About This Text
1.2 The Study of Chemistry
Chemistry has been called the “central science” because it is important to so many other fi elds of scientifi c study. So, even if you have never taken a chemistry course, chances are good that you have seen some chemistry before. This text and the course in which you are using it are designed to help you connect pieces of information you have already picked up, increase your understanding of chemical concepts, and give you a more coherent and systematic picture of chemistry. The ultimate goal of introductory college chemistry courses is to help you appreciate the chemical viewpoint and the way it can help you to understand the natural world. This type of perspective of the world is what enables chemists and engineers to devise strategies for refi ning metals from their ores, as well as to approach the many other applied problems we’ll explore.
This coherent picture involves three levels of understanding or perspectives on the nature of chemistry: macroscopic, microscopic, and symbolic. By the end of this course, you should be able to switch among these perspectives to look at prob- lems involving chemistry in several ways. The things we can see about substances and their reactions provide the macroscopic perspective. We need to interpret these events considering the microscopic (or “particulate”) perspective, where we focus on the smallest components of the system. Finally, we need to be able to communicate these concepts effi ciently, so chemists have devised a symbolic perspective that allows us to do that. We can look at these three aspects of chemistry fi rst, to provide a refer- ence for framing our studies at the outset.
The Macroscopic Perspective
When we observe chemical reactions in the laboratory or in the world around us, we are observing matter at the macroscopic level. Matter is anything that has mass and can be observed. We are so often in contact with matter that we tend to accept our intuitive feel for its existence as an adequate defi nition. When we study chemistry, however, we need to be aware that some of what we observe in nature is not matter.
For example, light is not considered matter because it has no mass.
When we take a close look at matter—in this case aluminum—we can see that various questions arise. The behavior of the aluminum in a can is predictable. If the can is tossed into the air, little will happen except that the can will fall to the earth under the force of The aluminum in bauxite is typically
found in one of three minerals: gibbsite, bohmite, and diaspore.
The aluminum in bauxite is typically found in one of three minerals: gibbsite, bohmite, and diaspore.
gravity. Aluminum cans and other consumer goods like those shown in Figure 1.2 do not decompose in the air or undergo other chemical reactions. If the aluminum from a soda can is ground into a fi ne powder and tossed into the air, however, it may ignite—chemically combining with the oxygen in air. It is now believed that the Hindenburg airship burned primarily because it was covered with a paint containing alumi- num powder and not because it was fi lled with hydrogen gas. (You can easily fi nd a summary of the evidence by doing a web search.)
One of the most common ways to observe matter is to allow it to change in some way. Two types of changes can be distinguished:
physical changes and chemical changes. The substances involved in a physical change do not lose their chemical identities. Physical properties are variables that we can measure without changing the identity of the substance being observed. Mass and density are familiar physical properties. Mass is measured by comparing the object given and some standard, using a balance. Density is a ratio of mass to volume. (This variable is sometimes called mass density).
To determine density, both mass and volume must be measured. But these values can be obtained without changing the material, so den- sity is a physical property. Familiar examples of physical properties also include color, viscosity, hardness, and temperature. Some other physical properties, which will be defi ned later, include heat capacity, boiling point, melting point, and volatility.
Chemical properties are associated with the types of chemical changes that a substance undergoes. For example, some materials burn readily, whereas others do not. Burning in oxygen is a chemical reaction called combustion. Corrosion—the degradation of metals in the presence of air and moisture—is another commonly observed chemical change. Treating a metal with some other material, such as paint, can often prevent the damage caused by corrosion. Thus an important chemical prop- erty of paint is its ability to prevent corrosion. Chemical properties can be determined only by observing how a substance changes its identity in chemical reactions.
Both chemical and physical properties of aluminum are important to its utility. A structural material is useful only if it can be formed into desired shapes, which requires it to be malleable. Malleability is a measure of a material’s ability to be rolled or ham- mered into thin sheets, and metals are valuable in part because of their malleability. It is a physical property because the substance remains intact—it is still the same metal, just in a different shape. An aluminum can is formed during its manufacturing process, but its shape can be changed, as you have perhaps done many times when you crushed a can to put it into a recycling bin. Similarly, the chemical properties of aluminum are important. Pure aluminum would be very likely to react with the acids in many popu- lar soft drinks. So aluminum cans are coated inside with a thin layer of polymer—a plastic—to keep the metal from reacting with the contents. This demonstrates how knowing chemical properties can allow product designers to account for and avoid potentially harmful reactions.
When we observe chemical reactions macroscopically, we encounter three common states, or phases, of matter: solids, liquids, and gases. At the macroscopic level, solids are hard and do not change their shapes easily. When a solid is placed in a container, it retains its own shape rather than assuming that of the container. Even a powdered solid demonstrates this trait because the individual particles still retain their shape, even though the collection of them may take on the shape of the container.
