Chemistry Reference
In-Depth Information
Lesson 1- 4: Chemical and Physical Changes
Matter can undergo many types of changes. In nuclear changes, atoms
can be split apart or fused together with other atoms, and thus transformed
into other elements. (We will learn more about these types of nuclear
changes in Lesson 6-5.) As you study chemistry, you are more likely to
observe matter undergoing various chemical and physical changes. The Law
of Conservation of Mass states that matter cannot be created or destroyed;
it can only change form. This law, as it is stated here, does not apply to
nuclear changes, yet it holds true with regard to physical and chemical
changes. It means that if you start off with 10.0 g of matter before a physi-
cal or chemical reaction, you will end up with 10.0 g of matter. These changes
don't create or destroy atoms; they only change how the atoms are con-
nected to each other. Even if we burn a piece of wood in a fire, we are not
destroying atoms; we are just breaking bonds between atoms and forming
new ones.
Physical changes are changes that do not result in the production of a
new substance. If I break a glass beaker, I won't be able to use it anymore,
but the material does not change into a new element or compound. If I
crumple up a ball of aluminum foil, I have changed its shape and size, but I
still have aluminum foil. Even a significant physical change, such as grind-
ing a piece of wood into sawdust, is not a chemical change, because it does
not result in the production of a new substance.
The “change-of-phase operations,” discussed in Lesson 1-2, are very
common physical changes. Imagine a beautiful ice sculpture in the shape
of a swan, on a table at an outdoor party. The sculpture is made up of
frozen water, which can be indicated with the notation H
2
O
(s)
, where the
subscript “s” stands for “solid.” As the day wears on and the ice sculpture
continues to absorb heat from its surroundings, much of the ice will melt,
producing a puddle of liquid water, H
2
O
(l)
. Now, the liquid water will con-
tinue to absorb energy from its surroundings, and, over time, much of it
will evaporate, becoming gaseous water, H
2
O
(g)
. Despite the fact that the
water has existed in three different phases, it has remained water, H
2
O.
Because none of these changes resulted in the production of a new sub-
stance, they are each examples of a physical change.
Sometimes, you will see a change-of-phase operation illustrated in a
form that you may associate with chemical reactions, such as the equation
for melting ice that you see following this paragraph. Don't be confused by
the format, it is still showing a physical change.
H
2
O
(s)
+ energy
H
2
O
(l)
