High School Chemistry/Chemistry is a Science of Materials
In the last chapter we discussed some of the goals of early alchemists and some of the roles of chemists today. What you might have noticed is that while methods of chemical experimentation have improved and while knowledge of chemical properties has increased, chemistry in the 21st century AD and chemistry in the 5th century BC were both concerned with the question: How does matter change from one form to another? Can we predict the properties of matter? And how can we control these properties in order to use them to our advantage? Chemistry is essentially concerned with the science of matter and materials. Therefore, we'll begin our discussion of chemistry by considering some of the chemical materials that have been important both to early civilizations and to society today.
- Give examples of chemical properties a scientist might measure or observe in a laboratory.
- Explain the difference between a physical change and a chemical change, giving examples of each.
- Identify the situations in which mass can be converted to energy and energy can be converted to mass.
Ancient Materials Versus Modern Materials
Before humans had any understanding of chemistry, they used whatever they could find in the world around them. One type of material that was easily accessible to early civilizations, at least in small amounts, was metal. Native gold, silver, and copper, and compounds of tin and iron can all be found occurring naturally in cliffs and caves (in fact, the discovery of natural gold in El Dorado County, California is what lead to the great Gold Rush of 1849) and, as a result, these metals became very important to people in early times.
Many ancient civilizations fashioned tools, jewelry, and weapons out of metal that they scavenged from rocks around them. After a while, however, people discovered that by mixing naturally occurring metals with other substances, they could create new materials that often had superior properties.
Some of the oldest materials produced by man include mixtures (or more specifically solutions) of metals known as alloys. One of the earliest alloys ever discovered was bronze. Bronze can be made by heating chunks of tin and copper until they are liquid and then mixing the two pure metals together. Bronze was very important to early civilizations because it was more resistant to rust than iron, harder than copper, and could hold an edge and be sharpened to create tools and weapons.
Another alloy, produced early in the history of civilization, is steel. As you learned in an earlier section, steel is an alloy of iron and carbon (or charcoal). Steel, particularly Wootz steel (which required a special technique that involved the addition of glass), was especially strong, and could be fashioned into very sharp edges, perfect for swords. Another old material whose production was known to early civilizations is brass. Again, brass is an alloy, made of two pure metals, copper, and zinc. Early Romans knew that if they melted copper, and a zinc ore known as calamine together, they could produce brass, which was both shiny like gold, and resistant to rust. Brass was a common material used to make coins.
What you might notice about these "old" materials is that they are mainly alloys. At the time when bronze and brass and steel were discovered, people didn’t know much about the composition of matter or about how matter was assembled on a microscopic scale. As a result, inventing materials was largely a matter of trial and error. Towards the end of the 19th century, however, scientists were beginning to understand the make-up of matter, and this understanding led to new insight into how to develop materials with desirable properties.
One of the huge breakthroughs in recent history has been the discovery of plastic and plastic products. Initially, plastic was made by chemically modifying cellulose, a naturally occurring chemical found in plants. As chemical knowledge developed, however, scientists began to realize that plastics had special properties because, on a microscopic scale, they were composed of thousands of tiny chains of molecules all tangled up together. Scientists reasoned that if they altered the chemicals in these chains, but still managed to keep the chains intact, they could make new plastics with new properties. Thus began the plastic revolution!
Semiconductors are another class of "new" materials whose development is largely based on our improved understanding of chemistry. Because scientists know how matter is put together, they can predict how to fine-tune the chemical composition of a semiconductor in order to make it absorb light and act as a solar cell or emit light and act as a light source. We've come a long way from our early days of producing bronze and steel. Nevertheless, as our understanding of chemistry improves, we will be able to create even more useful materials than we have today.
Chemists Study the Properties of Matter
Hopefully at this point you are fully convinced of how important and useful the study of chemistry can be. You may, however, still be wondering exactly what it is that a chemist does. Chemistry is the study of matter and the changes that matter undergoes. In general, chemists are interested in both characteristics that you can test and observe, like a chemical's smell or color, and characteristics that are far too small to see, like what the oxygen you breathe in or the carbon dioxide you breath out looks like under a microscope 1,000 times more powerful than any existing in the world today.
Wait a minute… how can a chemist know what oxygen and carbon dioxide look like under a microscope that doesn't even exist? What happened to the scientific method? What happened to relying on observations and careful measurements? In fact, because chemists can't see the underlying structure of different materials, they have to rely on the scientific method even more! Chemists are a lot like detectives. Suppose a detective is trying to solve a murder case – what does she do? Obviously, the detective starts by visiting the site of the crime and looking for evidence. If the murderer has left enough clues behind, the detective can piece together a theory explaining what happened.
