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? –D–Density is a physical property of elements and compounds. –D–Density is easy to measure using a balance and a ruler (for large objects) and a graduated cylinder & water (for small objects) –U–Unknown elements and compounds can be identified by finding their density!

Some common densities

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!

Density Formulas Density = mass ÷ volume D = m V m V VV D mV 1 D 1 D = D

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

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 = g / mL D = g/mL (only 3 significant digits allowed!)

Some common densities

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 g/mL (only found in neutron stars) Black hole: near-infinite density (5 solar masses compressed to a point in space)

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? 3.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. 4.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? 5.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?