Ch

emis

try

XX

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

Ch

emis

try

XX

I

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)

Ch

emis

try

XX

I

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.

Ch

emis

try

XX

I

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

Ch

emis

try

XX

I

To determine the density of an object we need to measure

VOLUME and MASS

What kind of tools can we use?

Measuring Density

Ch

emis

try

XX

I

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

Ch

emis

try

XX

I

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.

Ch

emis

try

XX

I

|-------|--------|-------|-------|-------|----------|----------|---------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

Ch

emis

try

XX

I

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

Ch

emis

try

XX

I

Vernier Caliper

10 mm

6 mm

0.25 mm 10 + 6 + 0.25 = 16.25 mm

Ch

emis

try

XX

I

Vernier Caliper

26.9 +/- 0.1

What’s the Reading?

160.5 +/- 0.1

Ch

emis

try

XX

I

|׀

׀׀

׀|

׀׀

׀׀

|׀

׀׀

׀|

׀׀

׀׀

|׀

׀׀

׀|

׀׀

׀׀

|׀

׀׀

|׀

׀׀

׀|

׀׀

׀׀

|׀

׀׀

׀|

׀׀

׀׀

|׀

׀׀

׀|

׀׀

׀׀

|׀

׀׀

׀|

……

...

0

1

0

2

0

3

0

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

Ch

emis

try

XX

I

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!

Ch

emis

try

XX

I

Read the volume in each

graduatedcylinder

|׀

׀׀

׀|

׀׀

׀׀

|׀

׀׀

׀|

׀׀

׀׀

|׀

׀׀

׀|

׀׀

׀׀

|׀

׀׀

|׀

׀׀

׀|

׀׀

׀׀

|׀

׀׀

׀|

׀׀

׀׀

|׀

׀׀

׀|

׀׀

׀׀

|׀

׀׀

׀|

0

1

2

3

4

5

|׀

׀׀

׀|

׀׀

׀׀

|׀

׀׀

׀|

׀׀

׀׀

|׀

׀׀

׀|

׀׀

׀׀

|׀

׀׀

|׀

׀׀

׀|

׀׀

׀׀

|׀

׀׀

׀|

׀׀

׀׀

|׀

׀׀

׀|

׀׀

׀׀

|׀

׀׀

׀|..

.0

10

20

30

40

50

|

…

…..

.0

1

2

3

4

5

6

7

8

9

10

How many sig figs?

Let′s Think!

Ch

emis

try

XX

I

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.

Ch

emis

try

XX

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?

Ch

emis

try

XX

I

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

Ch

emis

try

XX

I

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.

Ch

emis

try

XX

I

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!

Ch

emis

try

XX

I

Determining Density

Use this data to determine the density ( = m/V) of this plastic.

Ch

emis

try

XX

I

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!

Ch

emis

try

XX

I

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

Ch

emis

try

XX

I

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

Ch

emis

try

XX

I

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!

Ch

emis

try

XX

I

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

Ch

emis

try

XX

I

Separation Flow Chart