Transcript Physical Properties & Density

Unit 1: B.1-B.2
In which you will learn about:
•Physical v. chemical properties
•Physical v. chemical changes
•Density
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
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?
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!
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)
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.
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.
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/JCESof
t/CCA/CCA0/MOVIES/S1047.MOV
Physical vs. Chemical Properties
• Examples:
– melting point
physical
– flammable
chemical
– density
physical
– magnetic
physical
– tarnishes in air
chemical
Physical vs. Chemical Changes
• Examples:
– rusting iron
Chemical
– dissolving in water
Physical
– burning a log
Chemical
– melting ice
Physical
– grinding spices
Physical
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.
B.2 DENSITY - an important and
useful physical property
Density 
Mercury
mass (g)
volume (cm3)
Platinum
Aluminum
13.6 g/cm3
21.5 g/cm3
2.7 g/cm3
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).
mass
(g)
Density 
volume (cm3)
Strategy
1. Get dimensions in common units.
2. Calculate volume in cubic centimeters.
3. Calculate the density.
SOLUTION
1. Get ALL dimensions in common units.
1cm
0.95 mm •
= 0.095 cm
10 mm
2. Calculate volume in cubic centimeters.
(9.36 cm)(7.23 cm)(0.095 cm) = 6.4 cm3
Note only 2 significant figures in the answer!
3. Calculate the density.
57.54 g
3
=
9.0
g
/
cm
6.4 cm3
PROBLEM: Mercury (Hg) has a density of 13.6 g/cm3.
What is the mass of 95 mL of Hg in grams? In pounds?
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
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
13.6 g
3
3
95 cm •
= 1.3 x 10 g
3
cm
2.
Convert mass (g) to mass (lb)
3
1.3 x 10 g •
1 lb
= 2.8 lb
454 g
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
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
Volume Displacement
A solid displaces a matching volume of water when
the solid is placed in water.
33 mL
25 mL
Density Depends on Temperature
• Most density tables are given with a specific
temperature because substances expand
when heated.
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.
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
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.
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…
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.