Properties of Pure
Substances
Pure Substance

A substance that has a fixed chemical composition throughout is
called pure substance.

Ex: Water, nitrogen, helium, carbon dioxide
Pure Substance

However, it does not have to be of a single chemical element or
compound. A mixture of various chemical elements or compounds also
qualifies as a pure substance as long as the mixture is homogeneous.

Ex: Air (gaseous)… Although it is mixture of several gases, it is often
considered as pure substance because it has a uniform chemical
composition.

But oil and water (although mixture) is not a pure substance since oil
is not soluble in water and it will collect on top of the water, forming
two chemically dissimilar regions.
Pure Substance

A mixture of two or more phases of a pure substance is still a pure
substance as long as the chemical composition of all phases is the
same.

Ex: mixture of ice and water… It is a pure substance because
phases have the same chemical composition.

A mixture of liquid air and gaseous air, however, is not a pure
substance the composition of gaseous air is not the same as the
liquid air, making the mixture not homogeneous.
Phases of Pure Substance

There are three principal phases: solid, liquid and gas.

However, a substance may also have several phases within a
principal phase each with a different molecular structure.

Ex: carbon may exist as graphite or diamond in solid phase, helium
has two liquid phase and iron has three solid phase.
Phase: Solids

Solids have strongest molecular bonds.

Solids are closely packed three dimensional crystals.

Their molecules do not move relative to each other
Phase: Liquids

Intermediate molecular bond strength

Liquid molecular spacing is comparable to solids but their
molecules can float about in groups.

There is molecular order within the groups
Phase: Gas

Weakest molecular bond strength.

Molecules in the gas phases are far apart, they have no ordered
structure

The molecules move randomly and collide with each other.

Their molecules are at higher energy levels, they must release
large amounts of energy to condense or freeze.
Phase – Change Processes Of Pure
Substances

It is often important to consider the liquid to gas phase change
process. Not so much solid to liquid because thermodynamics deals only
with liquid to gases (or vice versa) to generate power.
Gibb’s Phase Rule:
The Phase Rule states that the number
of known properties of a pure substance
needed in order to also determine its
other properties may be computed if
the number of phases and the number
of components present in the said
substance is known.
Gibb’s Phase Rule:
P+F=C+2
P = number of phases (homogeneous region of
matter)
C = number of components (minimum number of
chemical species required to prepare the system)
F = degrees of freedom (number of properties that
can be independently varied)
For instance, if the degrees of
freedom for a given system is equal
to two, then, it means that knowing
only two properties of the system
will exactly determine the state of
the system, hence, all its other
properties will also be known.
Examples:
Determine the degrees of freedom for the
following systems:
1. liquid water in equilibrium with its vapor
2. liquid water in equilibrium with a mixture
of water vapor and nitrogen
3. liquid solution of alcohol in water in
equilibrium with its vapor
4. decomposition of calcium carbonate
5. mixture of ice, liquid water, and gaseous
water