Liquids can be distinguished from solids macroscopically because unlike solids, liquids adapt to the shape of the container in which they are held. They may not fi ll the entire volume, but the portion they do occupy has its shape defi ned by the container. Finally, gases can be distinguished macroscopically from both liquids and solids primarily because a gas expands to occupy the entire volume of its container.
Although many gases are colorless and thus invisible, the observation that a gas fi lls
We will discuss corrosion and its prevention in detail in Chapter 13.
We will discuss corrosion and its prevention in detail in Chapter 13.
Aluminum is generally found second, behind gold, in rankings of metal malleability.
Aluminum is generally found second, behind gold, in rankings of metal malleability.
Two other states of matter are plasmas and Bose-Einstein condensates.
But these do not exist at ordinary temperatures.
Two other states of matter are plasmas and Bose-Einstein condensates.
But these do not exist at ordinary temperatures.
Figure 1.2 ❚ All of the common kitchen items shown here are made of aluminum. The metal’s light weight, corrosion resistance, and low cost make it a likely choice for many consumer products.
© Cengage Learning/Charles D. Winters
the available volume is a common experience; when we walk through a large room, we are not concerned that we will hit a pocket with no air.
The aluminum that we encounter daily is a solid, but during the refi ning process, the metal must become molten, or liquid. Handling the molten metal, pouring it into containers, and separating impurities provide both chemical and engineering chal- lenges for those who design aluminum production plants.
Often, chemical and physical properties are diffi cult to distinguish at the macro- scopic level. We can assert that boiling water is a physical change, but if you do noth- ing more than observe that the water in a boiling pot disappears, how do you know if it has undergone a chemical or physical change? To answer this type of question, we need to consider the particles that make up the water, or whatever we observe, and consider what is happening at the microscopic level.
The Microscopic or Particulate Perspective
The most fundamental tenet of chemistry is that all matter is composed of atoms and molecules. This is why chemists tend to think of everything as “a chemical” of one sort or another. In many cases, the matter we encounter is a complex mixture of chemicals, and we refer to each individual component as a chemical substance. We will defi ne these terms much more extensively as our study of chemistry develops, but we’ll use basic defi nitions here. All matter comprises a limited number of “building blocks,” called elements. Often, the elements are associated with the periodic table of elements, shown inside the back cover of this textbook and probably hanging in the room where your chemistry class meets. Atoms are unimaginably small particles that cannot be made any smaller and still behave like a chemical system. When we study matter at levels smaller than an atom, we move into nuclear or elementary particle physics. But atoms are the smallest particles that can exist and retain the chemical identity of whatever element they happen to be. Molecules are groups of atoms held together so that they form a unit whose identity is distinguishably different from the atoms alone. Ultimately, we will see how forces known as “chemical bonds” are responsible for holding the atoms together in these molecules.
The particulate perspective provides a more detailed look at the distinction be- tween chemical and physical changes. Because atoms and molecules are far too small to observe directly or to photograph, typically we will use simplifi ed, schematic draw- ings to depict them in this book. Often, atoms and molecules will be drawn as spheres to depict them and consider their changes.
If we consider solids, liquids, and gases, how do they differ at the particulate level? Figure 1.3 provides a very simple but useful illustration. Note that the atoms The word atom comes from the Greek
word “atomos” meaning indivisible.
The word atom comes from the Greek word “atomos” meaning indivisible.
To correctly depict the relative densities of a gas and a liquid, much more space would need to be shown between particles in a gas than can be shown in a drawing like Figure 1.3.
Figure 1.3 ❚ Particulate level views of the solid, liquid, and gas phases of matter. In a solid, the molecules maintain a regular ordered structure, so a sample maintains its size and shape. In a liquid, the molecules remain close to one another, but the ordered array breaks down. At the macroscopic level, this allows the liquid to fl ow and take on the shape of its container. In the gas phase, the molecules are very widely separated, and move independently of one another. This allows the gas to fi ll the available volume of the container.
Solid Liquid Gas
in a solid are packed closely together, and it is depicted as maintaining its shape—here as a block or chunk. The liquid phase also has its constituent particles closely packed, but they are shown fi lling the bottom of the container rather than maintaining their shape. Finally, the gas is shown with much larger distances between the particles, and the par- ticles themselves move freely through the entire volume of the container. These pictures have been inferred from experiments that have been conducted over many years.
Many solids, for example, have well-ordered structures, called crystals, so a particulate representation of solids usu- ally includes this sense of order.
How can we distinguish between a chemical and a physical change in this perspective? The difference is much easier to denote at this level, though often it is no more obvious to observe. If a process is a physical change, the at- oms or molecules themselves do not change at all. To look at this idea, we turn to a “famous” molecule—water. Many people who have never studied chemistry can tell you that the chemical formula of water is “H two O.” We depict this molecule using different sized spheres; the slightly larger sphere represents oxygen and the smaller spheres represent hydrogen. In Figure 1.4, we see that when water boils, the composition of the individual molecules is the same in the liquid phase and the gas phase. Water has not been altered, and this fact is characteristic of a physical change.
