1. Chemistry XXI 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 1 How 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?

2. Chemistry XXI FOUR MAIN MODULES Unit 1 How do we distinguish substances? 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.

3. Chemistry XXI 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?

4. Chemistry XXI Unit 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.

5. Chemistry XXI 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? Analysis What is this?

6. Chemistry XXI Central 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);

7. Chemistry XXI How 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!

8. Chemistry XXI Measurable Properties In general, it is more useful to measure properties that do not depend on the size of the system (intensive properties): For some systems, its is useful to measure properties that are size dependent (extensive properties):

9. Chemistry XXI Chemical 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?

10. Chemistry XXI Differentiating Characteristics All 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. Liquid Nitrogen Let′s explore!

11. Chemistry XXI Differentiating Characteristics What other properties of a substance can be used as “differentiating characteristics”? Physical Properties DensityHeat Capacity Boiling PointConductivity Melting PointViscosity Surface TensionSolubility 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.

12. Chemistry XXI 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.

13. Chemistry XXI Phase Transitions A sample of solid water at -20 o C is heated at a constant rate to 120 o C. 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( o C) t E (kJ) T( o C)

14. Chemistry XXI Physical Change The observed behavior is characteristic of all substances undergoing phase changes. During a phase change, the physical properties of the substance change t T ( o C)

15. Chemistry XXI Energy Transfer Energy added (kilojoules- kJ) During a phase transition, energy is transferred between the system and the surroundings. Convention Energy added (+) Energy released (-)

16. Chemistry XXI Phase Stability For any given substance, different phases are stable at different values of temperature and pressure. P T S L G S

17. Chemistry XXI Phase Changes and Phase Diagrams

18. Chemistry XXI Phase Diagrams Important 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

19. Chemistry XXI Two Different Substances Carbon dioxide Water What are the major differences? Let′s think! Notice differences in scales

20. Chemistry XXI Separating 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.

21. Chemistry XXI Distillation- Based on the difference in boiling points. Separating Air STEP 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 Distillation 0.934Argon 20.9Oxygen 78.1Nitrogen % by VSubstance Air Composition

22. Chemistry XXI Assess what you know Let′s apply!

23. Chemistry XXI Let′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 A Liquid 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

24. Chemistry XXI A rich fuel mixture containing the following hydrocarbons has been extracted from underground: PropaneButane Neo-Pentane 2-Heptene The mixture is at an initial temperature of 5 o C. Fuel Separation Based on the data provided, design a procedure to separate each component. What differentiating characteristic would you use? Let′s apply! Propane Butane Neo-Pentane 2-Heptene

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

26. Chemistry XXI Searching for Differences Summary The 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.

27. Chemistry XXI 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?