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Density

    DENSITY is a physical property of matter, as each element and compound has a unique density associated with it. Density defined in a qualitative manner as the measure of the relative "heaviness" of objects with a constant volume.

    1. 1. Introduction
    2. 2. Examples
    3. 3. Mathematical Definition of Density
    4. 4. Contributors

    Introduction

    For example: A rock is obviously more dense than a crumpled piece of paper of the same size. A styrofoam cup is less dense than a ceramic cup. Density may also refer to how closely "packed" or "crowded" the material appears to be - again refer to the styrofoam vs. ceramic cup.

    Examples

    In chemistry, the density of many substances is compared to the density of water. Does an object float on water or sink in the water? If an object such as a piece of wood floats on water it is less dense than water vs. if a rock sinks, it is more dense than water.

    • Oil and vinegar salad dressing: The oil floats on the vinegar water mixture, while the solids sink to the bottom.
    • Oil spills: What happens when an oil tanker leaks on the ocean? The oil floats on the water since it is less dense, and this provides some opportunity to clean up the oil spills by skimming the oil from the surface of the water.
    • Ice: Everyone knows that ice floats on water, but did you know that this is an abnormal physical property of solid/liquid state of water? The more normal physical property is for the solid of a compound to sink in its own liquid.

    120density.GIF

    Figure 1: Examples of density

    Mathematical Definition of Density

    The formal definition of density is mass per unit volume. Usually the density is expressed in grams per mL or cc. Mathematically a "per" statement is translated as a division. cc is a cubic centimeter and is equal to a mL Therefore,

     

     Density =
     mass =
    		   g/mL
     
      volume
    		  

    Mass vs. Weight: Although the terms mass and weight are used almost interchangeably, there is a difference between them. Mass is a measure of the quantity of matter, which is constant all over the universe. Weight is proportional to mass but depends on location in the universe. Weight is the force exerted on a body by gravitational attraction (usually by the earth).

    Example: The mass of a man is constant. However the man may weigh: 150 lbs on earth, 25 lbs on the moon (because the force of gravity on the moon is 1/6 that of the earth), and be "weightless" in space.

    Demonstrations with Density

    • Mysterious Ice
    • Layers of Liquids
    • Egg Densities - sugar water/oil
    • Smart Eggs - salt water and acid
    • Floating Eggs - sugar and water
    • Floating Spheres
    • Lava Lamp
    • Underwater Smoke Stack
    • Floating objects in water
     Densities of Common Elements and Compounds
     Substance
     Density
    grams per mL
     Pine wood
    0.35 -0.50
     Water
    1.00
     Salt, NaCl
     2.16
     Aluminum, Al
     2.70
     Iron, Fe
     7.80
     Gold, Au
     19.30
     Mercury, Hg
     13.5

     In order to determine the density of an object, it is necessary to know: the mass, the volume of the substance, and the definition of density.

    Density = mass (g)
    volume (mL)
    Example: Calculate the density in g/mL of aluminum if a 50 mL block weighs135 g.

    Solution:
    Apply the definition:

     

     Density =
     135 g =
    		  2.70 g/mL
     
      50 mL
    		  
    If the density of a substance and either mass or volume is known, volume or mass, respectively, can be calculated using either simple algebra or dimensional analysis. The density must be translated as a conversion factor.
    Example: Calculate the mass in a 200 cc block of Titanium with a density of 4.51 g. per cc.

    Solution: The density translated as a conversion factor is:

    4.51 g = 1 cc - "per" is equivalent to an equal sign.

     

     200 cc x
     4.51 g =
    		  902 g
     

    Contributors

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