Holt Science Spectrum

Copyright  by Holt, Rinehart and Winston. All rights reserved.

Chapter 2

What Is Matter?

In this section, you will learn that matter, atoms, and elements share a relationship. You will also see how elements are combined in compounds. This section will show how chemical formulas are used to represent both elements and compounds. Finally, you will understand why elements and compounds are classified as pure substances and how they differ from mixtures.

Take a look at Figure 1 in Spectrum Book. This figure shows glass being made. Three materials—sand, limestone, and soda ash—are changed into a totally different material: glass. This is what chemistry is all about. Chemistry is the study of what things are made of and how things change.

Everything you use daily—from soap to food to glue—involves chemistry. Glass is a perfect material for making windows because it is transparent, solid, and waterproof. However, sand—one of the materials used to make glass—is not transparent, solid, or waterproof. Chemistry helps us recognize how the different properties of materials are related to what the materials are made of. In other words, chemistry is the study of matter. But what is matter?

You are made of matter. The book is also made of matter. In fact, anything you can hold or touch is matter. Even the air you breathe is matter, even though you cannot see the air. But light, sound, and electricity are not matter because they have no mass or volume. Matter can be defined as anything that has mass and volume.

Most matter consists of atoms. Wood is another example of matter. Wood is both rigid and lightweight, which is why it is a good material for making furniture and buildings. When wood gets hot enough, its surface chars or turns black. The intense heat breaks down wood to form another kind of material— carbon. Carbon and wood have different properties. No matter how much more heat carbon is exposed to, it will not break down any further. Carbon is an example of an element. An element is a substance that cannot be broken down into simpler substances.

Elements are made of atoms. An atom is the smallest particle that has the properties of an element. Figure 2 shows the atoms that make up the elements iron and copper. Carbon atoms make up a wide variety of substances, including diamonds. Aluminum atoms are also in a wide variety of substances, including the foil used to wrap a baked potato. Figure 3 shows the elements that are most abundant on Earth and those that are most abundant in the human body. Examine Figure 3 closely. Which is the most abundant element on Earth? If you said iron, you are correct. Now which element is the most abundant element in the human body? If you said oxygen, you are correct. Notice that although iron accounts for over 34 percent of the elements on Earth, it makes up only 0.004 percent of the human body.

Each element can be represented by a one- or two-letter symbol. For example, carbon is represented by C, iron as F-e, copper as C-u, and aluminum as A-l. Notice that if two letters are used to represent an element, the second letter is always written in lower case. More than 110 elements have been identified. Each element is unique and behaves differently from the others.

Now let’s take a look at how elements are combined into compounds. As you just learned, aluminum and iron are elements. But nylon, another familiar substance, is not an element. Nylon is a compound. A compound is a substance made of more than one element.

The unit that makes up nylon contains atoms of the elements carbon, hydrogen, nitrogen, and oxygen. Each nylon strand actually consists of hundreds of these units linked together.

Every compound is unique and different from the elements it contains. For example, nylon is a flexible solid. But three of the elements that make up nylon—hydrogen, oxygen, and nitrogen—are colorless gases.

Another example of a compound is iron-three-oxide. We often see iron-three-oxide as rust. Iron-three-oxide is made of two atoms of iron and three atoms of oxygen. When elements combine to make a specific compound, the elements always combine in the same proportions. For example, iron-three-oxide always has two parts of iron for every three parts of oxygen.

Atoms can join together to form a molecule. A molecule is the smallest unit of a substance that exhibits all the properties characteristic of that substance. One of the molecules you are most familiar with is water. Notice in Figure 4 that water can be shown as a chemical formula. The formula for water is written as H-two-O. This formula indicates that a water molecule is made of two hydrogen atoms and one oxygen atom. Figure 4 also shows three other ways of representing a water molecule: by a two-dimensional image printed on paper, by a physical model, or on a computer.

When one oxygen atom and two hydrogen atoms combine to form a molecule of water, the molecule acts as a unit. A molecule is simply the smallest unit that behaves like a substance. Water is a molecule that is made of atoms of two different elements—hydrogen and oxygen. Some molecules are made of multiple atoms of the same element. Oxygen, for example, is a molecule that is made of two oxygen atoms,  hydrogen and chlorine is another example. Notice also that phosphorus is a molecule made of four phosphorus atoms.

Just as elements are represented by chemical symbols, compounds and molecules are represented by chemical formulas. A chemical formula consists of chemical symbols and numbers that indicate the atoms contained in the basic unit of a substance. For example, take indigo, the dye that was originally used to make blue jeans blue. The formula for a molecule of indigo is written as C-sixteen-H-ten-N-two-O-two. The letters represent the elements. The numbers, which are written as subscripts, indicate the number of atoms of each element in the molecule. 

