### MEASUREMENT OF DENSITY

**Introduction**

In
order to classify and identify materials of a wide variety, scientists use
numbers called physical constants (e.g. density, melting point, boiling point,
index of refraction) which are characteristic of the material in question.
These constants do not vary with the amount or shape of the material, and are
therefore useful in positively identifying unknown materials. Standard reference works have been complied
containing lists of data for a wide variety of substances. The chemist makes
use of this in determining the identity of an unknown substance, by measuring
the appropriate physical constants in the laboratory, consulting the scientific
literature, and then comparing the measured physical constants with the values
for known materials. This experiment illustrates several approaches to the
measurement of the density of liquids and solids.

Density is a measure of the “compactness” of matter
within a substance and is defined by the equation:

Density = mass/volume eq
1.

^{}
The
standard metric units in use for mass and volume respectively are grams and
milliters or cubic centimeters. Thus,
density has the unit grams/milliter (g/ml) or grams/cubic centimenters (g/cc). The literature values are usually given in
this unit. Density may be calculated
from a separate mass and volume measurement, or, in the case of liquids, may be
determined directly by the use of an instrument called hydrometer.

Volume
measurements for liquids or gases are made using a graduated containers, for
example, a graduated cylinder. For
solids, the volume can be obtained either from the measurement of the
dimensions of the solid or by displacement.
The first method can be applied to solids with regular geometric shapes
for which the mathematical formulas can be used to calculate the volume of the
solid from the dimensions of the solid.
Alternatively, the volume of any solid object, irregular or regularly
shaped, can be measured by displacement.
The solid is submerged in a liquid in which it is not soluble, and the
volume of liquid displaced measured.

The
hydrometer measures density directly. An
object that is

__less__dense than a liquid will float in that liquid density to a depth such that the mass of the object submerged equals the mass of the of the liquid displaced (Archimedes' Principle). Since mass equals density X volume (see equation 1), an object floated in liquids of different densities will displace different volumes of liquid. A hydrometer is a tube of constant mass that has been calibrated to measure density by floating the hydrometer in liquids of known densities and recording on a scale the fraction of the hydrometer submerged. Any hydrometer can be used over a limited range of densities because the hydrometer must float in the liquid being studied and the hydrometer level must be sufficiently submerged to obtain an on scale reading. Hydrometers may be calibrated in g/ml or some other unit of density.
In
the following experiment, the identities of three colorless liquids will be
determined by measuring the densities of the liquids by two methods and then
comparing the density of the liquid to literature (reference) values for the
three liquids. The identity of an
unknown metal will be established in a similar manner.

**Procedure**

1) Weigh a clean, dry 50ml graduated
cylinder. Add approximately 30ml of
liquid to your weighed 50ml graduated cylinder without bothering to measuring
out the liquid accurately. Now carefully read and record whatever amount of
liquid there is in the cylinder. Weigh
the cylinder and liquid, and then calculate the density of the liquid. Repeat
this procedure to find the density of each liquid

2) Determine the density of each of the above,
using a hydrometer and an ungraduated cylinder.
Read the density from where the liquid crosses the hydrometer's scale.

3) Weigh and record the mass of an unknown metal
cylinder. Also record the identity of
the unknown metal cylinder (A, B, C, or D).
Calculate the volume of the metal cylinder by measuring (in cm) the
height (h) and diameter (d) of the metal cylinder and then applying the
formula: Volume (cc) = = h x 0.785d

^{2}. Also, measure the volume of the metal cylinder by displacement of water in a 50ml graduated cylinder. Calculate the density of the metal cylinder for each method of measuring volume and identify the metal by comparing the value obtained with the literature values for various metals.
4) Using any
appropriate procedure learned above, find the density of one of the following
more objects: a coin, a piece of chalk,
a small cork.,

Formulas
for volumes of regular shaped objects

Area of circle ¼ p d

^{2}, where d = diameter, and p = 3.14159
Volume of a cylinder = area of base x height

Volume of a sphere 1/6 p d

^{3}

**Data and Calulations**

Name___________________
Date_________________Lab section________

a) Weight of graduated
cylinder_______________g

Liquid A Liquid B Liquid C

Wt.of cyl + liquid ________g _________g _________g

Wt. of liquid ________g _________g _________g

Volume of liquid ________ml _________ml _________ml

Density _________g/ml _________g/ml _________g/ml

b) Density as determined with
hydrometer:

_________g/ml _________g/ml _________g/ml

Literature value:

_________g/ml _________g/ml __________g/ml

c) Data for metal cylinder

unknown No._________________

unknown color________________

weight _____________________g

height ____________________

__cm__
diameter __________________

__cm__
volume(a)_________________

__cc__(by calculation)
volume(b)_________________

__ml__(by displacement, 1 ml = 1 cc)
density(a)_________________(b)
_____________g/cc(g/ml)

identity of metal_______________

literature value of
density___________

literature source__________________

Density for special materials

1) Identity and description of material:

Mass of material ________________

Volume of material ________________

Density of material ________________

2) Identity and description of material:

Mass of material ________________

Volume of material ________________

Density of material ________________

3) Identity and description of material:

Mass of material ________________

Volume of material ________________

Density of material ________________

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