In this explainer, we will learn how to describe the conservation of elements during chemical reactions.
All matter is composed of chemical substances. There are millions of known chemicals, and they are all made from different arrangements of elements. An element is a collection of atoms with a specific and identical number of protons in their nuclei.
Definition: Element
An element is a substance that consists of one type of atom and 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 elements.
Some elements occur naturally such as copper (), oxygen (), and gold (), while others like americium () have been synthesized by scientists. Each element has its own unique set of properties. For example, copper is a soft metal that has a reddish-orange color and is a good conductor of electricity. Pure oxygen at room temperature is a colorless, odorless, and tasteless gas.
While elements can exist in their pure form, most chemical substances are a combination of atoms of different elements. This type of substance is called a compound.
Definition: Compound
A compound is any substance composed of atoms of two or more different elements that are chemically combined.
A chemical formula can be used to represent a compound. The formula lists the symbol of each element and may include subscripts to indicate the number of atoms of each element needed to make up one unit of the compound. For example, table salt is a compound composed of the elements sodium () and chlorine (). Its chemical formula is .
Definition: Chemical Formula
A chemical formula is an expression of chemical symbols and numerical subscripts that represents the composition of one unit of a compound.
Elements and compounds can be transformed into new compounds during a chemical reaction.
Definition: Chemical Reaction
A chemical reaction is a process in which one or more substances are changed into one or more new substances.
The new substances formed in the reaction will have a different composition and different properties than the original substances. For example, iron () is a silver-gray metal that is attracted to a magnet. When iron () reacts with oxygen () and water (), rust () is formed. Rust is a brownish-orange powdery substance that is not attracted to a magnet.
During a chemical reaction, atoms of the elements must be conserved. This means that no element can appear or disappear during a chemical reaction. The atoms are simply recombined in different arrangements.
Definition: Conservation of Elements
Atoms of elements can neither be created nor destroyed during a chemical reaction.
For example, hydrogen () and oxygen () can react to produce water ():
We can see from the reaction above that atoms of the elements hydrogen and oxygen are present in both the starting materials and the product.
Example 1: Identifying the Statement That Best Describes the Conservation of Elements
Which of the following statements best describes the conservation of elements during a chemical reaction?
- Elements can appear but not disappear during a chemical reaction.
- No element can appear or disappear during a chemical reaction.
- Elements can appear or disappear during a chemical reaction.
- Elements can disappear but not appear during a chemical reaction.
Answer
During a chemical reaction, the atoms of all elements must be conserved. This means that any elements that are present in the starting materials must be present in the products. The atoms of the elements may be recombined, but no element can appear or disappear during a chemical reaction. Therefore, the correct answer is choice B.
Example 2: Identifying the Reaction Equation That Does Not Obey the Concept of Conservation of Elements
In which of the following chemical reactions is the concept of conservation of elements not obeyed?
Answer
During a chemical reaction, the atoms of all elements must be conserved. This means that any elements that are present in the starting materials must be present in the products. The atoms of the elements may be recombined, but no element can appear or disappear during a chemical reaction.
Looking at choice A, we see that the elements magnesium (), copper (), sulfur (), and oxygen () appear on both sides of the reaction arrow, so this reaction obeys the conservation of elements. Looking at choice B, we see that the elements calcium (), oxygen (), and carbon () appear on both sides of the reaction arrow, so this reaction obeys the conservation of elements. Choices C and E also obey the conservation of elements as all of the elements found on the left side of the reaction arrow also appear on the right side of the reaction arrow. In choice D, the element zinc () appears on the left side of the arrow but not the right side of the arrow. In addition, the element copper () appears on the right side of the arrow but not the left side of the arrow. This chemical reaction does not follow the concept of conservation of elements, so the correct answer is choice D.
Example 3: Determining Which Statement about the Reaction of Magnesium with Oxygen Is Incorrect
Magnesium reacts explosively with oxygen to form magnesium oxide according to the following equation:
Which of the following is not true of this reaction?
- The concept of conservation of elements is obeyed.
- The reaction describes the formation of a new substance.
- The product of the reaction, magnesium oxide, is a compound.
- The reactants, magnesium and oxygen, are elements.
- Magnesium oxide has similar physical and chemical properties to both oxygen and magnesium.
Answer
To answer this question, we need to determine which statement does not correctly describe the reaction between magnesium and oxygen. Looking at the given reaction equation, we can see that only the elements magnesium () and oxygen () appear on both sides of the reaction arrow. This means that the reaction obeys the law of conservation of elements, which states that atoms of elements can neither be created nor destroyed in a chemical reaction. So, choice A is correct.
