If someone were to ask you to define the
word “chemistry,” what would you say? You might be tempted to answer that it is
just a subject that most students are required to study. However, chemistry is
much more than that, for it explains our world around us. By studying
chemistry, we learn about the composition, structure, properties, and changes
of matter that makes up our world. Equally important, it helps us understand
how the composition of matter relates to the kinds of elements it contains.
Chemistry, also, explains how the structure of matter relates to the ways the
atoms of these elements are arranged. Thus, without chemistry, we would not
understand the function of. Everyday life would be much different from what it
is now. Can you imagine hot summers without an air conditioner or a cold soda?
If you needed an operation, would you want your doctor to perform surgery
without anesthesia? Ouch!
Directions: Did you get all that? Let’s stop for
a minute to make sure. Click on the words “cloze exercise” to check if you can remember what you just read.
Once you have completed it, click on the back button of your browser to
return to this page to learn more about chemistry.
We
mentioned the word “matter” several times in paragraph 1. If you are just
beginning to study chemistry, you may wonder what the term “matter” refers to.
The easiest way to explain matter is to describe it. First, matter exists in
four physical states (phases). Oh, but you have always heard that there are
only three physical states – solid, liquid, and gas - right? Well,
there is actually a fourth, plasma, but we
will discuss it later since chemists and pharmacists deal primarily with the
three states commonly found here on the surface of the earth.
Each state can be further divided into pure
substances and mixtures. When scientists use the term “pure substance,” they
are referring to either an element or a compound. Oxygen is an example of an
element because it is composed of a single kind of atom. Scientists use a
chemical symbol consisting of one or two letters to represent the known
elements. For example, O2
stands for oxygen while chlorine is represented by Cl2. In contrast,
compounds – such as sulfuric acid – are composed of two or more elements joined
chemically. They, too, are represented by chemical symbols consisting of two or
more letters, e.g. H2SO4 for sulfuric acid. According to
the law of definite proportions, the elemental composition of a pure compound
never changes.
Do you
remember the second type of pure substance we mentioned earlier? If you
answered “mixtures,” you are correct. Some common mixtures are sugar and
concrete. Concrete is a good example of why mixtures are mixtures are not
compounds. Why? It is because the basic parts of compounds cannot be removed by
physical force. For example, if you were apply physical force (grind up solid
concrete), you would be able to separate the individual components. You cannot
do this with compounds. There are two types: homogeneous mixtures (solutions)
and heterogeneous mixtures. Mixtures that are uniform in composition,
properties, and appearance throughout the mixture are called homogeneous
mixtures. Many of these mixtures are soluble in water. Common examples of
homogeneous mixtures which are soluble are sugar and concrete. In contrast to
homogeneous mixtures, the composition, properties, and appearance of
heterogeneous mixtures vary throughout the mixture. Sand and copper sulfate are
examples of heterogeneous mixtures.
Did you get all that?
Let’s stop for a minute to make sure. Do the cloze exercise. Once
you have completed it, click on the back button of your browser to return to
this page. Then, answer the questions below.
Directions: Based on the reading above, classify each of the substances below as a pure substance, homogeneous mixture, or heterogeneous mixture.
|
1. dirt
|
2.
palm trees
|
|
3.
banana custard
|
4. mercury
|
|
5.
seawater |
6.
black ink in a pen
|
Directions:
Give the chemical symbol for the elements below, or give the name of the
chemical elements represented by the chemical symbol. If you need help, go to WebElementsTM, and
then click the back button to return to this page.
|
7.
tin |
8.
K |
|
9.
Hg |
10.
gold |
|
|
12.
magnesium |
Directions: After answering the questions below, check your answers.
13. Suggest a method for
determining if saltwater is a compound. Answer
14. Suggest a method
for separating a mixture of iron and sulfur. Answer
Directions: Answer the questions below. If you are unable to answer the question correctly, a link has been provided for each question to explain why the answer is correct.
15. Read the following
sentence: Common examples of homogeneous mixtures which are soluble are sugar
and salt. Based only on this sentence, how do you know that not all homogeneous
mixtures are soluble? Answer
16.
The
reading stated that “According to the law of definite proportions, the
elemental composition of a pure compound never changes.” Let’s revise this
sentence to read as follows: “According to the law of definite proportions; the
elemental composition of a pure compound never changes.” Why is the revised
sentence incorrect? Answer
17.
What
is the subject of the reading?
a.
matter
b.
pure
substances and mixtures
c.
the
states of matter
d.
heterogeneous
and homogeneous mixtures
18.
What
is the purpose of the reading?
a.
classification,
b.
comparison
c.
definition
19.
Which
audience was the reading most likely written for?
a.
doctors
b.
first-year
students studying pharmacy
c.
average
newspaper readers
d.
Arab
males
20. Referring to
question 20, why? Answer
Earlier,
we said that there are four states of matter. Do you remember what the fourth
one is? Yes, you are correct. It is plasma, which is especially important in
the sciences of astronomy and physics. All substances can exist in one of four
forms (states or phases). A good example of this is H2O. If it appears
as ice, it is a solid. However, if it exists as water, it is a liquid. It also
is present as a gas when it is in the form of water vapor. Its fourth state is
as plasma. Because H2O is a compound, it cannot be separated by
physical force, but it can be altered into another state by physical force. The
key is energy. Energy, in the case of H2O, relates to temperature
and pressure. At normal temperatures, H2O exists as water. When you
take away the energy (temperature), it becomes a solid, ice. However, if you
increase the temperature, it takes on the form of water vapor, a gas. If you
continue to add energy (e.g. increase the temperature as well as the pressure),
H2O begins to lose electrons, and it moves to a more active state,
plasma.
Directions: Click
on the words “self-test”
to check if you remember what you just read.
Chemistry, like other sciences, uses
measurements. While some measurements are precise, they are not necessarily
accurate. You may be wondering how this can be true. The answer lies in the
definitions of precise and accurate. Precise means that two or more
measurements of a quantity closely agree; however, how well the measurement of
a quantity agrees with the accepted (true) value of that quantity means it is
accurate. In other words, a measurement may be precise, but not accurate. Now
that you understand the difference between precision and accuracy, let’s move
on to conversions. In chemistry, many
measurements quite often must be converted from one unit to another, e.g. from
grams to kilograms. Do not read any further if you are a coward. What follows
is only for those brave individuals who seek to be the heroes of their chemistry
classes.
Directions:
Perform
the conversions below. If you do not have a calculator handy (nearby), use your
computer’s calculator.