Contrast this with Figure 1.5, which depicts a process called electrolysis at the particulate level; electrolysis occurs
when water is exposed to an electric current. Notice that the molecules themselves change in this depiction, as water molecules are converted into hydrogen and oxygen molecules. Here, then, we have a chemical change.
If we observe these two reactions macroscopically, what would we see and how would we know the difference? In both cases, we would see bubbles forming, only in one case the bubbles will contain water vapor (gas) and in the other they contain hydrogen or oxygen. Despite this similarity, we can make observations at the macro- scopic level to distinguish between these two possibilities. Example Problem 1.1 poses an experiment that could be set up to make such an observation.
Microscopic view Macroscopic view
H2O (liquid) H2O (gas)
Figure 1.4 ❚ The boiling of water is a physical change, in which liquid water is converted into a gas. Both the liquid and gas phases are made up of water molecules; each molecule contains two hydrogen atoms and one oxygen atom. The particulate scale insets in this fi gure emphasize that fact and also show that the separation between water molecules is much larger in the gas than in the liquid.
Photo: © Cengage Learning/Charles D. Winters
Figure 1.5 ❚ If a suitable electric current is passed through liquid water, a chemical change known as electrolysis occurs. In this process, water molecules are converted into molecules of hydrogen and oxygen gases, as shown in the particulate scale insets in the fi gure.
Hydrogen gas Oxygen gas
Liquid water
Photo: © Cengage Learning/Charles D. Winters
E X A M P L E P RO B L E M 1.1
Consider the experimental apparatus shown in the photo to the left, in which a candle is suspended above boiling water. This equipment could be used to test a hypothesis about the chemical composition of the gas in the bubbles that rise from boiling water.
What would be observed if the bubbles were composed of (a) water, (b) hydrogen, or (c) oxygen?
Strategy This problem asks you to think about what you expect to observe in an experiment and alternatives for different hypotheses. At this stage, you may need to do a little research to answer this question—fi nd out how hydrogen gas behaves chemi- cally in the presence of a fl ame. We also have to remember some basic facts about fi re that we’ve seen in science classes before. To be sustained, fi re requires both a fuel and an oxidizer—usually the oxygen in air.
Solution
(a) If the bubbles coming out of the liquid contain water, we would expect the fl ame to diminish in size or be extinguished. Water does not sustain the chemical reac- tion of combustion (as oxygen does), so if the bubbles are water, the fl ame should not burn as brightly.
(b) You should have been able to fi nd (on the web, for example) that hydrogen tends to burn explosively. If the bubbles coming out of the water were hydrogen gas, we would expect to see the fl ame ignite the gas with some sort of an explosion.
(Hopefully, a small one.)
(c) If the bubbles were oxygen, the fl ame should burn more brightly. The amount of fuel would remain the same, but the bubbles would increase the amount of oxygen present and make the reaction more intense.
Check Your Understanding Work with students in your class or with your in- structor to construct this apparatus and see whether or not your observations confi rm any of these hypotheses. Draw a picture showing a particulate level explanation for what you observe.
Symbolic Representation
The third way that chemists perceive their subject is to use symbols to represent the atoms, molecules, and reactions that make up the science. We will wait to introduce this perspective in detail in the next two chapters, but here we point out that you certainly have encountered chemical symbols in your previous studies. The famous
“H two O” molecule we have noted is never depicted as we have done here in the quotation marks. Rather, you have seen the symbolic representation of water, H2O.
In Chapter 2, we will look at chemical formulas in more detail, and in Chapter 3, we will see how we use them to describe reactions using chemical equations. For now, we simply note that this symbolic level of understanding is very important because it pro- vides a way to discuss some of the most abstract parts of chemistry. We need to think about atoms and molecules, and the symbolic representation provides a convenient way to keep track of these particles we’ll never actually see. These symbols will be one of the key ways that we interact with ideas at the particulate level.
How can we use these representations to help us think about aluminum ore or aluminum metal? The macroscopic representation is the most familiar, especially to the engineer. From a practical perspective, the clear differences between unrefi ned ore and usable aluminum metal are apparent immediately. The principal ore from which aluminum is refi ned is called bauxite, and bauxite looks pretty much like ordinary
Thomas Holme and Keith Krumnow Stan Celestian and Glendale Community College
A sample of bauxite.
rock. There’s no mistaking that it is different from aluminum metal. At the molecular level, we might focus on the aluminum oxide (also called alumina) in the ore and com- pare it to aluminum metal, as shown in Figure 1.6. This type of drawing emphasizes the fact that the ore is made up of different types of atoms, whereas only one type of atom is present in the metal. (Note that metals normally contain small amounts of impurities, sometimes introduced intentionally to provide specifi c, desirable proper- ties. But in this case, we have simplifi ed the illustration by eliminating any impurities.) Finally, Figure 1.6 also shows the symbolic representation for aluminum oxide—its chemical formula. This formula is slightly more complicated than that of water, and we’ll look at this type of symbolism more closely in Chapter 2.