Volume 1

Simple and Complex Substances • Simple Substances and Elements: There Is a Difference! Exercises

1.3.1. Simple and Complex Substances. As we already know, any substance is composed of identical molecules. There are two types of substances: simple and complex.

A simple substance is a substance that consists of molecules composed of only one element (atoms of just one type). For example, hydrogen gas, H2, is a simple substance because it consists of molecules made up of atoms of the same element, hydrogen (H). White phosphorus, P4, is also a simple substance, and so are nitrogen, N2, and oxygen, O2. As mentioned above, some simple substances, such as helium gas (He), consist of single atoms that are frequently called monoatomic molecules.

A complex substance, also known as a compound, is a substance that consists of identical molecules that are made up of more than one chemical element. For example, water is a complex substance because its molecule, H2O, consists of atoms of two different elements, hydrogen (H) and oxygen (O). More examples of both complex substances (compounds) and simple substances are presented below.

1. Table salt, sodium chloride, NaCl, is a complex substance (or a compound) because it is made up of two different chemical elements, sodium (Na) and chlorine (Cl). Find these elements in the periodic table.

2. Aspirin (acetylsalicylic acid), C9H8O4, is a complex substance (or a compound) whose molecule consists of three different elements: nine carbon atoms, eight hydrogen atoms, and four oxygen atoms. Find these elements in the periodic table. Read the formula of aspirin.

3. Soda ash, also known as washing soda, is sodium carbonate, Na2CO3, a complex substance (or a compound) formed by three different elements: sodium, carbon, and oxygen. Find these elements in the periodic table. Read the formula of soda ash.

4. Oxygen, O2, is a gas and a major component of air (about 21% by mass). Oxygen is a simple substance and must not be called a compound because O2 consists of only one element, oxygen. Find oxygen in the periodic table.

1.3.2. Simple Substances and Elements: There Is a Difference! Did you notice that gaseous oxygen (O2) is called by the same name, "oxygen", as the element (O) comprising it? This is also the case with the name "hydrogen": both the element (H) and hydrogen gas (H2) have the same name. So, when someone says "hydrogen" or "oxygen", how do we know if it is the chemical element that is meant, or the simple substance, gaseous hydrogen or oxygen? This problem that seems confusing at first glance is, in fact, easily avoided. All we have to do is just pay attention to the context in which the word "hydrogen" or "oxygen" is used.

Like complex substances (compounds), simple substances can be gaseous, liquid, or solid. For example, among simple substances, oxygen is a gas, mercury is a liquid, and sulfur is a solid. Substances can be produced, packed in cylinders or bottles, transported, used for a variety of purposes, etc. They are tangible because they are physical. In contrast, a chemical element is a type of atom. While atoms themselves are material, their types are not. One cannot put a type of atom in a bottle or a gas container. To further clarify the difference, some examples below show how, and explain why, the same name means either a type of atom (element) or a substance.

Example 1. All organic molecules contain carbon. What is meant here is the element carbon (C) because it is that particular type of atom (carbon atom) that is always found in all organic molecules. (With a very limited number of exceptions, carbon compounds are studied in a separate branch of chemistry that is called Organic Chemistry. Some basics of Organic Chemistry are presented in Volume 4 of this course.)

Example 2. Mixtures of hydrogen and oxygen are explosive. What is meant here are certainly substances. Such a mixture can be produced on mixing oxygen (O2) and hydrogen (H2) gases. Like other gases, O2 and H2 can be made, put in a balloon or a cylinder, stored, and used.
Digression. Hydrogen gas is no longer used to fill party balloons and blimps because if it leaks into the air, which contains oxygen, a flammable and explosive mixture of H2 and O2 is formed. Watch the live footage (Video 1-5 below) of the infamous Hindenburg disaster, the horrible explosion of the hydrogen-filled passenger airship LZ 129 Hindenburg, which occurred on May 6, 1937, in Manchester Township, New Jersey, U.S.A. These days, helium gas (He) is used for filling balloons and blimps, despite the fact that helium has a weaker lifting power and is vastly more expensive than hydrogen. Why? Because helium is non-flammable. "Safety first" is a good motto.
Video 1-5. Explosion of the hydrogen-filled passenger airship LZ 129 Hindenburg (1937) (source).

Example 3. Hemoglobin is responsible for carrying oxygen from the lung to all tissues of the body. Here oxygen refers to oxygen gas, a simple substance.

Example 4. Passing electricity through water, H2O, which consists of hydrogen and oxygen, results in the decomposition of water to hydrogen and oxygen, which bubble off. This is an interesting one. In the first part of the sentence, the terms "hydrogen and oxygen" are used to describe the elements, i.e., the two types of atoms that a molecule of water is composed of. However, for "hydrogen and oxygen" at the end of the sentence, simple substances are meant, the oxygen and hydrogen gases that effervesce (bubble off) on passing an electric current through water.

Still confused? Try to think of different breeds of dogs. A dog breed is a type of dog having a distinctive appearance and other characteristics, such as temper, weight, hair color, running speed, etc. For instance, when you hear someone say "German Shepherds are great dogs to keep", you picture immediately that particular type of athletic and agile dog, their approximate size, shape of the head, body, and tail, as well as many other characteristic features of the breed. It is the combination of the features that determines the breed and that you can easily visualize in your mind's eye. However, you cannot pet, feed, or walk a breed because a breed is a type of dog, not a particular dog. Now imagine that your neighbor calls you saying "I have to be away from home tonight. Could you walk my German Shepherd after dinner?" In this case "German Shepherd" means a particular dog, your neighbor's dog that you can walk and play with. Chemical elements and dog breeds are not tangible because they are just types of atoms and dogs. In contrast, simple substances and real existing dogs are material physical bodies (physical objects).

There is one more important difference between an element and a simple substance formed by atoms of this element. An element that is critically needed by a biological system for its proper functioning may, at the same time, form a simple substance that is highly toxic to the same system. For example, we would not exist without considerable amounts of phosphorus as a chemical element that is present in many complex substances that our body contains and needs. Yet as a simple substance, phosphorus (white phosphorus, P4) is a lethal poison.

1.3.3. Exercises. Determine if a chemical element or a simple substance formed by this element is meant in the following phrases.

1. Chlorine is a toxic gas. Answer

2. Spinach is rich in iron. Answer

3. Bromine is a liquid at room temperature and atmospheric pressure. Answer

4. Of all metals, tungsten (W) has the highest melting point. Answer

5. Your body needs phosphorus. Answer