Page 102 - Laboratory Manual in Physical Geology

WATER DISPLACEMENT METHOD How to Determine Mass. Earth materials do not just take
FOR DETERMINING VOLUME up space (volume). They also have a mass of atoms that
OF A MINERAL SAMPLE can be weighed. You will use a gram balance to measure
the mass of materials (by determining their weight under
the pull of Earth’s gravity). The gram (g) is the basic unit
mL Written mL or ml mL of mass in the metric system, but instruments used to
10 10 measure grams vary from triple-beam balances to spring
scales to digital balances (page viii ). Consult with your
laboratory instructor or other students to be sure that you
9 9
understand how to read the gram balance provided in
your laboratory.
8 8 7.8 mL
How to Calculate Density and Specific Gravity. Every

material has a mass that can be weighed and a volume of
7 7
C. Volume space that it occupies. However, the relationship between
of water is
2.8 mL a material’s mass and volume tends to vary from one
6 6 kind of material to another. For example, a bucket of
rocks has much greater mass than an equal-sized bucket
5.0 mL of air. Therefore a useful way to describe an object is to
5 5
determine its mass per unit of volume, called density .
Per refers to division, as in miles per hour (distance
4 4
divided by time). So density is the measure of an object’s
mass divided by its volume (density = mass ÷ volume).
3 3 Scientists and mathematicians use the Greek character
Mineral
sample rho (r) to represent density. Also, the gram (g) is the
basic metric unit of mass, and the cubic centimeter is
2 2
3
the basic unit of metric volume (cm ), so density (r) is
3
usually expressed in grams per cubic centimeter (g/cm ).
1 1 For example:
Mineral sample weighs 44.0 grams

Mineral sample takes up 11.0 ml of volume
A. Starting volume B. Ending volume
of water of water 44.0 g
3
= = 4.00 g>cm = r
11.0 cm 3
PROCEDURES
Specific gravity (SG) is the ratio of the density
A. Place water in the bottom of a graduated
cylinder. Add enough water to be able to totally of a substance divided by the density of water. Since
3

immerse the mineral sample. It is also helpful to water has a density of 1 g>cm and the units cancel out,
use a dropper bottle or wash bottle and bring specific gravity is the same number as density but without
the volume of water (before adding the mineral any units. In the example above, the specific gravity of
sample) up to an exact graduation mark like the the mineral sample would be 4.00 (i.e., SG = 4.00).
5.0 mL mark above. Record this starting volume 3
of water. The mineral quartz has a density of 2.65 g/cm so its
specific gravity is 2.65 (i.e., SG = 2.65).
B. Carefully slide the mineral sample down into
the same graduated cylinder, and record the
ending volume of the water (7.8 mL in the above Calculating Density and Specific Gravity—
example).
The Math You Need
C. Subtract the starting volume of water from You can learn more about calculating density
the ending volume of water to obtain the and specific gravity at this site featuring
displaced volume of water. In the above
example: 7.8 mL – 5.0 mL = 2.8 mL (2.8 mL is The Math You Need, When You Need It
3
the same as 2.8 cm ). This volume of displaced math tutorials for students in introductory
water is the volume of the mineral sample. geoscience courses: http://serc.carleton.edu/
mathyouneed/density/index.html
FIGURE 3.17 How to determine volume of a mineral sample.

Mineral Properties, Identification, and Uses ■ 87