Density and Archimedes' Principle Page Page has been visited times since March 21,2000

Density and Archimedes' Principle

SATURDAY COLLEGE

Instructor: Oliver Seely, Jr., Professor of Chemistry

Introduction

Tradition tells us that the king was suspicious about the purity of the gold in his crown and asked Archimedes to find a way to determine if it was the real thing. Solving the problem seemed to be impossible because in those days (3rd century B.C.) nothing was known about chemical analysis. One day Archimedes was thinking about the problem while taking a bath. As he lay floating in the bathtub he thought about his "weightless" body. Suddenly he realized that all bodies "lose" a little weight when placed in water, and the bigger their volume, the more weight they lose. He realized that the density of a metal can be found from its weight and its weight loss in water. The weight of the King's crown and its apparent loss of weight in water would tell him if it were made out of pure gold. Archimedes shouted "Eureka!" (I have found it!) and rushed out into the street naked to announce that he had solved the problem. Today the effect he observed is called Archimedes' Principle.

Objective

To determine the density of an object by two different methods and to compare the results.

Procedure

Method 1: Determination of density by direct measurement of volume.

The object you have is a cube of metal. The volume of a cube can be found from the formula V=a3, where a is the length of one edge in centimeters. The mass of the cube can be found by weighing it. Then the density can be determined by dividing the mass by the volume.

a. Weigh your cube on the electronic scale. Record your mass below.

b. Measure the edge of the cube in centimeters with your plastic ruler. Record the length below.

b. Calculate the volume. Record the volume below.

d. Calculate the density. Record the density below.

DATA

Mass of object: m1 = ______________ g

Length of edge of object = _____________ centimeters

Volume of object: V = edge x edge x edge = ______________ cubic centimeters (cc)

D = mass/volume = ____________ grams per cubic centimeter (cc)

(mass divided by volume)

Method 2: Archimedes' Principle

Archimedes' Principle says that the apparent weight of an object immersed in a liquid decreases by an amount equal to the weight of the volume of the liquid that it displaces. Since 1 mL of water has a mass almost exactly equal to 1g, if the object is immersed in water, the difference between the two masses (in grams) will equal (almost exactly) the volume (in mL) of the object weighed. Knowing the mass and the volume of an object allows us to calculate the density.

a. Record m1 below from the value on the previous page.

b. Set up balance arm hooked to wooden block and paper clip on other end.

c. Hang your cube on the paper clip.

d. Read the mass on the scale. This is m2. Don't worry if it is different from m1.

e. Fill the beaker with water up to within one inch of the top rim.

f. Immerse your cube in the water, being careful not to let it touch the walls or bottom.

g. Read the mass on the scale. This is m3. Record m3 below.

h. Subtract m2 from m3 and record the difference.

i. Calculate the density: divide m1 by the difference m2 - m3.

DATA

Mass of your object (m1 from the previous page): m1 = ______________ g

Electronic scale reading with object hanging on balance arm in the air: m2 = __________ g

Electronic scale reading with object on balance arm immersed in water: m3 = __________ g

Difference in mass of your object: m2 - m3 = _____________ g

m1

D = ---------- ________________ grams per cc

m2 - m3

Questions:

1. How do the two densities compare?

2. Why is Archimedes' Principle so important and well remembered if there is another perfectly good way (method 1) of measuring density?

Materials Required

5 Electronic lab scales (capacity 1200 g, ±0.1g)

5 250 mL beakers

30 metal objects -- all cubes, including lead!!

30 plastic metric rulers

5 wooden meter sticks

5 metal fulcrums

10 metal hangers

5 laboratory electronic calculators, placed in on benches as follows from left to right: 1,1,2,2

Box of regular size paper clips

5 special wooden blocks with hooks to be placed on electronic balances as counter weights.