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2.S: The Physical and Chemical Properties of Matter (Summary)
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A
compound
is defined as a substance that results from the combination of two or more elements in a constant ratio. In a compound such as water, we show the ratio of the elements (hydrogen and oxygen) by using
subscripts
on the chemical symbols for each element. Thus, water (two hydrogens and one oxygen) is written using the
chemical formula
H
2
O. In a
molecule
, the atoms are not only bonded together in a constant ratio, but they are also bonded in a
specific geometric arrangement
.
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A
pure substance
contains only one kind of matter;it can be a single element or a single chemical compound. Two or more pure substances mixed together constitute a mixture;you can always separate a mixture by simple physical means.
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A
heterogeneous mixture
is not uniform and different samples of the mixture will have a different compositions. A
homogeneous mixture
, is uniform and any sample that you examine will have
exactly
the same composition as any other sample. Within chemistry, the most common type of homogeneous mixture is a
solution
.
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Any pure substance, under appropriate conditions, can exist in three different
states
: solids, liquids and gases. States of matter are examples of
physical properties
of a substance. Other physical properties include appearance (shiny, dull, smooth, rough), odor, electrical conductivity, thermal conductivity, hardness and density, etc.
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Solids
have both a definite shape and volume.
Liquids
have a definite volume, but take on the shape of their container.
Gasses
have neither a definite shape nor volume, and both of these are defined by the shape and volume of their container.
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The
kinetic molecular theory (KMT)
is generally used to explain physical states of matter. The
KMT
suggests that atoms and molecules are
always
in motion and are loosely bound to each other by attractive called
intermolecular forces
. In a
solid
, the kinetic energy (energy of motion) associated with the atoms or molecules is insufficient to break these forces and the particles are essentially fixed in place, adjacent to each other. In a
liquid
, there is enough kinetic energy to break some of the attractive forces, allowing the particles to “slip and slide” next to each other, but there is not enough energy to allow them to escape. In a
gas
, there is sufficient kinetic energy to totally overcome the forces and the particles have no interactions with each other.
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A change of state from a solid to a liquid occurs at a defined temperature (which) called the
melting point
(or freezing point);this temperature is a unique physical property of the substance. The transition from a liquid to a gas, likewise, occurs at the
boiling point
. A direct transition from a solid to a gas is called
sublimation
.
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An
intensive property
is defined as a property that is
inherent
to the substance and is not dependent on the sample size.
Density
, the ratio of mass-to-volume for a substance, is a classic example of an intensive property.
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Density
is calculated by taking the
mass
of a sample of a substance, and dividing that by the
volume
of that sample. Density for solids is typically expressed using units of grams per cubic centimeter (g cm
-3
);liquids as grams per milliliter (g mL
-1
) and gasses as grams per liter (g L
-1
), although any mixture of mass and volume units may be used. Remember, a mL has the same volume as a cm
3
, and a L is simply 1000 mL.
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Physical changes
are changes in outward appearances that do not alter the chemical nature of the substance and produce no new substance. When a
chemical change
occurs, a new substance is produced. Just like physical properties describe the appearance or intensive properties of a substance,
chemical properties
describe the set of chemical changes that are possible for that substance.
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The
law of mass conservation
(conservation of mass) simply states, that there is no detectable change in the total mass of materials when they react chemically (undergo a chemical change) to form new substances.