1. Unit 1 How do we distinguish substances?
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.
M1. Searching for Differences
Identifying differences that allow us to separate components.
M2. Modeling Matter
Using 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.

2. How do we distinguish substances? Module 2: Modeling Matter
Unit 1
How do we distinguish substances?
Module 2: Modeling Matter
Central goal:
To explain the diversity in properties and behaviors of the different substances in a system based on the particulate model of matter.

3. The Challenge
Modeling How do I explain it?
As we have seen, each substance has at least one differentiating characteristic that makes it unique.
Carbon dioxide
Nitrogen
Water
What causes the differences?
How could we explain, predict, and even design, these differences?

4. Models of Matter
The central task of differentiating substances has been greatly simplified by the development models about their internal structure.
These models allow us to explain and predict the properties of matter, and to develop better techniques to detect and identify them.

5. Particulate Model of Matter
One of the most powerful models we have to explain and predict the physical properties and behavior of substances is the particulate model of matter.
Solid
Liquid
Gas
Basic Assumptions:
Any macroscopic sample of a substance is composed of a large number of very small particles;
1 mL of water = particles = 3.34 x 1022 particles

6. Most substances are made of particles of “nanometer” size.
How Small?
m = 10-7 m
0.01 m = 10-2 m
0.000,000,001 m = 1 x 10-9 m = 1 nm (1 nanometer)
Most substances are made of particles of “nanometer” size.
0.001 m = 10-3 m
0.1 m = 10-1 m
m = 10-5 m
1 m

7. Being Cautious What do these “particles” represent?
Gas
What do these “particles” represent?
What are the limitations of these representations?
Liquid
Solid

8. What determines the speed (v) of the particles?
Dynamic Nature
Basic Assumptions:
2. Particles are constantly moving;
What determines the speed (v) of the particles?

9. What is the “average pressure” in this model?
Dynamic Nature
Basic Assumptions:
2. Particles are constantly moving;
What is the “average pressure” in this model?

10. Interactions Basic Assumptions:
3. Particles interact with each other. The nature and strength of the interactions depend on the distance between particles;
Distance

11. Go to: http://www.chem.arizona.edu/chemt/C21/sim (Ideal Gas)
Let’s Explore
Go to: (Ideal Gas)
Explore the properties of the particulate model of matter when interactions among particles are neglected:
How does pressure depend on temperature, volume, and number of particles?
Build graphs of the type P vs. T, P vs. V, and P vs. N as part of your analysis.

12. Molecular Dynamics Simulation
Let’s Explore
Molecular Dynamics Simulation
F(r)  a(t)  v(t)  r(t)

13. The model predicts the following type of behavior:
Predictions
The model predicts the following type of behavior: P T P N P V
This behavior is observed in all gases at high temperatures and low pressures. Why?

14. Go to: http://www.chem.arizona.edu/chemt/C21/sim (Real Gas)
Let’s Explore
In the absence of interactions among particles (intermolecular forces) the model does not predict the existence of phase transitions as we change T.
Go to: (Real Gas)
Analyze the behavior of the model when intermolecular forces (IMF) among particles are introduced.
Discuss: What is the effect of the on the behavior of the particles and the system?

15. Let’s Explore

16. Force strength does not change; Size of particles does not change;
Phase Transitions
To explain the existence of phase transitions we have to assume that there are intermolecular forces among particles.
When temperature decreases, the average kinetic energy per particles decreases. Attractive forces between particles are then able to hold them together.
Force strength does not change; Size of particles does not change;
Let′s think!
Why does the temperature remain constant during a phase transition?

17. Potential Energy
Particles that attract each other are said to have negative potential energy compared to free particles. Ep r r1 r3 r4
More Negative r2

18. Potential vs. Kinetic EnergyEp r
Need to add energy to separate
Need to extract energy to get them closer r1 r4

19. Why do liquids get colder when they evaporate?
Explain and Predict
Let′s think!
Why does the boiling temperature of water decrease with decreasing external pressure?
Let′s think!
Why do liquids get colder when they evaporate?

20. Explain and Predict T1 < T2
Let′s think!
How do you explain that nitrogen condenses at a lower temperature than oxygen in terms of IMF?

21. Explain and Predict
Substance A
Substance B

22. Understanding Differences
In the particulate model of matter, many differences between substances are attributed to the presence of different intermolecular forces among particles.
Why are the intermolecular forces different?
We assume the composition and the structure of the “particles” are different.
What does this mean?

