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Unit 3: Forces and energy

Density — HIA International - Grade 8 Revision Notes & Topical Questions

5 min read 8 parts Lesson 9 of 16 1 quiz

📚 HIA International - Grade 8 Hub / Topical Revision
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Lesson summary

Density — HIA INTERNATIONAL notes: Define density, calculate it for regular and irregular objects, and use it to predict floating and sinking.Density = mass per unit volume.Not just weight — a large block of polystyrene can have more mass than a small piece of metal, but the same…

  • Hollow or solid
  • Rearranged
  • Conversion
  • Floating object
  • Anomalous results
  • Helium and hot air

Section 1 of 8Mass, volume and density

Density
Density mass per unit volume.
Not just weight
  • Not just weight a large block of polystyrene can have more mass than a small piece of metal,
    • but the same volume of iron has more mass than polystyrene.
Hollow or solid
  • Hollow or solid hollow means there is a space filled with air inside; solid means no space.
Examples
  • Examples iron is 7.9 g/cm3 and polystyrene is 0.05 g/cm3.

Section 2 of 8The density equation

Equation
  • Equation density = mass ÷ volume.
Rearranged
  • Rearranged mass = density × volume, and volume = mass ÷ density.
Units
  • Units g and cm3 give g/cm3; kg and m3 give kg/m3.
Conversion
  • 1 g/cm3 = 1000 kg/m3.
Water
  • Water has a density of 1.0 g/cm3.

Section 3 of 8Finding the volume

Regular object
Regular object its faces meet at right angles; volume = length × width × height.
Example
  • 10 mm × 6 mm × 4 mm = 240 mm3.
Irregular object
  • Irregular object use displacement; the rise in water level in a measuring cylinder is its volume.
Reading
  • Reading keep your eye level with the surface of the liquid.
Floating object
  • Floating object it must be pushed completely under water to get the correct volume.

Section 4 of 8Measuring density

1
Check the balance reads 0.0 g and measure the mass of the object.
2
Measure the volume with a ruler or by displacement.
3
Divide the mass by the volume.
4
Record the results in a table.
Anomalous results
  • Anomalous results a result such as 52.7 g/cm3 for wood is clearly anomalous.
Safety
  • Safety slide heavy objects into measuring cylinders at an angle.

Section 5 of 8Comparing densities

MaterialDensity (g/cm3)
Helium0.00018
Air0.0012
Soft wood0.35 – 0.52
Cooking oil0.91 – 0.93
Water1.0
Concrete2.4
Aluminium2.7
Iron7.9
Osmium22
Gases
  • Gases have the lowest densities because the particles are far apart.
Solids
  • Solids have the highest densities because the particles are packed closest together.
Elements
  • Elements density usually increases as atomic number increases.

Section 6 of 8Floating and sinking

Rule
  • Rule more dense than water sinks; less dense than water floats.
Ships
  • Ships made of steel (8–9 g/cm3) but with large air spaces, so the average density of the whole ship is less than water.
Cargo
  • Cargo adds mass and increases a ship’s density.
Liquids
  • Liquids less dense liquids float on denser ones, e.g. oil on water.
Gases
  • Gases bubbles in drinks rise because the gas is less dense than the liquid.
Helium and hot air
  • Helium and hot air a helium balloon floats because it is less dense than air;
    • heated air expands, so a hot-air balloon floats.
Compression
  • Compression compressing a gas makes it denser; expanding it makes it less dense.

Section 7 of 8Key terms

TermMeaning
DensityThe mass per unit volume of a substance.
VolumeThe amount of space an object takes up.
Displacement methodFinding the volume of an irregular object from the rise in water level.
RegularHaving faces that meet at right angles.
IrregularHaving sides or angles that are not equal.
HollowHaving an air-filled space inside.

Section 8 of 8Exam technique

1
Heavy materials always have high density — density depends on mass in a given volume.
2
Density changes when you cut a sample — cutting a block in half halves both mass and volume, so density stays the same.
3
Ships float because steel is light — they float because their average density is less than water.
4
Density is mass × volume — density is mass ÷ volume.

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