Transcript A-5 Notes

Science 8: Unit A: Mix and Flow of
Matter
Topic 5: Density
What is density?
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Density is a property of a MATERIAL. It is similar to
specific heat capacity.
Density is the mass of a substance in a given volume.
The standard volume for density is a ml (for gases and
liquids) and cm3 (for solids)
Note that 1 cm3 = 1 ml
The density of a material is a constant physical property.
Whether here or on Mars, the density of water is 1 g/ml
at 20C and at standard pressure.
The Standard for Density
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The density of water is 1 g/ml at sea level
and at room temperature. This is the
standard density that all other materials are
compared to.
Because water’s particles spread out when
they turn solid, ice’s density is 0.92 g/ml,
lower than water’s. This is why ice floats on
water.
This is weird because most materials’
densities increase when they become solid.
Why do different materials have
different densities?
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According to the particle model of matter,
different materials have different particles
and these particles can be larger or smaller,
heavier and lighter.
Also because of the different particle sizes
only a certain amount of particles can fit in a
given volume.
What are some example densities?
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Air: 0.0012 g/ml
Liquid mercury: 13.5 g/ml
Gold: 19.3 g/cm3
Copper: 9.0 g/cm3
Lead: 11.3 g/cm3
The Sun: 1.4 g/cm3 (average density)
White Dwarf Star: 1’000’000 g/cm3 (theory)
The White Dwarf Star: A Case Study
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A white dwarf star is a dead
star. Once our sun burns up all
of its hydrogen fuel its gravity
will continue to collapse its
mass until most of it is
squeezed into a volume about
the same size as our Earth.
Remember that the sun has the
mass of 332’946 Earths. This
makes the white dwarf very
dense, so dense in fact that you
would not be able to lift up a
pinch full.
How do we calculate density?
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Density = mass ÷ volume.
Remember units are g/ml (liquids/gases) or
g/cm3 (solids)
Short Version of formula:
D = m/v
Example
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Example: Tommy wants to know why he’s gained two
pounds weight even though he’s been working out four
days now. He takes a 18 g sample of fat (taken from a
steak) and measures its volume to be 20 cm3. What is
the density of body fat?
D = m/v = 18g/20 cm3 = 0.9 g/cm3 .
He then measures the muscle tissue from the steak and
measures the mass of 10 cm3. It turns out to be 10.6 g.
So…
D = m/v = 10.6g/10 cm3 = 1.06 g/ cm3
So he is replacing less dense fat with denser muscle.
Heat and Density
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Solids tend to be denser
than liquids which are
denser than gases.
When you heat a
substance the particles
gain thermal energy and
spread out. This lowers
density. So hot water is
less dense than cold
water. But remember
water is weird: it’s denser
as a liquid than a solid.