23. How many different substances are included in this representation?
Modeling Substances
This is a typical chemical representation at the particulate level of the main components of “pure” air.
Let′s think!
How many different substances are included in this representation?
What similarities and differences do you observe between the different types of particles present in the system?

24. The representation conveys the idea that we model air as a “mixture”:
Modeling Substances
Free atom of argon
The representation conveys the idea that we model air as a “mixture”:
a system composed of two or more types of independent particles present in proportions that may vary from sample to sample.
Atom or molecule?
Molecule of carbon dioxide
Molecule of oxygen
Bonded atom of nitrogen
Individual particles of different substances are modeled as made of free or bonded atoms of different types.
Molecules are made of two or more bonded atoms

25. Classifying Substances
As we have seen, the different components of a mixture can be separated by physical means (filtration, distillation):
Nitrogen (liquid)
Water
S L G
Argon (gas)
Chemists classify substances as “elements” or “compounds” based on particle composition.
Carbon dioxide (Solid, Dry ice)
Let′s think!
Which of these are elements/compounds? What makes the difference?

26. Chemical Elements
Elements are the most simple substances in Nature. They are composed of identical particles made of free or bonded atoms of the same type.
Macroscopic
Symbolic
Particulate
Atomic Element
Molecular Element

27. Chemical Elements Cl2 C Na P4
There are relatively few elements in Nature (fewer than 100), and a few more have been synthesized in lab.
None of them can be decomposed in simpler substances by physical or chemical means.
Cl2
Chlorine
Phosphorus P4 Na
Sodium
Carbon C

28. Atoms vs. Elements
We need to differentiate between the elements, as real substances, and the atoms they are made of. The Periodic Table summarizes the properties of the individual atoms, not of the actual elements.

29. Chemical Compounds CO2 H2O
Most substances in nature are chemical compounds. They are composed of identical particles made of bonded atoms of two or more different types.
Macroscopic
Symbolic
Particulate
Carbon dioxide
CO2
Water
H2O C H O N
Color Code
Molecular Formula

30. There is a wide variety of molecular compounds in Nature.
H2O and CO2 belong to a group of compounds called “molecular compounds”: They are made of molecules.
Methane CH4
There is a wide variety of molecular compounds in Nature.
This diversity is due to the possibility of having molecules with different compositions, sizes, and structures.
Caffeine C8H10N4O2
Hemoglobin
C2952H4664N812O832S8Fe4

31. Models and Formulas CH2O Space-filling Ball-and-stick Models
Keep in mind that molecules are represented in a variety of ways:
Space-filling
Ball-and-stick
Models
Formaldehyde (Air Pollutant)
Structural formula
CH2O
Molecular Formula
Formulas

32. Let’s Explore
Decide whether these particulate models correspond to an element, a compound or a mixture. C E M

33. Let′s apply!
Assess what you know

34. Analyze What does this system represent?
Let′s apply!
T = 280 K
P = 2 atm
What does this system represent?
How many phases are present in this system?
How many substances are in each phase?
How many elements? How many compounds?
How would you separate the different components in the system?

35. Predict In which of the phases:
Let′s apply!
T = 280 K
P = 2 atm
In which of the phases:
Are the intermolecular forces strongest?
Is the average potential energy per particle the lowest (most negative)?
Is the average particle speed the highest?
Is the average kinetic energy per particle the lowest?
Which of the two main substances, water or hexane, has the greater vapor pressure?

36. Write down one central idea that you learned in this module.
Share your idea with the members of your group.

37. Useful to analyze, synthesize, and transform chemical substances.
Modeling Matter
Summary:
The particulate model of matter allows us to explain and predict the properties of chemical substances.
Useful to analyze, synthesize, and transform chemical substances.
Differences in the intermolecular forces among the particles of different substances can be used to explain their different physical properties.

38. Modeling Matter Summary:
Based on the composition of their particles, substances are classified as elements or compounds.
N2 (Element)
CO2 (Compound)
The nature and strength of these interactions depend on the atomic composition and structure of a substance’s particles.

39. For next class,
Investigate how we can determine how many times heavier is one type of atom than another.
How can we use this knowledge to quantify, for example, how many molecules of O2 we breathe in a day?