Unit 1: B.1-B.2

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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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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

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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 its 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, 0C
• What others can you think of?

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• 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!

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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, whats a chemical property? A property that
  can only be observed and/or measured if the
  substance is chemically altered (Example
  flammability)

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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.

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Chemical Properties and Chemical Change

• Burning hydrogen (H2) in oxygen (O2) gives H2O.

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Chemical Properties and Chemical Change

• Burning hydrogen (H2) in oxygen (O2) gives H2O.

• Chemical change or chemical reaction
  transformation of one or more atoms or molecules
  into one or more different molecules.

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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/MOVI
ES/S1047.MOV
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Physical vs. Chemical Properties

• physical
• chemical
• physical
• physical
• chemical

• Examples
• melting point
• flammable
• density
• magnetic
• tarnishes in air

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Physical vs. Chemical Changes

• Examples
• rusting iron
• dissolving in water
• burning a log
• melting ice
• grinding spices

Chemical Physical Chemical Physical Physical
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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.

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B.2 DENSITY - an important and useful physical
property
13.6 g/cm3
21.5 g/cm3
2.7 g/cm3
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• Problem A 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).

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

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• 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!
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PROBLEM Mercury (Hg) has a density of 13.6
g/cm3. What is the mass of 95 mL of Hg in grams?
In pounds?
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PROBLEM Mercury (Hg) has a density of 13.6
g/cm3. What is the mass of 95 mL of Hg?
First, note that 1 cm3 1 mL

• 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

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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)
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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

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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

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Volume Displacement

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

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DENSITY

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

Brick
Styrofoam
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Density Depends on Temperature

• Most density tables are given with a specific
  temperature because substances expand when heated.

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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.

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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
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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.

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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

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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.