Elements and Compounds

Chemistry in History Elements and Compounds Periodic Table Chemical Reactions     Energy Transfer Table of Contents


Section 1: What is Matter?

Lesson Objectives

The student will:

  • define matter and explain how it is composed of building blocks known as atoms.
  • explain the differences between substances and mixtures.
  • classify mixtures as homogeneous or heterogeneous.
  • identify the chemical symbols of common elements.
  • explain the difference between an element and a compound by their symbols or formulas.
  • demonstrate the proper use of parentheses and subscripts in writing chemical formulas.
  • determine the number of atoms and name of each element in a compound.

Vocabulary

atom

compound


element


heterogeneous mixture


homogeneous mixture


law of constant composition


matter


molecule



Introduction

Matter is anything that has mass and volume. The entire universe is composed of matter, which is in turn composed of atoms. Anatom is the basic building block of all matter. All matter in the universe, from a teaspoon of salt to the Pacific Ocean, has mass and occupies space. The salt and ocean, however, have very different properties and behaviors. Since everything in the universe is composed of matter, there are clearly many types of matter. In this lesson, you will learn about how scientists classify the different types of matter.

Categories of Matter

Matter can be classified into two broad categories: mixtures and pure substances, as illustrated below.

Mixtures are physical combinations of two or more substances. The term “physical combination” refers to mixing together different substances that do not chemically react with each other. The physical appearance of the substances may change, but the atoms in the substances do not.

In comparison, a pure substance is a form of matter that has a constant composition and constant properties throughout the sample. Elements and compounds are both example of pure substances.

Mixtures: Homogeneous and Heterogeneous

One example of a mixture is sand and gravel stirred together. In this case, you can see that there are two different substances present, each with the same properties that it had before it was mixed. When substances do not mix thoroughly and evenly (like sand and gravel), the mixture is said to be heterogeneous. A heterogeneous mixture consists of visibly different substances.

Another example of a mixture is salt dissolved in water. In this case, you cannot see the different substances, but you can test the solution to show that each substance (salt and water) has the same chemical properties it had before being mixed. When substances mix thoroughly and evenly (like salt in water), the mixture is said to be homogeneous. Homogeneous mixtures are often referred to as solutions. Solutions often may appear to be one pure substance, but some simple tests can show that the solutions are indeed mixtures.

Pure Substances: Elements and Compounds

Elements are the simplest substances. An element is a substance that is made up of only one type of atom. It doesn’t matter if the atoms are in groups, as in P4 or S8, or isolated, as in Na. As long as there is only one kind of atom, the substance is an element. Elements cannot be chemically broken down into anything smaller and still retain the properties of the element. For example, an atom of iron can be smashed into electrons, protons, and neutrons, but those pieces would not have the properties of iron.

Atoms from two or more elements can chemically combine to form a new substance. Compounds are substances that are made up of more than one type of atom. In other words, compounds are chemical combinations of elements. These combinations form new substances with completely different properties than the atoms from which they were formed.

The image above is a model of water. Water is a compound consisting of one atom of oxygen and two atoms of hydrogen. Hydrogen is an explosive gas, and oxygen is a gaseous substance that supports combustion. Yet, when these two elements are chemically combined to form water, the product neither burns nor supports combustion. In fact, water is used to put out fires.

molecule is the smallest particle of a compound. If you break up the molecule, you no longer have the properties of the compound. Molecules, like atoms, are too small to be seen. Even with the most powerful microscopes, we have only seen the very largest of molecules.

The illustration above shows a single unit of the compound called sodium chloride on the left. This single unit is made up of one sodium ion and one chloride ion. Sodium is a very reactive metal that explodes in water and burns in air, while chlorine is a very deadly, poisonous gas. When these two are combined, we get table salt (sodium chloride). When sodium chloride is in solid form, many units join together, as illustrated above on the right.

Elements: Names and Symbols

Everything, from ants to galaxies, is composed of atoms. So far, scientists have discovered or created 118 different types of atoms. Scientists have given a name to each different type of element and organized them into a chart called the periodic table. As you can see in the table below, each square contains one of the elements.