The subscript 16 after the letter C means that there are 16 carbon atoms in a molecule of indigo. The chemical formula for table sugar is C-twelve-H-twenty-two-O-eleven. Thus, a molecule of table sugar contains 12 carbon atoms, 22 hydrogen atoms, and 11 oxygen atoms.

Keep in mind that no subscript is written when only one atom of the element is present. For example, the formula for carbon dioxide is C-O-two, not C-one-O-two.

Now let us examine the difference between pure substances and mixtures.

What does the word pure mean to you? To most people, the word pure usually means "not mixed with anything else." For example, pure grape juice contains only the juice of grapes. In chemistry, however, the word pure has a different meaning. In chemistry, a pure substance is matter with a fixed composition and definite properties.

A chemist would definitely not consider pure grape juice a pure substance. Rather, a chemist would classify grape juice as a mixture. A mixture is a combination of more than one pure substance. Grape juice, for example, is actually a mixture of several different pure substances, including water, sugars, acids, and vitamins. In addition, the composition of grape juice is not fixed. Grape juice can contain different amounts of water, sugars, or other compounds, depending on how the juice was made.

Elements and compounds are pure substances. Mixtures, however, are not pure substances. Many things we encounter in our daily lives are mixtures, including most foods we eat and the air we breathe. Air is a mixture of nitrogen, oxygen, and other elements and compounds.

A mixture can be separated into the parts that make it up. For example, the water in a mixture like grape juice can evaporate. The sugars, acids, and other compounds that make up grape juice will be left. The water molecules that were evaporated were not chemically changed. These water molecules simply turned from a liquid into a gas. A pure substance such as water cannot be broken down by boiling or melting.

Recall that a compound is very different from the elements that make it. But a mixture may have some properties similar to the pure substances that make it. For example, grape juice is wet like water and sweet like sugar.

Mixtures are classified by how thoroughly the substances mix.

Some mixtures are made by combining solids and liquids. The mixture shown on the left in Figure 7 was made by combining flour and water. The mixture shown on the right in Figure 7 was made by combining salt and water. Both flour and salt are white solids. But the mixtures they form with water are very different. Flour does not mix well with water. As a result, a mixture of flour and water has a cloudy white appearance. A chemist would say that the flour is suspended in the water. This is because flour does not dissolve in water. Over time, the flour will slowly settle toward the bottom of the beaker. A mixture of flour and water is called a heterogeneous mixture because the flour and water are not uniformly mixed.

Now consider what happens when salt and water are mixed. In this case, the salt dissolves in the water. Because the salt particles become very small as they dissolve, you cannot see them in the water and the salt-water mixture is clear. Unlike the flour, the salt will not settle toward the bottom, no matter how much time passes. Instead, the salt particles will remain evenly spread throughout the water. Salt and water form a homogeneous mixture because the composition of the mixture is the same throughout.

Compounds in a mixture are classified by how well they mix in the mixture. Gasoline is a homogeneous liquid mixture that consists of at least 100 compounds in various quantities. All these compounds are miscible. This means that all these compounds dissolve in each other. As a result, gasoline looks like a pure substance, but it isn’t. Figure 8A shows a homogeneous mixture of water and rubbing alcohol. The water and the alcohol are miscible because they mix together well. As you can see from the zoom window, the water and the alcohol are mixed together even at the molecular level.

Unlike the compounds in gasoline, oil and water do not mix well. If you mix oil and water and then shake the two, the water will settle out after a while. Notice in Figure 8B that the oil is floating on top of the water. Oil and water form a heterogeneous mixture. Because these two liquids do not mix well, they are said to be immiscible.

Gases can mix with liquids.

Air is a homogeneous mixture of gases, mostly nitrogen and oxygen. Because these two gases in air form a homogeneous mixture, you get oxygen every time you breathe. A carbonated drink is also a homogeneous mixture. In this case, the mixture that makes up a carbonated drink consists of sugar, flavorings, and carbon dioxide gas, C-O-two, dissolved in water. The carbon dioxide gas is mixed into the liquid under pressure to form a solution. Gases can mix with liquids even when they are not subjected to high pressure. For example, liquid water usually contains gases.

Consider what would happen if you let a glass of cold water stand overnight. The next morning, you might see bubbles on the sides of the glass. These bubbles contain air that was previously dissolved in the cold water. When you pour a carbonated drink, you get a foam on top. The foam is a different kind of gas-liquid mixture. In the carbonated drink, the gas is dissolved in the liquid. But in the foam, the gas is not dissolved; instead, it has formed tiny bubbles in the liquid. Eventually, the tiny bubbles form bigger bubbles that can escape from the foam. When this happens, the foam collapses.

Some foams are more stable than those in carbonated drinks. If you whip egg whites with enough air, you get a foam. If you heat this foam in an oven, the liquid foam will dry and harden. What you now have is a solid foam that you can eat. This solid foam is called meringue.

Matter has mass and occupies space.