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The central goal of this unit is to help you understand and apply basic ideas that can be used to distinguish

the different substances present in a system.

Unit 1How do we distinguish substances?

In which relevant contexts would be important for humans to distinguish

between different substances?

What would the consequences be of failing to do so?

In your groups, discuss your answers to the following two questions:

Why do we care?

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IUnit 1

How do we distinguish substances?

Food control

Pollution detection and

control

Health monitoring

Crime investigation

Drug development

Resource exploration and

management

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FOUR MAIN MODULES

Unit 1How do we distinguish substances?

M1. Searching for Differences Identifying differences that allow us to separate components.

M2. Modeling MatterUsing the particulate model of matter to explain differences.

M3. Comparing Masses Characterizing differences in particle’s mass and number.

M4. Determining Composition Characterizing differences in particle’s composition.

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Integration

To illustrate the power of chemical ideas and models in distinguishing and identifying chemical substances we will focus our attention in our own

planet’s atmosphere.

What is it made of?

How do we identify them?

How do we explain their properties?

How do we model their behavior?

How do we separate its components?

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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.

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The Challenge

Imagine that you are interested in investigating

the properties and composition of certain

system, from the Earth’s atmosphere to an unknown

material found in a crime scene or polluting the ocean.

How would you analyze the system?

What properties would you measure?

How would you separate its different components?

AnalysisWhat is this?

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ICentral Problems

Most systems, natural or synthetic, are complex.

They tend to be a combination of hundred or thousands of different substances (mixture vs. pure substance)

Smell > 800

different substances

Their properties change as a function of position (heterogeneous vs. homogeneous).

Their properties change as a function of time (dynamic vs static);

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IHow do we characterize the

state of a system?

We need to identify and measure

properties that allow us to determine how a system changes in

time and space.

Consider, for example, our atmosphere.

What properties could we use to study its state and behavior?

Let′s think!

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Let′s think!

We use properties such as:

Temperature

Pressure

Density

Concentration

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Measurable Properties

In general, it is more useful to measure properties that do not depend on the size of the system

(intensive properties):

Temperature Pressure Density Concentration

For some systems, its is useful to measure properties that are size dependent (extensive properties):

Mass Volume Energy

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IChemical Composition

From a chemical point of view, it is of crucial interest to find the “composition” of the system

(the nature of the substances present in it).

How do we do it?

A first common step is to try to separate the different

components.

Let′s think!

How would you propose to separate the different

substances present in our atmosphere?

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IDifferentiating CharacteristicsAll chemical analysis is based on the assumption

that each substance, no matter how simple or complex, has a “differentiating characteristic” that

can be used to separate/detect/identify it.

For example, in the case of the atmosphere, we can take advantage of

the different boiling/condensation

points of the substances

present in it.

77.36-195.79Nitrogen

87.36-189.35Argon

90.20-182.95Oxygen

373.15100Water

KoCSubstance

T(K) = T(oC) + 273.15

Liquid NitrogenLet′s explore!

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IDifferentiating CharacteristicsWhat other properties of a substance can be used

as “differentiating characteristics”?

Physical Properties

Density Heat CapacityBoiling Point ConductivityMelting Point ViscositySurface Tension Solubility

We look for properties that:

Do not depend on the size of the sample (Intensive Properties);

Can be measured without changing the chemical nature of the substances.

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The separation of the main components of our atmosphere

can be accomplished using boiling/condensation points as “differentiating characteristic.”

Phase Behavior

However, to implement a successful separation we need to understand the phase behavior of

substances.

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IPhase Transitions

A sample of solid water at -20 oC is heated at a constant rate to 120 oC. During the experiment you

measure the temperature of the system as a function of time, as well as the amount of energy absorbed by

the system during the heating process.What would you expect to see?

Let′s think!

T(oC)

t

100

0

0

E (kJ)

T(oC)1000

Absorbed (+)

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IPhysical Change

The observed behavior is characteristic of all substances undergoing phase changes.

During a phase change, the physical properties of the

substance changet

T (oC)

100

0 Constant T

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IEnergy Transfer

Energy addedEnergy added ((kilojoules- kJkilojoules- kJ))

During a phase transition, energy is

transferred between the

system and the surroundings.

Convention

Energy added (+)

Energy released (-)

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GGGGGGGGGGGGG

GGGGGGGGGGGGS

GGGGGGGGGGSSS

GGGGGGLLLSSSS

GGGLLLLLSSSSS

GLLLLLLSSSSSS

LLLLLLSSSSSSS

LLLLLSSSSSSSS

LLLLSSSSSSSSS

LLLSSSSSSSSSS

Phase StabilityFor any given substance, different phases are stable

at different values of temperature and pressure.

P

T

S

L

G

S

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IPhase Changes and

Phase Diagrams

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Phase DiagramsImportant Points

for Water

T(°C) P(mmHg)

Normal boiling point 100 760 =1 atm

Normal freezing point 0 760

Triple point 0.0098 4.58

Vapor Pressure Curve

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Two Different Substances

Carbon dioxide Water

What are the major differences?Let′s think!

Notice differences in scales

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ISeparating Air

A variety of people are interested in separating air components for different purposes:

Purification, environmental analysis, mining.How do they do it?

What differentiating characteristics may they use?

Used to separate

solids from liquids too.

STEP 1

Filtration – Based on differences in particle size.

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Distillation- Based on the difference in boiling points.

Separating AirSTEP 2

Air is normally cooled in stages until it reaches –200°C.

Water vapor condenses, and is removed using absorbent filters; Carbon dioxide freezes at –79 °C, and is removed; Oxygen liquefies at –183 °C; Nitrogen liquefies at –196 °C.

Argon

Fractional Distillation0.934Argon

20.9Oxygen

78.1Nitrogen

% by VSubstance

Air Composition

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I Assess what you know

Let′s apply!

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ILet′s apply!

Boiling of a liquid occurs when its vapor pressure (the

pressure exerted by the vaporize substance) equals the atmospheric pressure.

Notice log scale

ALiquid

Gas

Which liquid, A or B, is more volatile (higher vapor

pressure at same T)?

Which liquid boils at a lower temperature?

What is its normal boiling temperature?

B

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A rich fuel mixture containing the following hydrocarbons has been

extracted from underground:

Propane ButaneNeo-Pentane 2-Heptene

The mixture is at an initial temperature of 5 oC.

Fuel Separation

Based on the data provided, design a procedure to

separate each component.

What differentiating characteristic would you use?

Let′s apply!

PropaneButane

Neo-Pentane 2-Heptene

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Propane (G) Butane (G)

Neo-Pentane (L)

2-Heptene (L)

Fuel Separation

At 5 oC and 1 atm:

We can distill the liquids (heat up)

and the gases (cool down).

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I In your groups, come up with one question about something you don’t understand at this point?

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Searching for Differences

SummaryThe identification of a differentiating characteristics is crucial to: identify, detect, separate, or quantify

the amount of substances is a system.

Phase properties are particularly useful because they can be

measured/induced by changing temperature and pressure.

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For next class,

Investigate what model of matter would allow us to explain why two substances, such as water and carbon dioxide, have different

phase behaviors.

What assumptions about the submicroscopic structure of matter do we need to make

to explain it?