Chemistry Reference
In-Depth Information
2H
2
O
(l)
+ energy
2H
2(g)
+ O
2(g)
Two water molecules are broken down into two hydrogen molecules and
one molecule of oxygen.
Figure 1-4a
water evaporates to form water vapor, it only requires a temperature change
to bring it back to liquid water. In the reaction shown in Figure 1-4a, the
hydrogen and oxygen won't join to make water again unless another chemi-
cal reaction is carried out.
Examples of Chemical Changes
Photosynthesis:
6O
2
+ C
6
H
12
O
6
6H
2
O + 6CO
2
Zinc reacting with an acid:
Zn + 2HCl
ZnCl
2
+ H
2
Methane gas burning:
CH
4
+ 2O
2
CO
2
+ 2H
2
O
Synthesis of magnesium oxide:
Mg + O
2
2MgO
You may have noticed that chemical equations are sometimes written
with more information than at other times. For example, I don't always
include the subscripts (for example, (s) or (g)), which indicate the state of
the matter at the time of the reaction. Also, I don't always indicate whether
energy is present as a reactant or as a product. When you use these nota-
tions depends upon the type of information that you want to convey at a
given time. In general, it is always okay to show more information than you
need to. In the next lesson you will learn how to show energy changes in
chemical equations.
Try the following practice questions and check your answers at the end
of the chapter before moving on to the next lesson.
Lesson 1- 4 Review
1.
_______________ changes produce new substances, with new
properties.
2.
The Law of _______________ of Mass states that matter cannot be
created or destroyed.
