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Density

Density. Section 2.3. High population density (large population per square mile). Low population density (small population per square mile). Density in Science. Definition: Mass per unit volume Why do we need to know about density?

kirby-taylor
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Density

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  1. Density Section 2.3

  2. High population density (large population per square mile) Low population density (small population per square mile)

  3. Density in Science • Definition: Mass per unit volume • Why do we need to know about density? • Density is a physical property of elements and compounds. • Density is easy to measure using a balance and a ruler (for large objects) and a graduated cylinder & water (for small objects) • Unknown elements and compounds can be identified by finding their density!

  4. Some common densities

  5. Density and Buoyancy • Water has a density of 1.0 g/mL • Any substance with a density greater than 1.0 g/mL will sink in water (all metals, for example) • Any substance with a density less than 1.0 g/mL will float in water (ice, alcohol, all gases) • Mercury has a density of 13.6 g/mL • Any substance with a density greater than 13.6 g/mL will sink in mercury (don’t try this with gold!) • Any substance with a density less than 13.6 g/mL will float in mercury, including lead!

  6. Density Formulas • Density = mass ÷ volume m D = V 1 D 1 D m V D = V V D = m D V

  7. Units of Density m g Density has units of D = mL V

  8. Sample problem • An unknown liquid filled up to the 50.1 mL mark on a graduated cylinder has a mass of 40.5 g. What is the density of the substance? What is the unknown liquid? • D = m/V • D = 40.5 g / 50.1 mL • D = 0.80838 g / mL • D = 0.808 g/mL (only 3 significant digits allowed!)

  9. Some common densities

  10. More densities • No density: vacuum (0 g/mL) • Water: 1.0 g/mL • Rocks: 2 to 7 g/mL • Copper: 8.92 g/mL • Highest density element: Osmium (22.5 g/mL) • Neutron degenerate matter: 1 x 1015 g/mL (only found in neutron stars) • Black hole: near-infinite density (5 solar masses compressed to a point in space)

  11. Group work 1. A 20.0 g sample of lead causes the water level in a graduated cylinder to rise from 50.0 mL to 51.8 mL. Calculate the density of lead. 2. A platinum ring has a mass of 40.0 g and a density of 21.1 g/mL. If placed into a graduated cylinder filled to the 42.0 mL mark with water, to what level will the water rise? • A sample of aluminum has a volume of 2.0 L and a density of 2.7 g/mL. Calculate the mass of the sample. • An unknown material has a volume of 51,000 mL and a mass of 50,000 g. What is its density? Will it float in water? • A stack of 15 nickels has a volume of 4.0 mL and a density of 5.2 g/mL.What is the mass of each nickel?

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