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

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Presentation on theme: "Unit 1: B.1-B.2 In which you will learn about: Physical v. chemical properties Physical v. chemical changes Density."— Presentation transcript:

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 color melting and boiling point melting and boiling point odor 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 cm 3 = 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 boiling of a liquid melting of a solid melting of a solid dissolving a solid in a liquid to give a homogeneous mixture — a SOLUTION. dissolving a solid in a liquid to give a homogeneous mixture — a SOLUTION.

7 Chemical Properties and Chemical Change Burning hydrogen (H 2 ) in oxygen (O 2 ) gives H 2 O.Burning hydrogen (H 2 ) in oxygen (O 2 ) gives H 2 O.

8 Chemical Properties and Chemical Change Chemical change or chemical reaction — transformation of one or more atoms or molecules into one or more different molecules.Chemical change or chemical reaction — transformation of one or more atoms or molecules into one or more different molecules. Burning hydrogen (H 2 ) in oxygen (O 2 ) gives H 2 O.Burning hydrogen (H 2 ) in oxygen (O 2 ) gives H 2 O.

9 Sure Signs of a Chemical Change HeatHeat Odor changeOdor change Gas Produced (not from boiling!)Gas Produced (not from boiling!) Precipitate – a solid formed by mixing two liquids togetherPrecipitate – a solid formed by mixing two liquids together Color changeColor change t/CCA/CCA0/MOVIES/S1047.MOV

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

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

12 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.

13 B.2 DENSITY - an important and useful physical property Mercury 13.6 g/cm g/cm 3 Aluminum 2.7 g/cm 3 Platinum

14 Problem Problem A piece of copper has a mass of g. It is 9.36 cm long, 7.23 cm wide, and 0.95 mm thick. Calculate density (g/cm 3 ).

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

16 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 cm 3 Note only 2 significant figures in the answer!

17 PROBLEM: Mercury (Hg) has a density of 13.6 g/cm 3. What is the mass of 95 mL of Hg in grams? In pounds?

18 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 PROBLEM: Mercury (Hg) has a density of 13.6 g/cm 3. What is the mass of 95 mL of Hg? First, note that 1 cm 3 = 1 mL

19 1.Convert volume to mass PROBLEM: Mercury (Hg) has a density of 13.6 g/cm 3. What is the mass of 95 mL of Hg? 2.Convert mass (g) to mass (lb)

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

21 Solution 2) Placing the mass and volume of the osmium metal into the density setup, we obtain D = mass = g = volume2.22 cm 3 volume2.22 cm 3 = g/cm 3 = 22.5 g/cm 3 = g/cm 3 = 22.5 g/cm 3

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

23 DENSITYDENSITY INTENSIVE Density is an INTENSIVE property of matter. – does NOT depend on quantity of matter. – Temperature is also intensive EXTENSIVE Contrast with EXTENSIVE – depends on quantity of matter. – mass and volume are extensive Styrofoam Brick

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

25 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.

26 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

27 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.

28 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 kg. Assuming it to be a sphere of average radius 7.0 x 10 5 km, calculate the average density of the star in units of grams per cubic centimeter. CONTINUED…

29 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.


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