Science 8: Unit A: Mix and Flow of Matter

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Science 8 Unit A Mix and Flow of Matter

• Topic 5 Density

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What is density?

• 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.

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The Standard for Density

• 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 waters particles spread out when they
  turn solid, ices density is 0.92 g/ml, lower
  than waters. This is why ice floats on water.
• This is weird because most materials densities
  increase when they become solid.

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Why do different materials have different
densities?

• 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.

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What are some example densities?

• 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 1000000 g/cm3 (theory)

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The White Dwarf Star A Case Study

• 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 332946
  Earths. This makes the white dwarf very dense, so
  dense in fact that you would not be able to lift
  up a pinch full.

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How do we calculate density?

• Density mass volume.
• Remember units are g/ml (liquids/gases) or g/cm3
  (solids)
• Short Version of formula
• D m/v

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Example

• Example Tommy wants to know why hes gained two
  pounds weight even though hes 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.

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Heat and Density

• 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 its denser
  as a liquid than a solid.