1.2 The Classification of Matter

Part of understanding matter is being able to describe it. One way chemists describe matter is to assign different kinds of properties to different categories.

Physical and Chemical Properties

The properties that chemists use to describe matter fall into two general categories. Physical properties are characteristics that describe matter. They include characteristics such as size, shape, color, and mass. Chemical properties are characteristics that describe how matter changes its chemical structure or composition. An example of a chemical property is flammability—a material’s ability to burn—because burning (also known as combustion) changes the chemical composition of a material.

Elements and Compounds

Any sample of matter that has the same physical and chemical properties throughout the sample is called a substance. There are two types of substances. A substance that cannot be broken down into chemically simpler components is an element. Aluminum, which is used in soda cans, is an element. A substance that can be broken down into chemically simpler components (because it has more than one element) is a compound (Figure 1.2). Water is a compound composed of the elements hydrogen and oxygen. Today, there are about 118 elements in the known universe. In contrast, scientists have identified tens of millions of different compounds to date.

Note

Sometimes the word pure is added to substance, but this is not absolutely necessary. By definition, any single substance is pure.

The smallest part of an element that maintains the identity of that element is called an atom. Atoms are extremely tiny; to make a line 1 inch long, you would need 217 million iron atoms. The smallest part of a compound that maintains the identity of that compound is called a molecule. Molecules are composed of atoms that are attached together and behave as a unit. Scientists usually work with millions and millions of atoms and molecules at a time. When a scientist is working with large numbers of atoms or molecules at a time, the scientist is studying the macroscopic view of the universe. However, scientists can also describe chemical events on the level of individual atoms or molecules, which is referred to as the microscopic viewpoint. We will see examples of both macroscopic and microscopic viewpoints throughout this book (Figure 1.3).

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Figure 1.3 How Many Particles Are Needed for a Period in a Sentence?

Although we do not notice it from a macroscopic perspective, matter is composed of microscopic particles so tiny that billions of them are needed to make a speck we can see with the naked eye. The ×25 and ×400,000,000 indicate the number of times the image is magnified.

Mixtures

A material composed of two or more substances is a mixture. In a mixture, the individual substances maintain their chemical identities. Many mixtures are obvious combinations of two or more substances, such as a mixture of sand and water. Such mixtures are called heterogeneous mixtures. In some mixtures, the components are so intimately combined that they act like a single substance (even though they are not). Mixtures with a consistent composition throughout are called homogeneous mixtures (or solutions). Sugar dissolved in water is an example of a solution. A metal alloy, such as steel, is an example of a solid solution. Air, a mixture of mainly nitrogen and oxygen, is a gaseous solution.

Example

How would a chemist categorize each example of matter?

  1. saltwater
  2. soil
  3. water
  4. oxygen

Skill-Building Exercise

How would a chemist categorize each example of matter?

  1. coffee
  2. hydrogen
  3. an egg

Phases

Another way to classify matter is to describe it as a solid, a liquid, or a gas, which was done in the examples of solutions. These three descriptions, each implying that the matter has certain physical properties, represent the three phases of matter. A solid has a definite shape and a definite volume. Liquids ordinarily have a definite volume but not a definite shape; they take the shape of their containers. Gases have neither a definite shape nor a definite volume, and they expand to fill their containers. We encounter matter in each phase every day; in fact, we regularly encounter water in all three phases: ice (solid), water (liquid), and steam (gas).

We know from our experience with water that substances can change from one phase to another if the conditions are right. Typically, varying the temperature of a substance (and, less commonly, the pressure exerted on it) can cause a phase change, a physical process in which a substance goes from one phase to another (Figure 1.4). Phase changes have particular names depending on what phases are involved, as summarized in Table 1.1.

Table 1.1 Phase Changes
Change Name
solid to liquid melting, fusion
solid to gas sublimation
liquid to gas boiling, evaporation
liquid to solid solidification, freezing
gas to liquid condensation
gas to solid deposition

Figure 1.5 illustrates the relationships between the different ways matter can be classified.

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Figure 1.5 The Classification of Matter

Matter can be classified in a variety of ways, depending on its properties.

Concept Review Exercises

  1. Explain the differences between the physical properties of matter and the chemical properties of matter.

  2. What is the difference between a heterogeneous mixture and a homogeneous mixture? Give an example of each.

Give at least two examples of a phase change and state the phases involved in each.

Key Takeaways

  • Matter can be described with both physical properties and chemical properties.
  • Matter can be identified as an element, a compound, or a mixture.

Exercises

  1. Does each statement refer to a chemical property or a physical property?

    1. Balsa is a very light wood.
    2. If held in a flame, magnesium metal burns in air.
    3. Mercury has a density of 13.6 g/mL.
    4. Human blood is red.

  2. Does each statement refer to a chemical property or a physical property?

    1. The elements sodium and chlorine can combine to make table salt.
    2. The metal tungsten does not melt until its temperature exceeds 3,000°C.
    3. The ingestion of ethyl alcohol can lead to disorientation and confusion.
    4. The boiling point of isopropyl alcohol, which is used to sterilize cuts and scrapes, is lower than the boiling point of water.

  3. Define element. How does it differ from a compound?

  4. Define compound. How does it differ from an element?

  5. Give two examples of a heterogeneous mixture.

  6. Give two examples of a homogeneous mixture.

  7. Identify each substance as an element, a compound, a heterogeneous mixture, or a solution.

    1. xenon, a substance that cannot be broken down into chemically simpler components
    2. blood, a substance composed of several types of cells suspended in a salty solution called plasma
    3. water, a substance composed of hydrogen and oxygen

  8. Identify each substance as an element, a compound, a heterogeneous mixture, or a solution.

    1. sugar, a substance composed of carbon, hydrogen, and oxygen
    2. hydrogen, the simplest chemical substance
    3. dirt, a combination of rocks and decaying plant matter

  9. Identify each substance as an element, a compound, a heterogeneous mixture, or a solution.

    1. air, primarily a mixture of nitrogen and oxygen
    2. ringer’s lactate, a standard fluid used in medicine that contains salt, potassium, and lactate compounds all dissolved in sterile water
    3. tartaric acid, a substance composed of carbon, hydrogen, and oxygen

  10. Identify each material as an element, a compound, a heterogeneous mixture, or a solution.

    1. equal portions of salt and sand placed in a beaker and shaken up
    2. a combination of beeswax dissolved in liquid hexane
    3. hydrogen peroxide, a substance composed of hydrogen and oxygen

  11. What word describes each phase change?

    1. solid to liquid
    2. liquid to gas
    3. solid to gas

  12. What word describes each phase change?

    1. liquid to solid
    2. gas to liquid
    3. gas to solid

    answers