Video Transcript
In this video, we’ll learn how to describe the conservation of elements during chemical reactions. We’ll examine the difference between elements and compounds and recognize how elements and compounds can be transformed into new substances during chemical reactions. Before we can discuss the conservation of elements, we first need to identify what an element is.
An element is a substance that consists of one type of atom that cannot be broken down into anything simpler through a chemical reaction. There are 118 known elements that have been organized into the periodic table of the elements. Each element has its own unique set of physical and chemical properties. Elements may be metals, nonmetals, or metalloids. They may be naturally occurring or artificially made. At room temperature, they may exist as solids, liquids, or gases. They may be highly reactive or inert.
There are many pure elements that we may encounter every day. For example, oxygen in the air we breathe is an element. Likewise, copper in electrical wires, aluminum in soda cans, carbon in diamonds, and gold in jewelry are all elements. While elements can exist in their pure form, most chemical substances, like water and sugar, are compounds.
A compound is a species composed of atoms of two or more different elements that are chemically combined. We can represent a compound with a chemical formula. A chemical formula is an expression of chemical symbols and numerical subscripts that represents the composition of one unit of a compound. For example, one molecule or unit of water consists of two atoms of the element hydrogen and one atom of the element oxygen. This can be expressed by the chemical formula H2O. A molecule of sugar or sucrose consists of 12 atoms of the element carbon, 22 atoms of the element hydrogen, and 11 atoms of the element oxygen. So it can be expressed by the chemical formula C12H22O11.
All around us, elements and compounds are being transformed into new substances. Water, sugar, flour, and yeast can be transformed into bread. The oxygen we inhale is transformed into carbon dioxide we exhale, and iron nails left outside will rust over time. In each of these examples, a chemical reaction or several chemical reactions are occurring.
A chemical reaction is a process in which one or more substances are changed into one or more new substances. These new substances will have a different composition and different properties than the original substances. Let’s consider the example of iron rusting. Iron is a silver-gray metal that is attracted to a magnet. But rust, also called iron(III) oxide, is a brownish-orange powdery substance that is not attracted to a magnet.
By understanding chemical reactions, chemists can combine elements and compounds to create substances that have new properties. But when a chemical reaction occurs, atoms of elements must be conserved. This means that atoms of elements cannot be created or destroyed during a chemical reaction. Instead, the atoms are simply recombined in different arrangements. In other words, no element can appear or disappear during a chemical reaction. For example, the elements hydrogen and oxygen can be reacted to produce the compound water. As atoms of elements must be conserved during this reaction, the elements hydrogen and oxygen must appear on both sides of the reaction arrow. But atoms of these elements can be combined in different arrangements. This chemical reaction cannot suddenly produce an element that did not exist in the starting materials, as this would not obey the conservation of elements.
Let’s take another look at the conservation of elements by considering a series of reactions involving the element copper. In the first reaction, copper is reacted with nitric acid to form copper ions, nitrate ions, nitrogen dioxide gas, and water. If we examine both sides of the reaction arrow, we can see the elements copper, hydrogen, nitrogen, and oxygen on both sides of the reaction equation. All of the elements listed appear in the starting materials and products. So, even though the atoms of these elements have been combined in a different way during the reaction, the elements have been conserved.
Now, let’s examine the second reaction. In this reaction, copper ions are reacted with sodium hydroxide to produce solid copper(II) hydroxide and sodium ions. If we examine both sides of the reaction arrow, we see the elements copper, sodium, oxygen, and hydrogen on both sides of the reaction equation. All of the elements listed appear in the starting materials and products. So the elements have been conserved.
In reaction 3, copper(II) hydroxide decomposes into copper(II) oxide and water. In this reaction, the elements copper, oxygen, and hydrogen are conserved. And finally, in reaction 4, copper(II) oxide is reacted with carbon to regenerate copper metal and produce carbon dioxide gas. In this reaction, the elements copper, oxygen, and carbon are conserved.
Before we take a look at some questions, let’s consider one more chemical reaction. Butane is a compound that is commonly found in liquid form in lighters. It is composed of carbon and hydrogen atoms. When the lighter is used, the butane is burned and a chemical reaction occurs. After all of the butane has burned, nothing remains in the lighter. So we might assume that no new substances have been formed. We might even assume that the carbon and hydrogen in the butane were destroyed. However, a chemical reaction did occur. So new substances must have been formed, and the conservation of elements must have been obeyed. So, what happened during this reaction?
When butane is burned, a chemical reaction occurs between the butane and oxygen in the air. The elements carbon, hydrogen, and oxygen are recombined to form carbon dioxide gas and water vapor, which we cannot see with the naked eye. So, although we cannot see all of the starting materials or the products involved in this reaction, the reaction does produce new substances and the conservation of elements is obeyed.
Before we summarize what we’ve learned in this video, let’s take a look at a few questions.
Which of the following statements best describes the conservation of elements during a chemical reaction? (A) Elements can appear but not disappear during a chemical reaction. (B) No element can appear or disappear during a chemical reaction. (C) Elements can appear or disappear during a chemical reaction. Or (D) elements can disappear but not appear during a chemical reaction.