Even though the detective wasn't at the crime scene when the crime was committed and even though the detective didn't actually see the murderer kill the victim, with the right evidence, the detective can be pretty sure he or she knows how it took place. It's the same with chemistry. When chemists go into the laboratory, they collect evidence by making measurements. Once they've collected enough clues from the properties that they can observe, they use that evidence to piece together a theory explaining the properties that they can't observe – the properties that are too small to see.
What kinds of properties do chemists actually measure in the laboratory? Well, you can probably guess a few. Imagine that you go to dinner at a friend's house and are served something that you don't recognize – what types of observations might you make to determine exactly what you've been given? You might smell the food. You might note the color of the food. You might try to decide whether the food is a liquid or a solid because if it's a liquid, it's probably soup or a drink. The temperature of the food could be useful if you wanted to know whether or not you'd been served ice cream! You could also pick up a small amount of food with your fork and try to figure out how much it weighs – a light dessert might be something like an angel cake, while a heavy dessert is probably a pound cake. The quantity of food you've been given might be a clue too. Finally, you might want to know something about the food's texture – is it hard and granular like sugar cubes, or soft and easy to spread, like butter?
Believe it or not, the observations you are likely to make when trying to identify an unknown food are very similar to the observations that a chemists makes when trying to learn about a new material. Chemists rely on smell, color, state (that is, whether it is a solid, liquid or gas), temperature, volume, mass (which is related to weight, as you'll discover in a later section), and texture. There is, however, one property you might use to learn about a food, but that you should definitely not use to learn about a chemical - taste!
In The Atomic Theory, you'll see exactly how measurements of certain properties helped early scientists to develop theories about the chemical structure of matter on a scale much smaller than they could ever hope to see. You’ll also learn how these theories, in turn, allow us to make predictions about new materials that we haven't even created yet.
The learner.org website allows users to view streaming videos of the Annenberg series of chemistry videos. You are required to register before you can watch the videos but there is no charge. After you register the first time, you can return to the website (from the same computer) and view videos without registering again. The website has two videos that apply to this lesson. One is a video called The World of Chemistry that relates chemistry to other sciences and daily life. Another video called Thinking Like Scientists relates to the scientific method. The audience on the video is young children but the ideas are full grown. Video on Demand – The World of Chemistry.
Chemists Study of How and Why Matter Changes
In the last section, we discussed the properties of matter and how scientists use these properties to deduce certain facts about the structure of matter. However, if chemists only studied properties such as color and smell, they would only be collecting half of the evidence. While the properties of matter can tell us a lot, so too can the changes that matter undergoes. Suppose you've been served a slice of cake that you've noticed is cold to the touch. You might guess that you're dealing with an ice cream cake. But then again, maybe it's just a normal cake that's been kept in the freezer. Can you think of some way to tell between a frozen slice of ice cream cake and a frozen slice of regular cake? Well, one possibility is to wait for a while and see whether your slice melts. If the slice melts, it was an ice cream cake, and if it doesn't, it was just regular cake. In this case, you aren't observing a property, but rather a change in a property. The property being changed in the example is state.
Similarly, chemists learn a lot about the nature of matter by studying the changes that matter can undergo. Chemists make a distinction between two different types of changes that they study – physical changes and chemical changes. Physical changes are changes that do not alter the identity of a substance. Some types of physical changes include:
- Changes of state (changes from a solid to a liquid or a gas and vice versa)
- Separation of a mixture
- Physical deformation (cutting, denting, stretching)
- Making solutions (special kinds of mixtures)
When you have a jar containing a mixture of pennies and nickels and you sort the mixture so that you have one pile of pennies and another pile of nickels, you have not altered the identity of either the pennies or the nickels – you've merely separated them into two groups. This would be an example of a physical change. Similarly, if you have a piece of paper, you don't change it into something other than a piece of paper by ripping it up. What was paper before you starting tearing is still paper when you’re done. Again, this is an example of a physical change.
You might find it a little harder to understand why changes in state are physical changes. Until we discuss chemicals in terms of the smaller units (atoms and molecules) that make them up, it probably won’t be clear to you why freezing a substance or boiling a substance is only a physical change.
For now, though, you just have to trust that changes in state are physical changes. If you're ever in doubt, remember this: when a lake freezes in the winter, the water doesn’t disappear or turn into something else – it just takes on a new form. Liquid water and solid water (ice) are just different forms of the substance we know as water. For the most part, physical changes tend to be reversible – in other words, they can occur in both directions. You can turn liquid water into solid water through cooling; you can also turn solid water into liquid water through heating.
