1. How do we distinguish substances? Module 1: Searching for Differences
Unit 1
How do we distinguish substances?
Module 1: Searching for Differences
DISCUSSION
Central goal:
To identify distinctive properties of substances present in a system that can be used to identify and separate them.
Remind your class that there will be a post-discussion quiz on Friday, so they should take notes.

2. Plastic Identification Code (PIC)
Separating Plastics
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)
These codes appear on most recyclable plastics. The standard plastics included PETE (polyethylene terephthalate), HDPE (high density polyethylene), V (polyvinylchloride), LDPE (low density polyethylene), PP (polypropylene) and PS (polystyrene). Therefore one way a recycling company can separate plastics is finding the number, but what is the plastic is melted and reformed? How can plastics be recycled without numbers? In Tucson, only plastics 1 and 2 are currently recycled.
Main substance from which the plastic is made

3. Separating Plastics
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.
Students should come prepared with some ideas of how to use density to separate plastics. Involve them in the discussion.

4. Density
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)
Ask your class what an intensive versus an extensive property is.
Intensive properties do not depend on the size of the sample, and depend on the chemical composition of the material. For example melting points, boiling points, density, molar mass, etc. are intensive properties.
Extensive properties vary with the size of the sample and tend to be independent of chemical composition. For example, the mass of a beaker of water is an extensive property, and so is the volume of a beaker of water, but dividing the mass by the water gives density which is intensive.

5. Measuring Density
To determine the density of an object we need to measure
VOLUME and MASS
What kind of tools can we use?
Involve students with questions.

6. Measuring Mass Measure mass to one gram: 1 g to one milligram: 0.001 g
to one microgram: g
Home Postal scale
Cost $ 3.00
Mettler Analytical Balance
Cost $ 750-$1000
Do you really need a microbalance to check the weight of a letter?
Sartorius Microbalance
Cost $ 20,000

7. They can be used to measure the volume of solids too. How?
Measuring Volume
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?
Discuss that volume of solids can be measured in different ways.
For geometric solids, we can use length measurements and calculations to determine V.

8. Write your answers down. Then, compare and discuss.
Let′s Think!
Everyone measure the nail ! cm = centimeter; m-meter
Write your answers down.
Then, compare and discuss.
| | | | | | | | cm A
Have each group report then move on to next slide.
Ruler B = cm
|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀| ……... B cm
Length of the nail using
Ruler A = cm

9. Let′s Think!
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.
The Rule: Find the smallest division -- estimate into that division.
Record all the measured numbers and
one estimated digit.
5.2
5.15 2 3
| | | | | | | | cm
Once you discuss this, give each group a caliper and show them how to use it to measure length. A B
How many sig figs?
|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀| ……...

10. Vernier Caliper 6 mm 10 mm 0.25 mm 10 + 6 + 0.25 = 16.25 mm
Guide them in how to read the measurement. Discuss number of sig figs that can be assigned to the measurement (4).
Most students struggle with the caliper (even I do). Usually I use a regular ruler in addition to the caliper to explain the concept. You can also get 1 known from the preproom and use it as an example for the class.

11. Vernier Caliper 26.9 +/- 0.1 160.5 +/- 0.1 What’s the Reading?
It is important that all the students understand this before moving on.
/- 0.1

12. by displacement you need to:
Measuring Volume
Read the volume of the liquid
To measure volume
of a solid
by displacement you need to:
|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀| ……...
2) Carefully let the solid sink
3) Read the new volume
Volume by displacement.
4) The increase in volume is the volume of the solid.

13. Let′s Think! 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.
8 ml
6 ml
What is the volume of liquid in this graduated cylinder ?
Explain how to read the volume.
How many sig figs?

14. Read the volume in each graduated
Let′s Think!
|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|
|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|׀׀׀׀|...
| ……...
Read the volume in each graduated
cylinder
2.30 (3 sig. figs.), 39.1 (3 sig. figs.), and 8.9 (2 sig. figs.)
How many sig figs?

15. Calculating
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.
Now you will discuss how to correctly report numbers obtained by combining different measurements (multiplication or division)

16. Imagine 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 scale
5.025 ml ml g g OR
How many sig figs?

17. 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.

18. Let′s Think! Densities A B C Compute density
DATA
5.025 mL +/ mL +/ g +/ g +/- 0.1
Capillary bottle Hand Analytical balance Fish scale
Compute density
Using the analytical balance and the capillary bottle data
Using the analytical balance and the hand data
Using the fish scale and capillary bottle
Densities A B C
0.8002
Have them do the exercise in groups. Then, discuss the results.
0.8
0.80
If you want to learn more: Tro pp.

19. Use this data to build a graph of mass vs. volume using Excel.
Determining Density
Consider 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 (r = m/V).
Generate the best fit line for the data and use it to determine the density of the material.
Let′s Think!
Open Excel and make the whole graph in front of your students. They need to learn these skills to complete the lab reports properly.
Go over how to label graphs and adjust the number or significant figures on the graph to match the precision of measurements made.

20. Use this data to determine the density (r = m/V) of this plastic.
Determining Density
Before telling your students everything, challenge them to think how to do it.
Use this data to determine the density (r = m/V) of this plastic.

21. Determining Density
The best approach is to find the equation for the best fit line:
Y = mX
M = r V
The slope of the line should be the density of the material.
If there is time, have them do the conversion and discuss the results.
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)
Let′s Think!

22. Changing Units 1.4 x 103 kg/m3 r = 1.4 g/mL ? kg/m3
Stop at this slide and only finish the rest of this if you complete the whole lab and they finish their reports.
1.4 x 103 kg/m3

23. Separating Plastics
Imagine that you are assigned the task of designing a strategy to separate the plastics listed in the following table:
Plastic
PETE (1)
HDPE (2)
PP (5)
PS (6)
Density (g/cm3)
For that purpose you have access to the following substances: Water (r = 1.00 g/cm3), Ethanol (r = g/cm3), and Salt. You can use these substances as provided, or use them to prepare mixtures.

24. 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)
Use this information to design a strategy to separate the plastics using the minimum number of steps and liquid mixtures.

25. Relevant Data Plastic PETE (1) HDPE (2) PP (5) PS (6) Density (g/cm3)

26. Separation Flow Chart