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What Is Chemistry?

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

11.    Pb

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

d.       process

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.

  1. 7.58 X 10-2 g to mg

  1. 8 X 10-10 m to nm

  1. 675 mm to mm