Experiment 1: Physical Properties of Organic Compounds
Introduction
Physical properties are defined as a property that can be measured without changing the
composition of the substance. The physical properties that will be measured during this
lab are solubility, density, melting point, boiling point and refractive index. These
physical properties can help elucidate the identity of an unknown substance. These same
properties can also be used to determine the purity of a particular compound. Let us look
at each of these properties individually.
Solubility is defined as the maximum mass of a substance that can be dissolved in a fixed
mass of a solvent at a given temperature. A substance will have a different solubility in
different solvents depending on polarity of both the substance and the solvent. The old
adage is that like dissolves like, hence a polar substance will be more soluble in a polar
solvent and vice versa. The solvents used in this experiment will be water, ethanol,
acetone and benzene.
Density is defined as the mass of a substance per a unit volume. In this experiment,
grams per milliliter will be the density units. A dense liquid such as mercury has a
density of 13.546 g/mL. In contrast, hexane has a density of only 0.659 g/mL. Hence,
one can use the density in determining the identity of an unknown substance.
The melting point is defined as the temperature at which a substance goes from a solid to
a liquid. For example, water (ice) changes from a solid to a liquid at 0 Celsius at STP.
Although the melting point is usually listed at a certain temperature, most substances melt
over a temperature range. The temperature range is defined as the temperature at which a
substance begins to melt to the temperature at which the whole sample is completely a
liquid. A smaller temperature range would indicate a purer substance.
The boiling point is defined as the temperature at which a substance goes from a liquid to
a gas. For example, water changes to steam at 100 Celsius at STP. However, unlike the
melting point, boiling points are significantly affected by atmospheric pressure.
The refractive index can also be used to determine the identity of an unknown substance.
Refraction is the changing in the direction of a ray of light passing from one medium to
another. You have probably observed this looking down into the water while standing in
it. The refraction of light causes you legs to look shorter. As with the other physical
properties, a substance has unique refractive index.
