = 1.3524V
4.6,6.26
6.26
4.6,
3.4,4.59
4.59
3.4,
2.1,2.81
2.81
2.1,
1.1,1.48
1.48
1.1,
00
11
22
33
44
55
66
Volume(mL)
(mL)
Volume
r = 1.4 g/mL
What is the density of this plastic in kg/m3?
(1 kg = 1000 g; 1 mL = 1 cm3; 1 m3 = 1 x 106 cm3)
77
Changing Units
r = 1.4 g/mL
? kg/m3
6
Chemistry XXI
1 mL 1x10 cm
g
1 kg
x
x
1.4
x
3
3
1
cm
1
m
mL 1000 g
1.4 x 103 kg/m3
3
Separating Plastics
Imagine that you are assigned the task of
designing a strategy to separate the plastics
listed in the following table:
Chemistry XXI
Plastic
Density
(g/cm3)
PETE (1)
HDPE (2)
PP (5)
PS (6)
1.29-1.40
0.95-0.97
0.90-0.91
1.04-1.07
For that purpose you have access to the following
substances: Water (r = 1.00 g/cm3), Ethanol
(r = 0.789 g/cm3), and Salt.
You can use these substances as provided, or use
them to prepare mixtures.
Let′s Think!
The following graphs represent measurements of the
density of mixtures of water-ethanol and water-salt at
different concentrations (T= 20 oC)
Chemistry XXI
Water-Salt
1.02
1
0.98
0.96
0.94
0.92
0.9
0.88
0.86
0.84
0.82
0.8
0.78
Density (g/mL)
Density (g/mL)
Water-Ethanol
0
10
20 30
40
50
60
% Mass Ethanol
70 80
90 100
1.2
1.18
1.16
1.14
1.12
1.1
1.08
1.06
1.04
1.02
1
0
5
10
15
20
25
% Mass Salt
Use this information to design a strategy to separate the plastics
using the minimum number of steps and liquid mixtures.
Relevant Data
Plastic
PETE (1)
HDPE (2)
PP (5)
PS (6)
Density
(g/cm3)
1.29-1.40
0.95-0.97
0.90-0.91
1.04-1.07
Water-Salt
1.02
1
0.98
0.96
0.94
0.92
0.9
0.88
0.86
0.84
0.82
0.8
0.78
Density (g/mL)
Density (g/mL)
Chemistry XXI
Water-Ethanol
0 10 20 30 40 50 60 70 80 90 100
% Mass Ethanol
1.2
1.18
1.16
1.14
1.12
1.1
1.08
1.06
1.04
1.02
1
0
5
10
15
% Mass Salt
20
25
Chemistry XXI
Separation Flow Chart