- Identify the chemical properties of a substance.
- Describe chemical changes and differentiate them from physical changes.
- Use various visual clues to identify whether a chemical reaction is taking place.
- chemical property
- chemical reaction
Check Your Understanding
Recalling Prior Knowledge
- What is a chemical change?
- What are the three common states of matter?
As you have seen, physical changes are those that do not alter the identities of the substances involved. In contrast, chemical changes produce substances with different compositions. In this lesson, you will learn how to recognize chemical changes.
When exposed to air, an object made of iron will eventually begin to rust (Figure below).
Rust (iron oxide) forms on an unprotected iron surface.
As the rust forms on the surface of the iron, it flakes off to expose more iron, which will continue to rust. Rust is clearly a substance that is different from iron. Rusting is an example of a chemical change.
A chemical property describes the ability of a substance to undergo a specific chemical change. A chemical property of iron is that it is capable of combining with oxygen to form iron oxide, the chemical name of rust. A more general term for rusting and other similar processes is corrosion. Other terms that are commonly used in descriptions of chemical changes are burn, rot, explode, decompose, and ferment. Chemical properties are very useful as a way of identifying substances. However, unlike physical properties, chemical properties can only be observed as the substance is in the process of being changed into a different substance.
A chemical change is also called a chemical reaction. A chemical reaction is a process that occurs when one or more substances are changed into one or more new substances. Zinc (Zn) is a silver-gray element that can be ground into a powder. If zinc is mixed at room temperature with powdered sulfur (S), a bright yellow element, the result will simply be a mixture of zinc and sulfur. No chemical reaction occurs. However, if energy is provided to the mixture in the form of heat, the zinc will chemically react with the sulfur to form the compound zinc sulfide (ZnS). Pictured below are the substances involved in this reaction (Figure below).
Zinc (A) and sulfur (B) are two elements that undergo a chemical reaction when heated to from the compound zinc sulfide (C).
The reaction between zinc and sulfur can be depicted in something called a chemical equation. In words, we could write the reaction as:
zinc + sulfur → zinc sulfide
A more convenient way to express a chemical reaction is to use the symbols and formulas of the substances involved:
Zn + S → ZnS
The substance(s) to the left of the arrow in a chemical equation are called reactants. A reactant is a substance that is present at the start of a chemical reaction. The substance(s) to the right of the arrow are called products. A product is a substance that is present at the end of a chemical reaction. In the equation above, zinc and sulfur are the reactants that chemically combine to form zinc sulfide as a product.
Recognizing Chemical Reactions
How can you tell if a chemical reaction is taking place? Certain visual clues indicate that a chemical reaction is likely (but not necessarily) occurring, including the following examples:
- A change of color occurs during the reaction.
- A gas is produced during the reaction.
- A solid product, called a precipitate, is produced in the reaction.
- A visible transfer of energy occurs in the form of light as a result of the reaction.
Mercury(II) oxide is a red solid. When it is heated to a temperature above 500°C, it easily decomposes into mercury and oxygen gas. The red color of the reactant, mercury oxide, is gradually replaced by the silver color of the product, mercury. The color change is one sign that this reaction is occurring. Watch this decomposition take place at http://www.youtube.com/watch?v=_Y1alDuXm6A (1:12).
When zinc reacts with hydrochloric acid, the reaction bubbles vigorously as hydrogen gas is produced (Figure below). The production of a gas is also an indication that a chemical reaction may be occurring.
Zinc reacts with hydrochloric acid to produce bubbles of hydrogen gas.
When a colorless solution of lead(II) nitrate is added to a colorless solution of potassium iodide, a yellow solid called a precipitate is instantly produced (Figure below). A precipitate is a solid product that forms from a reaction and settles out of a liquid mixture. The formation of a precipitate may also indicate the occurrence of a chemical reaction.
A yellow precipitate of solid lead(II) iodide forms immediately when solutions of lead(II) nitrate and potassium iodide are mixed.
All chemical changes involve a transfer of energy. When zinc reacts with hydrochloric acid, the test tube becomes very warm as energy is released during the reaction. Some other reactions absorb energy. While energy changes are a potential sign of a chemical reaction, care must be taken to ensure that a chemical reaction is indeed taking place. Physical changes may also involve a transfer of energy. A solid absorbs energy when it melts, and the condensation of a gas releases energy. The only way to be certain that a chemical reaction has occurred is to test the composition of the substances after the change has taken place to see if they are different from the starting substances.
Combustion is a chemical change. Watch as magnesium reacts with oxygen to form magnesium oxide. View this lab-scale fireworks display at www.youtube.com/watch?v=EZ3JT2nWfMA (1:31).
- The composition of matter changes during a chemical reaction. The ability of a substance to react with other substances is reflected in its chemical properties.
- In a chemical reaction, the reacting substances are called reactants, and the substances that are produced by the reaction are called products. A chemical equation shows reactants becoming products.
- Four clues to a possible chemical reaction include a color change, the production of a gas, the formation of a precipitate, and an observable transfer of energy.
Lesson Review Questions
- What is the main difference between a physical change and a chemical change?
- How must the chemical properties of a given substance be observed?
- Give an example of a situation where a color change occurs, but not because of a chemical reaction.
- Classify the following as physical properties or chemical properties of the element nickel.
- It is gray in color.
- It melts at 1455°C.
- It reacts with sulfuric acid.
- It is a conductor of electricity.
- It forms a compound with oxygen.
- Combustion, also called burning, is a reaction with oxygen gas. When propane is combusted, carbon dioxide and water are formed. Identify the reactants and the products of this combustion reaction.
- Classify each of the following as a chemical change or a physical change.
- Sugar dissolves in water.
- A peach rots.
- Icicles melt in the warm sunlight.
- A baking cake rises in the oven.
- A leaf changes its color in the fall.
- Food coloring is added to a glass of water.
- Describe clues that you might observe during the following situations that would indicate that a chemical reaction is likely occurring.
- A log burns in the fireplace.
- A mixture of baking soda and vinegar fizzes.
- A silver fork tarnishes.
- A plumbing pipe becomes clogged with lime.
Points to Consider
A chemist must do more than simply make observations about chemical changes that are occurring in his or her experiments. He or she must also take measurements in order to fully understand what is taking place during chemical reactions.
- What types of measurements are routinely made by chemists?
- How does a chemist indicate the degree of confidence in a measurement?
- How are different numerical measurements manipulated when performing calculations?