Interactive Periodic Table

Each element not only has its own name, it also has its own symbol. Scientists use abbreviations called chemical symbols to represent the elements. Many of these symbols are the first one or two letters of the modern name of the element. The first letter of a chemical symbol must always be a capital letter, and the second letter, when there is a second letter, must always be a lowercase letter. Tablebelow shows some examples of elements and their symbols.

Examples of Elements
ElementSymbol
HydrogenH
OxygenO
CarbonC
CalciumCa
AluminumAl

As seen in Table below, the symbols for some of the elements consist of the first letter of the name and another letter (not the second letter) that comes later in the name.

More Examples of Elements
ElementSymbol
ZincZn
MagnesiumMg
ChlorineCl
ArsenicAs
ZirconiumZr

For other elements, the symbols were already used for other elements. When trying to decide on a symbol for silver, for example, the symbol S was already used for sulfur, and the symbol Si was already used for silicon. Since silver has been known to man for over a thousand years, it had a Latin name from ancient times. The old Latin name for silver was argentum, so it was decided that the symbol for silver would be Ag. There are a number of symbols chosen in this same manner, as seen in Table below.

Examples of Elements Whose Symbol Comes from Latin
ElementAncient NameSymbol
SilverArgentumAg
PotassiumKaliumK
SodiumNatriumNa
GoldAurumAu
LeadPlumbumPb
CopperCuprumCu
IronFerrumFe

Element WebQuest

Compounds: Chemical Formulas

The chemical symbols are not only used to represent the elements, they are also used to write chemical formulas for the millions of different compounds. For a given chemical compound, the law of constant composition states that the ratio by mass of the elements in the compound is always the same, regardless of the source of the compound. The law of constant composition can be used to distinguish between compounds and mixtures. Compounds have a constant composition, and mixtures do not. Pure water is always 88.8% oxygen and 11.2% hydrogen by weight, regardless of the source of the water. Brass is an example of a mixture. Brass consists of two elements, copper and zinc, but it can contain as little as 10% or as much as 45% zinc.

The formula for a compound uses the symbols to indicate the type of atoms involved and uses subscripts to indicate the number of each atom in the formula. For example, aluminum combines with oxygen to form the compound aluminum oxide. Forming aluminum oxide requires two atoms of aluminum and three atoms of oxygen. Therefore, we write the formula for aluminum oxide as \text{Al}_2\text{O}_3. The symbol \text{Al} tells us that the compound contains aluminum, and the subscript 2 tells us that there are two atoms of aluminum in each molecule. The \text{O} tells us that the compound contains oxygen, and the subscript 3 tells us that there are three atoms of oxygen in each molecule. It was decided by chemists that when the subscript for an element is 1, no subscript needs to be used. Thus the chemical formula \text{MgCl}_2 tells us that one molecule of this substance contains one atom of magnesium and two atoms of chlorine. The formula for sodium chloride is \text{NaCl}, which indicates that the compound contains one atom each of sodium and chlorine. The formula for sodium carbonate, \text{Na}_2\text{CO}_3, indicates that there are two atoms of sodium, one atom of carbon, and three atoms of oxygen. In formulas that contain parentheses, the subscript outside of the parentheses applies to everything inside. For example, the subscript 2 in\text{Ca(OH)}_2, the subscript 2 applies to the (OH). Therefore, this molecule of calcium hydroxide contains one atom of calcium, two atoms of oxygen, and two atoms of hydrogen.

Lesson Summary

  • All matter has mass and occupies space.
  • Matter can be classified into two broad categories: pure substances and mixtures.
  • A pure substance is a form of matter that has constant composition and constant properties throughout the sample.
  • Mixtures are physical combinations of two or more substances.
  • Elements and compounds are both example of pure substances.
  • Compounds are substances that are made up of more than one type of atom.
  • Elements are the simplest substances made up of only one type of atom.
  • The elements are organized into a chart called the periodic table.
  • Scientists use abbreviations called chemical symbols to represent the elements.
  • The first letter of a chemical symbol is capitalized, and the second letter is not.

