chemistry xxi unit 1 how do we distinguish substances? module 1: searching for differences central...
TRANSCRIPT
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IUnit 1
How do we distinguish substances?
Module 1: Searching for Differences
Central goal:
To identify distinctive properties of substances present in a system that can be used to identify
and separate them.
DISCUSSION
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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.
Separating Plastics
Main substance from which the plastic is made
Plastic Identification
Code (PIC)
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Each of these plastics differs in density and this difference can be used to separate them when mixed
in the garbage. How?
Separating Plastics
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.
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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:
= m/V (mass and volume are extensive properties)
Density
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To determine the density of an object we need to measure
VOLUME and MASS
What kind of tools can we use?
Measuring Density
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Measure massto one gram: 1 g
Measure massto one milligram: 0.001 g
Measure mass to one microgram: 0.000001 g
Home Postal scale Cost $ 3.00
Mettler Analytical Balance Cost $ 750-$1000
Sartorius Microbalance Cost $ 20,000
Measuring Mass
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We have a variety of beakers and flasks
to determine the volume of liquids
and gases.
Measuring Volume
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.
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|-------|--------|-------|-------|-------|----------|----------|---------0 1 2 3 4 5 6A
Length of the nail using
cm
Everyone measure the nail ! cm = centimeter; m-meter
Ruler A = cm
Write your answers down.Then, compare and discuss.
Let′s Think!
Ruler B = cm
| ׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀ | ……...0 1 2 3 4 5 6B cm
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Did everyone get the same numbers? Is there a possible range of valid results ? About what range? How many numbers should you write down? Which ruler was “best”? As a group create a rule for properly reading any marked device.
| ׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀ | ……...0 1 2 3 4 5 6
|-------|--------|-------|-------|-------|----------|----------|---------0 1 2 3 4 5 6A
B
cm
cm
The Rule: Find the smallest division -- estimate into that division.
Record all the measured numbers and one estimated digit.
Let′s Think!
How many
sig figs?
5.2
5.15
2
3
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Vernier Caliper
10 mm
6 mm
0.25 mm 10 + 6 + 0.25 = 16.25 mm
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Vernier Caliper
26.9 +/- 0.1
What’s the Reading?
160.5 +/- 0.1
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40
50To measure
volumeof a solid
by displacement you need to:
1) Read the volume of the liquid
2) Carefully let the solid sink
4) The increase in volume is the volume of the solid.
3) Read the new volume
Measuring Volume
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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 ?
8 ml
6 mlHow many sig figs?
Let′s Think!
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Read the volume in each
graduatedcylinder
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How many sig figs?
Let′s Think!
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Many times we need to multiply and divide measured data to calculate other quantities,
such as density.
Calculating
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.
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IImagine you gather data on the density of a liquid
using the four devices below
Measure volume Measure Mass
Capillary Pour it in your analytical fish bottle hand balance scale5.025 ml 5 ml 4.020 g 4.0 g
OR OR
How many sig figs?
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As in a chain – when MULTIPLYINGor DIVIDING the answer toa computation can haveno more measured digits
than the SMALLEST numberof measured digits in any of theseparate measurements used.
Sig Figs
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DATA 5.025 mL +/- 0.001 5 mL +/- 1 4.021 g +/- 0.001 4.0 g +/- 0.1 Capillary bottle Hand Analytical balance Fish scale
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
Let′s Think!
If you want to learn more: Tro 23-27 pp.
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Determining DensityConsider the following data for the mass and
volume of different samples of the same plastic:
m (g) 1.48 2.81 4.59 6.26 7.83
V (ml) 1.1 2.1 3.4 4.6 5.8
Use this data to build a graph of mass vs. volume using Excel.
The slope of this graph is a direct measure of the density of the plastic ( = m/V).
Generate the best fit line for the data and use it to determine the density of the material.
Let′s Think!
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Determining Density
Use this data to determine the density ( = m/V) of this plastic.
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Determining Density
The best approach is to find the equation for the best fit line:
Y = mX
M = V
The slope of the line should be the
density of the material. = 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)
Let′s Think!
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Changing Units
= 1.4 g/mL ? kg/m3
g
kgx
mL
g
1000
14.1 3
36
3 1
101
1
1
m
cmxx
cm
mLx
1.4 x 103 kg/m3
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Imagine that you are assigned the task of designing a strategy to separate the plastics
listed in the following table:
For that purpose you have access to the following substances: Water ( = 1.00 g/cm3), Ethanol
( = 0.789 g/cm3), and Salt. You can use these substances as provided, or use
them to prepare mixtures.
Separating Plastics
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
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The following graphs represent measurements of the density of mixtures of water-ethanol and water-salt at
different concentrations (T= 20 oC)
Water-Ethanol
0.780.8
0.820.840.860.88
0.90.920.940.960.98
11.02
0 10 20 30 40 50 60 70 80 90 100
% Mass Ethanol
De
ns
ity
(g
/mL
)
Water-Salt
11.021.041.061.08
1.11.121.141.161.18
1.2
0 5 10 15 20 25
% Mass Salt
De
ns
ity
(g
/mL
)
Use this information to design a strategy to separate the plastics using the minimum number of steps and liquid mixtures.
Let′s Think!
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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-Ethanol
0.780.8
0.820.840.860.88
0.90.920.940.960.98
11.02
0 10 20 30 40 50 60 70 80 90 100
% Mass Ethanol
Den
sity
(g/m
L)
Water-Salt
11.021.041.061.08
1.11.121.141.161.18
1.2
0 5 10 15 20 25
% Mass Salt
Den
sity
(g/m
L)
Relevant Data
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Separation Flow Chart