Mixtures, Solubility, and Acid/Base Solutions

Transcription

1 CHAPTER 9 Mixtures, Solubility, and Acid/Base Solutions Substances and Mixtures What do you think? Read the two statements below and decide whether you agree or disagree with them. Place an A in the Before column if you agree with the statement or a D if you disagree. After you ve read this lesson, reread the statements to see if you have changed your mind. Before Statement After 1. You can identify a mixture by looking at it without magnification. 2. A solution is another name for a homogeneous mixture. LESSON 1 Key Concepts How do substances and mixtures differ? How do solutions compare and contrast with heterogeneous mixtures? In what three ways do compounds differ from mixtures? Matter: Substances and Mixtures Many different types of matter exist around you. On your way to school, you might see metal, plastic, rocks, concrete, plants, fabric, water, and skin. You might notice that you can group some types of matter together into one category. For example, keys, coins, and paper clips are made of metal. Grouping matter into categories helps you understand how some things are similar to each other, but different from other things. Nearly all types of matter can be sorted into just two major categories substances and mixtures. What is a substance? A substance is matter that is always made up of the same combination of atoms. There are two types of substances elements and compounds. Recall that an element is matter made of only one type of atom, such as carbon (C), oxygen (O 2 ), and chlorine (Cl 2 ). A compound is matter made of atoms of two or more elements that are chemically bonded together. For example, water (H 2 O) always contains two atoms of hydrogen bonded to one atom of oxygen. Table salt (NaCl) always contains one atom of sodium bonded to one atom of chlorine. The composition of elements and compounds doesn t change. Therefore, all elements and compounds are substances. Make a Table with two columns to contrast substances and mixtures. Label one column Substances and the other column Mixtures. Complete the table as you read this lesson. Reading Check 1. Analyze Is table salt an element or a compound? Explain. Reading Essentials Mixtures, Solubility, and Acid/Base Solutions 139

2 Combine a two-tab and a four-tab book to organize your notes on matter. Two Types of Matter Substances Elements Compounds Mixtures Heterogeneous Homogeneous 2. Contrast How do substances and mixtures differ? 3. Distinguish How can you determine whether a mixture is homogeneous or heterogeneous? Visual Check 4. Interpret Can a mixture be made only of elements? Explain. What is a mixture? A mixture is two or more substances that are physically blended but are not chemically bonded together. Therefore, the relative amounts of each substance in a mixture can vary. Granite, a type of rock, is a mixture. Granite contains bits that are white, black, and other colors. Two pieces of granite will have different amounts of each color bit. The composition of rocks varies. Air is also a mixture. Air contains about 78 percent nitrogen, 21 percent oxygen, and 1 percent other substances. But this composition varies. Air in a scuba tank can have more than 21 percent oxygen and less of the other substances. Rocks and air are examples of the two different types of mixtures heterogeneous (he tuh roh JEE nee us) and homogeneous (hoh muh JEE nee us). It s not always easy to identify a mixture. A rock looks like a mixture, but air does not. Heterogeneous Mixtures A heterogeneous mixture is a mixture in which substances are not evenly mixed. For example, granite is a heterogeneous mixture. The substances that make up granite are unevenly mixed. When you look at a piece of granite, you can easily see the different parts. Often, you can see the different substances and parts of a heterogeneous mixture with unaided eyes. Sometimes you can see them only with a microscope. For example, blood looks evenly mixed its color and texture are the same throughout. But suppose you compare two samples of blood under a microscope. They will look different. The samples will contain different amounts of the components. Solutions Homogeneous Mixtures Many mixtures look evenly mixed, even when you view them with a powerful microscope. These mixtures are homogeneous. A homogeneous mixture is a mixture in which two or more substances are evenly mixed on the atomic level but not bonded together. The individual atoms or compounds of each substance are mixed. The mixture looks the same throughout under a microscope because individual atoms and compounds are too small to see. Air is a homogeneous mixture. If you view air under a microscope, you can t see the individual substances that make it up. Another name for a homogeneous mixture is solution. As you read, remember that the terms homogeneous mixture and solution mean the same thing. Review the figure on the next page. It summarizes the characteristics of substances and mixtures. 140 Mixtures, Solubility, and Acid/Base Solutions Reading Essentials

3 Chara cteristics of Substances and Mixtures Matter anything that has mass and takes up space Most matter on Earth is made up of atoms. Substances matter with a composition that is always the same two types of substances: elements and compounds Elements consists of just one type of atom organized on the periodic table Elements can exist as single atoms or as a diatomic molecule two atoms bonded together. Chemical changes Compounds made of two or more types of atoms bonded together can t be separated by physical methods properties are different from the properties of the elements that make them up two types: ionic and covalent Physical changes Separating mixtures filtering boiling using a magnet Combining substances mixing dissolving Heterogeneous mixtures two or more substances unevenly mixed uneven mixing is visible with unaided eyes or a microscope Mixtures matter that can vary in composition made of two or more substances mixed but not bonded together can be separated into substances by physical methods Two types of mixtures: heterogeneous and homogeneous Homogeneous mixtures (solutions) two or more substances evenly mixed Homogeneous mixtures appear uniform under a microscope. Reading Essentials Mixtures, Solubility, and Acid/Base Solutions 141

4 5. Apply Seawater is a mixture of mainly salts and water. What properties of these substances can you observe in seawater? 6. Identify In what three ways do compounds differ from mixtures? How do compounds and mixtures differ? You have read that a compound contains two or more elements that are chemically bonded together. In contrast, the substances that make up a mixture are not chemically bonded. Therefore, mixing is a physical change. The substances that exist before mixing still exist in the mixture. This leads to two important differences between compounds and mixtures. Substances keep their properties. Substances that make up a mixture are not changed chemically. Therefore, you can observe some of their properties in the mixture. Sugar water is a mixture of two compounds sugar and water. After the sugar is mixed in, you can t see the sugar in the water, but you can still taste its property of sweetness. You can also observe some properties of the water, such as its liquid state. In contrast, the properties of a compound can be different from the properties of the elements that make it up. Sodium and chlorine bond and form table salt. Sodium is a soft, opaque, silvery metal. Chlorine is a greenish, poisonous gas. You cannot observe any of these properties in table salt. Mixtures can be separated. The substances that make up a mixture are not bonded together. As a result, you can separate them from each other using physical methods. The physical properties of one substance are different from those of another. You can use these differences to separate the substances. In contrast, you can separate compounds only by a chemical change that breaks the bonds between the elements. 142 Mixtures, Solubility, and Acid/Base Solutions Reading Essentials

5 Mini Glossary heterogeneous (he tuh roh JEE nee us) mixture: a mixture in which substances are not evenly mixed homogeneous (hoh muh JEE nee us) mixture: a mixture in which two or more substances are evenly mixed on the atomic level but not bonded together solution: a homogeneous mixture substance: matter that is always made up of the same combination of atoms mixture: two or more substances that are physically blended but are not chemically bonded together 1. Review the terms and their definitions in the Mini Glossary. Write a sentence that explains how a homogeneous mixture and a solution are related. 2. Complete the concept web by writing the types of matter in the correct boxes. Matter a. What do you think Reread the statements at the beginning of the lesson. Fill in the After column with an A if you agree with the statement or a D if you disagree. Did you change your mind? ConnectED mixtures b. c. d. e. oxygen, carbon water, sodium chloride granite, blood Log on to ConnectED.mcgraw-hill.com and access your textbook to find this lesson s resources. example: air END OF LESSON Reading Essentials Mixtures, Solubility, and Acid/Base Solutions 143