Further Reading / Supplemental Links

You may listen to Tom Lehrer’s humorous song “The Elements” with animation at this website.

This website provides a review about matter and the categories of matter.

 

Section 2: Properties and Changes of Matter

Lesson Objectives

The student will:

  • explain the difference between physical and chemical properties of matter.
  • list examples of physical properties.
  • list examples of chemical properties.
  • classify properties as chemical properties or physical properties.
  • explain the difference between physical and chemical changes in matter.
  • list examples of physical changes.
  • list examples of chemical changes.
  • classify changes as physical changes or chemical changes.

Vocabulary

chemical change


chemical property


physical change


physical property

 

Introduction

What kinds of properties do chemists actually measure in the laboratory? Well, you can probably guess a few. Imagine that you are having dinner at a friend’s house and are served something that you don’t recognize. What types of observations might you make to determine what you’ve been given? You might note the smell or color of the food. You might observe whether the food is a liquid or a solid. 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. You might also 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. In general, chemists are interested in characteristics that you can test and observe, such as a chemical’s smell or color, and characteristics that are far too small to see, such as what the oxygen you breathe in or the carbon dioxide you breathe out looks like. Chemists rely on color, state (solid, liquid, or gas), temperature, volume, mass, 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!

Physical and Chemical Properties

There are two basic types of properties that are used to identify or describe matter: physical properties and chemical properties.Physical properties are properties that can be observed without changing the identity of the substance. In the image below, we have water molecules that are held in liquid form on the left. Each molecule contains two atoms of hydrogen chemically bounded with one atom of oxygen. When we heat the liquid water, it changes to water vapor. The physical properties change - we can see the liquid water, but the water vapor cannot be seen. Liquid water has a higher density than water vapor, and so on. But even though the physical properties have changed, the molecules are exactly the same as before. Each water molecule still contains two hydrogen atoms and one oxygen atom chemically bounded together.

On the other hand, chemical properties can only be observed when a substance is changed into a new substance. In the image below, on the left we have a molecule of methane (\mathrm{CH}_4) and two molecules of oxygen (\mathrm{O}_2). On the right, we have two molecules of water (\mathrm{H}_2\mathrm{O}) and one molecule of carbon dioxide (\mathrm{CO}_2). In this case, not only has the appearance changed, but the structures of the molecules have also changed. The new substances do not have the same chemical properties as the original ones. Therefore, this is a chemical change. The chemical properties, such as how they react and what they react with, however, will still be the same as before.

Physical and Chemical Changes

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)

 

If 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. Similarly, if you have a piece of paper and you rip it up, you don’t change the paper into something other than a piece of paper. These are examples of a physical change. For the most part, physical changes tend to be reversible, or capable of occurring in both directions. You can turn liquid water into solid water (ice) through cooling, and you can also turn solid water into liquid water through heating (Figure below).

 

Melting lake side

Melting snow is an example of a physical change.

 

Chemical changes are changes that occur when one substance is turned into another substance. 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. Another example of a chemical change, illustrated in Figure below, is the explosion of fireworks.

 

Fireworks

Fireworks are an example of a chemical change.

 

Lesson Summary

  • There are two basic types of properties that are used to identify or describe matter: physical properties and chemical properties.
  • Physical properties are those that can be observed without changing the identity of the substance.
  • Chemical properties are those that can be observed only when a substance is changed into a new substance.
  • 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
  • Chemical changes are changes that occur when one substance is turned into another substance.
  • Chemical changes are frequently harder to reverse than physical changes.

Further Reading / Supplemental Links

This website provides some free PowerPoint presentations. The presentation on “Matter and Energy" provides a review of some properties of matter, as well as provide examples of the topics covered in this lesson.

This website has lessons, worksheets, and quizzes on various high school chemistry topics. Lesson 1-5 is on physical and chemical properties, as well as physical and chemical changes.

 


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