The reaction occurs between magnesium () and oxygen (). Magnesium and oxygen are elements, a collection of atoms with a specific and identical number of protons in their nuclei. So, choice D is correct.
The product of this reaction is a new substance: magnesium oxide (). Magnesium oxide is a compound, a substance composed of atoms of two or more elements that are chemically combined. So, choices B and C are correct.
Magnesium is a brittle, lightweight, silver-white metal. Oxygen is a colorless, odorless, tasteless gas. Magnesium oxide is a powdery, white substance with a high melting point. Unlike magnesium and oxygen, magnesium oxide is hygroscopic and can react with moisture in the air. As with all chemical reactions, the product, magnesium oxide, has a different composition and different properties than the reactants, magnesium and oxygen. Thus, the statement that is not true is choice E, magnesium oxide has similar physical and chemical properties to both oxygen and magnesium.
The scheme below represents the conservation of copper through a series of transformations.
Although the full reactions are not shown in the figure, we can see that the element copper is conserved throughout all four reactions.
In reaction 1, a piece of copper metal is placed into a test tube. A small amount of concentrated nitric acid is then added to the test tube. The copper metal reacts with the concentrated nitric acid to form ions, in the form the copper nitrate. The color of the solution changes to blue because of the presence of ions.
The chemical equation for the transformation shown in reaction 1 is
In reaction 2, the ions are reacted with an aqueous solution of sodium hydroxide. The ions can be in the form of copper nitrate, , or copper sulfate, . Upon reacting the ions with sodium hydroxide, a blue precipitate of copper(II) hydroxide is produced. The copper(II) hydroxide can be extracted by filtering the solution, leaving behind the precipitate.
The chemical equation for the transformation shown in reaction 2 is
In reaction 3, a test tube containing the copper(II) hydroxide precipitate is carefully heated over a Bunsen burner. After heating for a while, a black solid of copper(II) oxide is produced, at which point the test tube is removed from the flame.
The chemical equation for the transformation shown in reaction 3 is
In the final reaction, a small amount of the black copper(II) oxide is added to a test tube along with a small amount of carbon powder. The test tube is then heated strongly until a reddish-orange substance, copper metal, is produced.
The chemical equation for the transformation shown in reaction 4 is
In reaction 1, we can see that the elements copper (), hydrogen (), nitrogen (), and oxygen () are conserved as they appear on both sides of the reaction arrow. In reaction 2, we can see that the elements copper (), sodium (), oxygen (), and hydrogen () are conserved. We can also see that the law of conservation of elements is obeyed in both reaction 3 and reaction 4.
Example 4: Identifying the Element Missing from a Chemical Equation
By using the concept of conservation of elements, identify the element missing in the following chemical equation:
Answer
During a chemical reaction, the atoms of all elements must be conserved. This means that any elements that are present in the starting materials must be present in the products. In the reaction equation provided, we can see the elements magnesium (), hydrogen (), sulfur (), and oxygen () on the left-hand side of the reaction arrow. On the right-hand side of the reaction arrow, we see the elements sulfur (), oxygen (), and hydrogen (). As any element present in the starting materials (left-hand side of the reaction arrow) must be present in the products (right-hand side of the reaction arrow), the missing element is choice C, .
When performing a chemical reaction, it may seem like some elements disappear during the reaction. For example, gasoline is a liquid that contains a compound called octane (). Octane is composed of the elements carbon () and hydrogen (). When a sample of gasoline is burned, it may seem like the gasoline simply disappears as no visible products will remain. However, when gasoline is burned, oxygen () gas in the air reacts with the octane. The atoms are recombined to form carbon dioxide gas () and water vapor (), two colorless gases:
So, although to our eye it seems as if the gasoline disappears and the elements are destroyed, in reality, the atoms of carbon, hydrogen, and oxygen are conserved and recombined into new compounds that we cannot see with the naked eye.
Key Points
- An element is a substance that consists of one type of atom and cannot be broken down into anything simpler through a chemical reaction.
- Atoms of two or more elements can chemically combine to form compounds.
- Elements and compounds can be transformed into new compounds during a chemical reaction.
- The new substances formed during a chemical reaction will have a different composition and different properties than the original substances.
- Elements must be conserved in a chemical reaction, meaning that atoms of elements can neither be created nor destroyed during a chemical reaction.