A chemical reaction is a process in which one or more substances is changed into one or more new substances. The new substances produced will have a different composition and different properties than the original substances. For example, sodium is a soft, silvery, highly reactive metal. It can undergo a chemical reaction with chlorine, a toxic yellow-green gas, to produce sodium chloride, a white crystalline solid, more commonly known as table salt.
During this reaction and all chemical reactions, elements must be conserved. The conservation of elements means that atoms of elements cannot be created or destroyed during a chemical reaction. Looking at the reaction of sodium with chlorine, we can see that the element sodium appears on both sides of the reaction arrow, as does the element chlorine. As all of the elements involved in the reaction appear on both sides of the reaction arrow, the elements have been conserved.
Another way to describe the conservation of elements is to say that atoms of elements cannot appear or disappear during a chemical reaction. Looking at the answer choices, we can see that the statement that best describes the conservation of elements during a chemical reaction is the statement shown in answer choice (B). No element can appear or disappear during a chemical reaction.
By using the conservation of elements, identify the element missing in the following chemical equation: Mg plus H2SO4 reacts to produce blank SO4 plus H2. (A) H, (B) S, (C) Mg, (D) O, or (E) Na.
The conservation of elements means that atoms of elements cannot be created or destroyed during a chemical reaction. In other words, elements that appear on one side of the reaction arrow in a chemical equation must appear on the other side as well. To identify the element that is missing in the chemical equation, we can make a list of the elements that appear on both sides of the reaction arrow. On the left side of the reaction arrow, we see the elements magnesium, hydrogen, sulfur, and oxygen. On the right side of the reaction arrow, we see the elements sulfur, oxygen, and hydrogen. All of the elements appear on both sides of the reaction arrow, except for the element magnesium. So the element missing in the chemical equation must be answer choice (C), Mg.
Magnesium reacts explosively with oxygen to form magnesium oxide according to the following equation: two Mg plus O2 react to produce two MgO. Which of the following is not true of this reaction? (A) The conservation of elements is obeyed. (B) The reaction describes the formation of a new substance. (C) The reactants, magnesium and oxygen, are elements. (D) The product of the reaction, magnesium oxide, is a compound. Or (E) magnesium oxide has similar physical and chemical properties to both oxygen and magnesium.
This question describes the reaction of magnesium with oxygen, which produces the product magnesium oxide. We need to evaluate each of the statements given in the answer choices and identify which statement is not true of this reaction.
Statement (A) says that the conservation of elements is obeyed during this reaction. The conservation of elements means that atoms of elements cannot be created or destroyed during a chemical reaction. In other words, any element which appears on the left side of the reaction arrow must also appear on the right side of the reaction arrow. On the left side of the reaction arrow in this equation, we see the elements magnesium and oxygen. And on the right side of the reaction arrow, we see the elements magnesium and oxygen. So the elements in this reaction are conserved. Since the statement in answer choice (A) is true and we need to identify the statement which is not true, answer choice (A) cannot be the answer to this question.
Answer choice (B) states that the reaction describes the formation of a new substance. A chemical reaction is a process in which one or more substances are changed into one or more new substances. In this chemical reaction, magnesium and oxygen are changed into the new substance magnesium oxide. Thus, statement (B) is true and cannot be the answer to this question.
Statement (C) states that magnesium and oxygen are elements. An element is a substance that only consists of one type of atom and cannot be broken down into anything simpler through a chemical reaction. We can see that magnesium only consists of magnesium atoms and oxygen only consists of oxygen atoms. In addition, both magnesium and oxygen can be found on the periodic table of the elements, a table which displays all 118 known elements. So statement (C) is true and cannot be the answer to this question.
Statement (D) states that magnesium oxide is a compound. A compound is a species composed of atoms of two or more different elements that are chemically combined. Magnesium oxide is composed of one atom of magnesium and one atom of oxygen, which are chemically combined. So magnesium oxide is a compound. Therefore, statement (D) is true and cannot be the answer to this question.
Statement (E) states that magnesium oxide has similar physical and chemical properties to both oxygen and magnesium. Magnesium is a lightweight, silver-white metal, and oxygen is a colorless, odorless, tasteless gas. Magnesium oxide is a powdery white solid. It has a significantly higher melting point than magnesium and oxygen. And it is hygroscopic, meaning that it can react with moisture in the air. So magnesium oxide does not have similar physical or chemical properties to either oxygen or magnesium. So the statement which is not true of the reaction of magnesium with oxygen to form magnesium oxide is statement (E). Magnesium oxide has similar physical and chemical properties to both oxygen and magnesium.
Now, let’s summarize what we’ve learned with the key points. An element consists of one type of atom and cannot be broken down into anything simpler through a chemical reaction. Examples of elements include copper in electrical wires, oxygen we breathe, and gold in jewelry. Atoms of two or more different elements can be chemically combined to form compounds. Examples of compounds include water, carbon dioxide, and sugar. In a chemical reaction, one or more substances are transformed into one or more new substances with a different composition and properties. Chemical reactions must obey the conservation of elements, which means that atoms of elements cannot be created or destroyed during a chemical reaction. In other words, any element which appears on one side of the reaction arrow in a chemical equation must appear on the other.
