Unit 1: B.1-B.2

1. Unit 1: B.1-B.2 In which you will learn about: Physical v. chemical properties Physical v. chemical changes Density

2. B.1 Physical Properties of Water Matter: anything that occupies spaced and has mass Matter can be distinguished by its physical properties Physical property: a property that can be observed/or measured without changing the chemical makeup of the substance What are some physical properties? • color • melting and boiling point • odor

3. Other Physical Properties • Density: the mass of a material within a given volume • The density of liquid water is usually given as 1 g/mL, but it’s actually temperature dependent • 1 cm3 = 1 mL (this is super useful for the rest of the year so MEMORIZE it now!) • Freezing point: the temperature at which a substance changes from a liquid to a solid • For water, it is of course, 0°C • What others can you think of?

4. Graphite — layer structure of carbon atoms reflects physical properties. This allows layers to easily be removed. This easy transfer of layers is why we use it in pencils!

5. Water Is Never Pure • Water is the only ordinary liquid found in naturally in our environment • Because so many substances dissolve readily in water, quite a few liquids are actually water solutions • A water-based solution is an aqueous solution • BTW, what’s a chemical property? A property that can only be observed and/or measured if the substance is chemically altered (Example: flammability)

6. Physical Changes • can be observed without changing the identity of the substance Some physical changes would be • boiling of a liquid • melting of a solid • dissolving a solid in a liquid to give a homogeneous mixture — a SOLUTION.

7. Chemical Properties and Chemical Change • Chemical change or chemical reaction — transformation of one or more atoms or molecules into one or more different molecules. • Burning hydrogen (H2) in oxygen (O2) gives H2O.

8. Sure Signs of a Chemical Change • Heat • Odor change • Gas Produced (not from boiling!) • Precipitate – a solid formed by mixing two liquids together • Color change http://jchemed.chem.wisc.edu/JCESoft/CCA/CCA0/MOVIES/S1047.MOV

9. Physical vs. Chemical Properties • physical • chemical • physical • physical • chemical • Examples: • melting point • flammable • density • magnetic • tarnishes in air

10. Physical vs. Chemical Changes • Examples: • rusting iron • dissolving in water • burning a log • melting ice • grinding spices Chemical Physical Chemical Physical Physical

11. Most of Chemistry Concerns Chemical Properties &Changes • BUT, physical properties & changes are important, too! • ALL mixtures can be separated physically. • They can be separated based on their PHYSICAL properties.

12. Platinum Mercury Aluminum B.2 DENSITY - an important and useful physical property 13.6 g/cm3 21.5 g/cm3 2.7 g/cm3

13. ProblemA piece of copper has a mass of 57.54 g. It is 9.36 cm long, 7.23 cm wide, and 0.95 mm thick. Calculate density (g/cm3).

14. Strategy 1. Get dimensions in common units. 2. Calculate volume in cubic centimeters. 3. Calculate the density.

15. SOLUTION 1. Get ALL dimensions in common units. 2. Calculate volume in cubic centimeters. 3. Calculate the density. (9.36 cm)(7.23 cm)(0.095 cm) = 6.4 cm3 Note only 2 significant figures in the answer!

16. PROBLEM: Mercury (Hg) has a density of 13.6 g/cm3. What is the mass of 95 mL of Hg in grams? In pounds?

17. PROBLEM: Mercury (Hg) has a density of 13.6 g/cm3. What is the mass of 95 mL of Hg? Strategy 1. Use density to calc. mass (g) from volume. 2. Convert mass (g) to mass (lb) Need to know conversion factor = 454 g / 1 lb First, note that1 cm3 = 1 mL

18. PROBLEM: Mercury (Hg) has a density of 13.6 g/cm3. What is the mass of 95 mL of Hg? 1. Convert volume to mass 2. Convert mass (g) to mass (lb)

19. Learning Check Osmium is a very dense metal. What is its density in g/cm3 if 50.00 g of the metal occupies a volume of 2.22cm3? 1) 2.25 g/cm3 2) 22.5 g/cm3 3) 111 g/cm3

20. Solution 2) Placing the mass and volume of the osmium metal into the density setup, we obtain D = mass = 50.00 g = volume 2.22 cm3 = 22.522522 g/cm3 =22.5 g/cm3

21. Volume Displacement A solid displaces a matching volume of water when the solid is placed in water. 33 mL 25 mL

22. Density Depends on Temperature • Most density tables are given with a specific temperature because substances expand when heated.

23. Direct vs. Inverse Proportions • Directly proportional – the relationship between two variables can be expressed as y/x = k where k is a constant. • Graphs of directly proportional variables are linear.

24. How do mass and volume relate? • If mass is your y variable, and volume is your x variable, y/x = k! (m/V = D) • The graph is linear, showing a directly proportional relationship between mass and volume. • Notice that the slope = density, a CONSTANT! Mass Volume

25. Inverse proportions will come later • In inversely proportional relationships, yx = k • This type of graph is curved. • We will see this a lot more when we get to the gas laws later in the year.

26. HOMEWORK EXERCISES • 1) What is a physical property? • 2) Identify three physical properties of water. • 3) How does the density of solid water compare to the density of liquid water? • 4) Describe a setting where you might observe water as a solid, a liquid, and a gas all at the same time. • 5) Distinguish between physical changes and chemical changes. • 6) A star is estimated to have a mass of 2 x 1036 kg. Assuming it to be a sphere of average radius 7.0 x 105 km, calculate the average density of the star in units of grams per cubic centimeter. • CONTINUED…

27. HOMEWORK END • 7) Classify the following as physical or chemical changes. • a) Moth balls gradually vaporize in a closet. • b) Hydrofluoric acid attacks glass, and is used to etch calibration marks on glass laboratory utensils. • c) A French chef making a sauce with brandy is able to burn off the alcohol from the brandy, leaving just the brandy flavoring. • d) Chemistry majors sometimes get holes in the cotton jeans they wear to lab because of acid spills.