Introductory Chemistry: A Foundation - Zumdahl S.S., DeCoste D.J. 2019
Physical and Chemical Properties and Changes
· To learn to distinguish between physical and chemical properties.
· To learn to distinguish between physical and chemical changes.
When you see a friend, you immediately respond and call him or her by name. We can recognize a friend because each person has unique characteristics or properties. The person may be thin and tall, may have blonde hair and blue eyes, and so on. The characteristics just mentioned are examples of physical properties . Substances also have physical properties. Typical physical properties of a substance include odor, color, volume, state (gas, liquid, or solid), density, melting point, and boiling point. We can also describe a pure substance in terms of its chemical properties , which refer to its ability to form new substances. An example of a chemical change is wood burning in a fireplace, giving off heat and gases and leaving a residue of ashes. In this process, the wood is changed to several new substances. Other examples of chemical changes include the rusting of the steel in our cars, the digestion of food in our stomachs, and the growth of grass in our yards. In a chemical change a given substance changes to a fundamentally different substance or substances.
Interactive Example 3.1. Identifying Physical and Chemical Properties
Classify each of the following as a physical or a chemical property.
a. The boiling point of a certain alcohol is .
b. Diamond is very hard.
c. Sugar ferments to form alcohol.
d. A metal wire conducts an electric current.
Items (a), (b), and (d) are physical properties; they describe inherent characteristics of each substance, and no change in composition occurs. A metal wire has the same composition before and after an electric current has passed through it. Item (c) is a chemical property of sugar. Fermentation of sugars involves the formation of a new substance (alcohol).
Self-Check: Exercise 3.1
· Which of the following are physical properties, and which are chemical properties?
a. Gallium metal melts in your hand.
b. Platinum does not react with oxygen at room temperature.
c. The page of the textbook is white.
d. The copper sheets that form the “skin” of the Statue of Liberty have acquired a greenish coating over the years.
See Problems 3.11, 3.12, 3.13, and 3.14.
Matter can undergo changes in both its physical and its chemical properties. To illustrate the fundamental differences between physical and chemical changes, we will consider water. As we will see in much more detail in later chapters, a sample of water contains a very large number of individual units (called molecules), each made up of two atoms of hydrogen and one atom of oxygen—the familiar . This molecule can be represented as
where the letters stand for atoms and the lines show attachments (called bonds) between atoms, and the molecular model (on the right) represents water in a more three-dimensional fashion.
What is really occurring when water undergoes the following changes?
We will describe these changes of state precisely in Chapter 14, but you already know something about these processes because you have observed them many times.
An iron pyrite crystal.
When ice melts, the rigid solid becomes a mobile liquid that takes the shape of its container. Continued heating brings the liquid to a boil, and the water becomes a gas or vapor that seems to disappear into “thin air.” The changes that occur as the substance goes from solid to liquid to gas are represented in Fig. 3.2. In ice the water molecules are locked into fixed positions (although they are vibrating). In the liquid the molecules are still very close together, but some motion is occurring; the positions of the molecules are no longer fixed as they are in ice. In the gaseous state the molecules are much farther apart and move randomly, hitting each other and the walls of the container.
The three states of water (where red spheres represent oxygen atoms and blue spheres represent hydrogen atoms).
The most important thing about all these changes is that the water molecules are still intact. The motions of individual molecules and the distances between them change, but molecules are still present. These changes of state are physical changes because they do not affect the composition of the substance. In each state we still have water , not some other substance.
Now suppose we run an electric current through water as illustrated in Fig. 3.3. Something very different happens. The water disappears and is replaced by two new gaseous substances, hydrogen and oxygen. An electric current actually causes the water molecules to come apart—the water decomposes to hydrogen and oxygen. We can represent this process as follows:
Electrolysis, the decomposition of water by an electric current, is a chemical process.
This is a chemical change because water (consisting of molecules) has changed into different substances: hydrogen (containing molecules) and oxygen (containing molecules). Thus in this process, the molecules have been replaced by and molecules. Let us summarize:
Physical and Chemical Changes
1. A physical change involves a change in one or more physical properties but no change in the fundamental components that make up the substance. The most common physical changes are changes of state: .
2. A chemical change involves a change in the fundamental components of the substance; a given substance changes into a different substance or substances. Chemical changes are called reactions : silver tarnishes by reacting with substances in the air; a plant forms a leaf by combining various substances from the air and soil; and so on.
Interactive Example 3.2. Identifying Physical and Chemical Changes
Classify each of the following as a physical or a chemical change.
a. Iron metal is melted.
b. Iron combines with oxygen to form rust.
c. Wood burns in air.
d. A rock is broken into small pieces.
a. Melted iron is just liquid iron and could cool again to the solid state. This is a physical change.
b. When iron combines with oxygen, it forms a different substance (rust) that contains iron and oxygen. This is a chemical change because a different substance forms.
c. Wood burns to form different substances (as we will see later, they include carbon dioxide and water). After the fire, the wood is no longer in its original form. This is a chemical change.
d. When the rock is broken up, all the smaller pieces have the same composition as the whole rock. Each new piece differs from the original only in size and shape. This is a physical change.
Self-Check: Exercise 3.2
· Classify each of the following as a chemical change, a physical change, or a combination of the two.
a. Milk turns sour.
b. Wax is melted over a flame and then catches fire and burns.
See Problems 3.17 and 3.18.