The properties of a substance are those
characteristics that are used to identify or describe it. When we say that water is
"wet", or that silver is "shiny", we are describing materials in terms
of their properties. Properties can be divided into the categories of physical
properties and chemical properties. Physical properties
are readily observable, like; color, size, luster, or smell. Chemical
properties are only observable during a chemical reaction. For example, you
might not know if sulfur is combustible unless you tried to burn it.
Another way of separating kinds of
properties is to think about whether or not the size of a sample would affect a
particular property. No matter how much pure copper you have, it always has the same
distinctive color. No matter how much water you have, it always freezes at zero
degrees Celsius under standard atmospheric conditions. Methane gas is combustible,
no matter the size of the sample. Properties, which do not depend on the size of the
sample involved, like those described above, are called intensive properties.
Some of the most common intensive properties are; density, freezing point, color, melting
point, reactivity, luster, malleability, and conductivity.
Extensive properties
are those that do depend on the size of the sample involved. A large sample of
carbon would take up a bigger area than a small sample of carbon, so volume is an
extensive property. Some of the most common types of extensive properties are;
length, volume, mass and weight.
Pieces of matter undergo various changes
all of the time. Some changes, like an increase in temperature, are relatively
minor. Other changes, like the combustion of a piece of wood, are fairly drastic.
These changes are divided into the categories of Physical and Chemical change.
The main factor that distinguishes one category form the other is whether or not a
particular change results in the production of a new substance.
Physical changes are
those changes that do not result in the production of a new substance. If you melt a
block of ice, you still have H2O at the end of the change. If you break a
bottle, you still have glass. Painting a piece of wood will not make it stop being
wood. Some common examples of physical changes are; melting, freezing, condensing,
breaking, crushing, cutting, and bending. Special types of physical changes where
any object changes state, such as when water freezes or evaporates, are sometimes called change
of state operations.
Chemical changes, or
chemical reactions, are changes that result in the production of another substance.
When you burn a log in a fireplace, you are carrying out a chemical reaction that releases
carbon. When you light your Bunsen burner in lab, you are carrying out a chemical
reaction that produces water and carbon dioxide. Common examples of chemical changes
that you may be somewhat familiar with are; digestion, respiration, photosynthesis,
burning, and decomposition.
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