Problemsolving

Interpret Scientific Illustrations

How do compounds form? Many atoms gain or lose electrons in order to have eight electrons in the outermost energy level. In the diagram, energy levels are indicated by the circles around the nucleus of each element. The colored spheres in the energy levels represent electrons, and the spheres in the nucleus represent protons and neutrons.

Hco3 Losing Electrons

Analysis

1. How many electrons are present in atoms of Element A? Element B?

2. How many protons are present in the nuclei of these atoms?

3. Use the periodic table on page 61 to determine the name and symbol of Element A and Element B.

Think Critically

4. Decide if these elements can form ions. If so, what would be the electric charges (magnitude and sign) and chemical symbols of these ions?

5. Formulate a compound from these two elements. What is the chemical formula of the compound?

Mixtures and Solutions

Unlike a compound, in which the atoms combine and lose their identities, a mixture is a combination of two or more components that retain their identities. When a mixture's components are easily recognizable, it is called a heterogeneous mixture. For example, beach sand, shown in Figure 3.14, is a heterogeneous mixture because its components are still recognizable—shells, small pieces of broken shells, grains of minerals, and so on. In a homogeneous mixture, which is also called a solution, the component particles cannot be distinguished, even though they still retain their original properties.

A solution can be liquid, gaseous, or solid. Seawater is a solution consisting of water molecules and ions of many elements that exist on Earth. Molten rock is also a liquid solution; it is composed of ions representing all atoms that were present in the crystals of the rock before it melted. Air is a solution of gases, mostly nitrogen and oxygen molecules together with other atoms and molecules. Metal alloys, such as bronze and brass, are also solutions. Bronze is a homogeneous mixture of copper and tin atoms; brass is a similar mixture of copper and zinc atoms. Such solid homogeneous mixtures are called solid solutions. You will learn more about solid solutions in Chapters 4 and 5.

^ Reading Check Describe three examples of solutions.

Acids Many chemical reactions that occur on Earth involve solutions called acids and bases. An acid is a solution containing a substance that produces hydrogen ions (H+) in water. Recall that a hydrogen atom consists of one proton and one electron. When a hydrogen atom loses its electron, it becomes a hydrogen ion (H+). The pH scale, shown in Figure 3.15, is based on the amount of hydrogen ions in a solution. This amount is referred to as the concentration. A value of 7 is considered neutral. A solution with a pH reading below 7 is considered to be acidic. The lower the number, the more acidic the solution.

Scale Glencoe
Figure 3.14 Not all mixtures of beach sand and shells are alike. Mixtures from the Atlantic Ocean will contain components that are different from mixtures that form in the Pacific Ocean.

C^fKtpts in MOHin

Interactive Figure To see an animation of the pH scale, visit glencoe.com.

Interactive Figure To see an animation of the pH scale, visit glencoe.com.

Scale Glencoe

Section 2 • Combining Matter 71

(tr)Gregor Schuster/zefa/CORBIS, (l to r)Studiohio, (2)Mark Burnett, (3)Studiohio, (4)Matt Meadows, (5)Amanita Pictures, (6)Studiohio, (7)Aaron Haupt

Section 2 • Combining Matter 71

(tr)Gregor Schuster/zefa/CORBIS, (l to r)Studiohio, (2)Mark Burnett, (3)Studiohio, (4)Matt Meadows, (5)Amanita Pictures, (6)Studiohio, (7)Aaron Haupt

Careers In Earth scienci

Geochemistry Some geochemists study the interaction of rocks, minerals and the environment. They can help mining companies reduce the amount of contamination from waste piles by understanding how the rocks and minerals break down and how toxic the byproducts might be. For more information on Earth science careers, visit glencoe.com.

The most common acid in Earth's environment is carbonic acid (H2CO3), which is produced when carbon dioxide (CO2) is dissolved in water (H2O) by the following reaction.

Some of the carbonic acid (H2CO3) in the water ionizes, or breaks apart, into hydrogen ions (H+) and bicarbonate ions (HCO3), as represented by the following equation.

These two equations play a major role in the dissolution and precipitation of limestone and the formation of caves, discussed in Chapter 10. Many of the reaction rates involved in geological processes are very slow. For example, it might take thousands of years for enough carbonic acid in limestone to dissolve in groundwater and produce a cave. ^

Bases When a solution contains hydroxide ions (OH"), the solution is called a base. A base can neutralize an acid because hydrogen ions (H+) from the acid react with the hydroxide ions (OH) from the base to form water through the following reaction.

Refer again to Figure 3.15. A solution with a reading above 7 is considered to be basic. The higher the number, the more basic the solution. Distilled water usually has a pH of 7, but rainwater is slightly acidic, with a pH of 5.0 to 5.6. The pH values of some common substances are shown in Figure 3.15.

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