The other type of change that chemists are concerned with is chemical change. A chemical change occurs when one substance is turned into an entirely new substance as a result of a chemical reaction. Again, as we learn more about chemicals, and what chemicals look like, the meaning of a chemical change and the distinction between a chemical change and a physical change will become more obvious. For now, realize that chemicals are made up of tiny units known as atoms. Some of these atoms are bonded (or "glued") together, but during a chemical change, some of the bonds are broken and new bonds are formed.
You're probably wondering how you know when a chemical change has occurred. Sometimes it can be pretty tricky to tell, but there are several evidences of chemical changes to look for. There has probably been a chemical change if:
- A change in color has occurred
- Light, heat or sound has been given off from the material itself
- A precipitate (a solid formed when two liquids are mixed) has appeared
- A gas has been produced (detected by bubbling or a new odor)
Chemical changes are frequently harder to reverse than physical changes. One good example of a chemical change is burning paper. In contrast to the act of ripping paper, the act of burning paper actually results in the formation of new chemicals (carbon dioxide and water, to be exact). Notice that whereas ripped paper can be at least partially reassembled, burned paper cannot be "unburned". In other words, burning only goes in one direction. The fact that burning is not reversible is another good indication that it involves a chemical change.
Chemists Study the Interchange of Matter and Energy
Chemists are concerned with the properties of matter, and the changes that matter undergoes. For the most part, though, the changes that chemists are interested in are either physical changes, like changes in state, or chemical changes like chemical reactions. In either case, Lavoisier's Law of Conservation of Mass, applies. In both physical and chemical changes, matter is neither created nor destroyed. There is, however, another type of change that matter can undergo that actually disobeys Lavoisier's Law of Conservation of Mass, and that is the conversion of matter into energy, and vice versa.
Back when Lavoisier was studying chemistry, the technology didn’t exist that would allow scientists to turn matter into energy and energy into matter, but it can be done. This is the concept that Einstein proposed in his famous equation E = mc2. (This equation states that the energy in a given amount of matter is equal to the mass of the matter times the speed of light squared.) Chemical reactions don't involve changing measurable amounts of energy to mass or mass to energy. Mass-energy conversion is, however, important in chemistry that deals with radioactivity and particularly in the production of electricity by nuclear power plants.
- Some of the earliest materials invented by humans were alloys such as bronze, steel, and brass.
- With improved understanding of chemistry comes the ability to design new and useful materials, like plastics and semiconductors.
- Chemists can't actually see the underlying structure of most materials. As a result, they measure properties that they can see or observe and use this evidence to develop theories that explain how chemicals are organized on a sub-microscopic (smaller than you can see with a microscope) scale.
- Some of the physical properties that scientists observe pertain to state (solid, liquid, or gas), temperature, volume, mass, and texture.
- Chemists also study the changes that different materials undergo; this can give them valuable information about the chemicals involved.
- There are two types of changes that are important in chemistry – physical changes and chemical changes.
- Physical changes are changes that do not alter the identity of a substance; they are usually reversible.
- Chemical changes are changes that occur when one substance is turned into another substance as a result of a chemical reaction. They are usually difficult to reverse.
- It is also possible to change matter into energy and energy into matter.
- Name the two types of changes that chemists are primarily interested in.
- Decide whether each of the following statements is true or false.
- (a) Plastics were developed in Rome around 300 AD
- (b) Bronze is an example of an alloy
- (c) Plastic is an example of an alloy
- (d) Brass is an example of an alloy
- Decide whether each of the following statements is true or false.
- (a) Physical changes are typically accompanied by a color change
- (b) A burning campfire is an example of a chemical change
- (c) When you heat your house with coal, the coal undergoes a chemical change
- (d) When you drop a plate, and it breaks, the plate undergoes a physical change
- In each of the following examples, determine whether the change involved is a physical change or a chemical change.
- (a) Flattening a ball of silly putty
- (b) Combining a bowl of cherries and a bowl of blueberries
- (c) Boiling water
- (d) Cooking an egg
- Judy has two beakers filled with clear liquids, and she needs to know whether the liquid in the first beaker is the same as the liquid in the second beaker. In which scenario does Judy use physical properties to answer her question, and in which scenario does Judy use changes in chemical properties to answer her question?
- (a) Judy smells the two liquids and notices that the liquid in the first beaker has a strong odor, while she can't smell the liquid in the second beaker at all.
- (b) Judy mixes some table salt into the first beaker and notices that a white precipitate forms. She then mixes some table salt into the second beaker, but nothing happens.
- A solution (or a special kind of mixture), in which at least one of the components is a metal.
- chemical change
- A change that occurs when one substance is turned into an entirely new substance as a result of a chemical reaction.
- physical change
- Changes that do not alter the identity of the substance.