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A Closer Look: Chemical Vs. Physical Change
Physical change: Although some extensive properties (like shape, phase,
etc.) of the material change, the material itself is the same before and
after the change. The change can be “undone.”
Ice melting: an example of
physical change.
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Chemical change: The substances present at the beginning of the change are not present
at the end; new substances are formed. The change cannot
be “undone.”
A nail rusting: an example of
chemical change.
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Mass conservation: Mass is neither created nor
destroyed.
What are the macroscopic and microscopic differences between
physical and chemical changes?
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Macroscopic Definition |
Microscopic Definition |
| Physical Change |
The matter is the same.
The original matter can be recovered. |
The particles of the substance are rearranged. |
| Chemical Change |
The matter is different.
The old matter is no longer present. The original matter cannot be
recovered.
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The particles of the substance are broken apart, and the atoms are
rearranged into new particles, forming a new substance. |
Examples:
Many changes of state are easy to characterize based on the definitions
above, including the following:
| Physical Changes |
Chemical Changes |
| Aluminum foil is cut in half. |
Milk goes sour. |
| Clay is molded into a new shape. |
Jewelry tarnishes. |
| Butter melts on warm toast. |
Bread becomes toast. |
| Water evaporates from the surface of the ocean. |
Rust forms on a nail left outside. |
| A juice box in the freezer freezes. |
Gasoline is ignited. |
| Rubbing alcohol evaporates on your hand. |
Hydrogen peroxide bubbles in a cut. |
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Food scraps are turned into compost in a compost pile. |
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A match is lit. |
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You take an antacid to settle your stomach. |
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Your body digests food. |
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You fry an egg. |
Is the distinction always “clear-cut”?
There are many cases
where the distinctions between physical changes and chemical changes
are unclear. For example:
The dissolution of salt in water: This seems like
a physical change because we know we can recover the salt from the water.
However, if we look at
the microscopic level, we see that the two types of atoms in salt,
sodium and chlorine, separate from one another. In this example, we don’t
have a new substance, therefore this salt in solution doesn’t
fit the microscopic definition of a chemical change; but we also don’t
have the substance in its original form — a stack of alternating
sodium and chlorine atoms. Does this mean the change is half chemical
and half physical? Though it has aspects of a chemical change, scientists
would
still classify the dissolution of salt as a physical change.
The creation
of a metal alloy: If we melt two types of metal together, we create
an alloy metal that has different properties than either
of its components (e.g., heat conductivity, electrical conductivity,
density,
etc.). This might lead us to think that we’ve witnessed a chemical
change. In fact, a new particle is not created by melting two metals
together. This indicates they did not undergo a chemical reaction.
Brass, for example,
is about 60% copper and 40% zinc, and is composed of individual copper
and zinc atoms (i.e., there is no “smallest unit” that
is still brass). There is no such thing as a brass molecule.
The heating
and cooling of certain rubbers and plastics: One might think that
exposing certain rubbers and plastics to heat or cold
would cause
a chemical reaction because the properties change (e.g., the materials
become more rigid and brittle). While chemical reactions do take
place, they simply bind together different parts of the large molecules
that
compose rubber and plastic. These new bonds add to the rigidity
of the material,
but the particles of the substances remain the same.
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