Matter and Its Properties

Transcription

1 SECTION 2 Plan and Prepare Objectives Upon completion of this section, students should be able to do the following: Distinguish between the physical properties and chemical properties of matter. Classify changes of matter as physical or chemical. Explain the gas, liquid, and solid states in terms of particles. Explain how the law of conservation of energy applies to changes in matter. Distinguish between a mixture and a pure substance. Classroom Catalyst Determine the mass of a deflated balloon. Then blow up the balloon, tie it, and find its new mass. Because the balloon increases in mass, the matter in the balloon must have mass. Point out to students that matter does not need to be visible to us and that it may be made of particles that we cannot see. Teach Ask students to consider the following questions: Which is heavier, a pound of lead or a pound of feathers? What does a pound represent? What happens to the matter of an astronaut who is in microgravity? Divide the class into small groups to design a tool that could be used to weigh something aboard the space shuttle. Use the Internet to find out how NASA really does it. FIGURE 2.1 Have students compare the models shown. Point out that chemists have different ways of representing the structures of substances. The diamond model is a ball-and-stick representation. Sucrose is shown as a space-fill model. Inform students that they will see both types throughout this book. Main Ideas Atoms are the building blocks of matter. All substances have characteristic properties. Matter can be a pure substance or a mixture. > VIRGINIA STANDARDS CH.2.h The student will investigate and understand that the placement of elements on the periodic table is a function of their atomic structure. The periodic table is a tool used for the investigations of: chemical and physical properties. CH.2.EKS-14; CH.2.EKS-15 FIGURE 2.1 SECTION 2 Atoms as Building Blocks Both elements and compounds are made of atoms, as shown in these models of diamond and table sugar. 6 Chapter 1 Differentiated Instruction Matter and Its Properties Key Terms mass physical change chemical reaction matter change of state reactant atom solid product element liquid mixture compound gas homogeneous extensive property plasma solution intensive property chemical property heterogeneous physical property chemical change pure substance All things are made up of matter, but exactly what is matter? What characteristics, or properties, make matter what it is? In this section, you will learn the answers to these questions. Explaining what matter is involves finding properties that all matter has in common. That may seem difficult, given that matter takes so many different forms. For the moment, just consider one example of matter a rock. The first thing you might notice is that the rock takes up space. In other words, it has volume. Volume is the amount of three - dimensional space an object occupies. All matter has volume. All matter also has a property called mass. Mass is a measure of the amount of matter. Mass is the measurement you make using a balance. Matter can thus be defined as anything that has mass and takes up space. These two properties are the general properties of all matter. MAIN IDEA Atoms are the building blocks of matter. Matter comes in many forms. The fundamental building blocks of matter are atoms and molecules. These particles make up elements and compounds. An atom is the smallest unit of an element that maintains the chemical identity of that element. An element is a pure substance that cannot be broken down into simpler, stable substances and is made of one type of atom. Carbon is an element and contains one kind of atom. The model of diamond in Figure 2.1a consists of carbon atoms. (a) Diamond English Learners Offer students the following suggestion to help guide their reading on physical and chemical properties. Fold a large piece of paper in half vertically. Fold the paper in half the other way, to form two columns and two rows on the paper. Label the left-hand side Physical and the right-hand side Chemical. Students should fill in the upper row with examples of physical and chemical properties. They should fill in the lower row with physical and chemical changes. A few classic examples should help them remember the difference between the two. Carbon atom Oxygen atom Hydrogen atom Carbon atom (b) Sucrose (table sugar) Inclusion Provide students with labeled pictures of an atom, an element, and a compound. Prepare a concept map that shows the relationships between these images. Include empty squares large enough to fit the labeled pictures. For example: A [blank square] --> is made of two or more kinds of [blank square] --> which are made up of only one kind of [blank square]. Have students place the pictures in the appropriate spots on the chart. Check their placement. When correct, have them tape or paste the pictures in place. Untitled /29/2011 6:41:16 6 Chapter 1

2 titled-82 7 A compound is a substance that can be broken down into simple stable substances. Each compound is made from the atoms of two or more elements that are chemically bonded. Sucrose, in Figure 2.1b, is an example of a compound. It is made of three elements: carbon, hydrogen, and oxygen. The atoms are chemically bonded to form a molecule. You will learn more about the particles that make up compounds when you study chemical bonding. For now, you can think of a molecule as the smallest unit of an element or compound that retains all of the properties of that element or compound. MAIN IDEA All substances have characteristic properties. Every substance, whether it is an element or a compound, has characteristic properties. Chemists use properties to distinguish between substances and to separate them. Most chemical investigations are related to or depend on the properties of substances. A property may be a characteristic that defines an entire group of substances. That property can be used to classify an unknown substance as a member of that group. For example, many elements are classified as metals. The distinguishing property of metals is that they conduct electricity well. Therefore, if an unknown element is tested and found to conduct electricity well, it is a metal. Properties can help reveal the identity of an unknown substance. However, conclusive identification usually cannot be made based on only one property. Comparisons of several properties can be used together to establish the identity of an unknown. Properties are either intensive or extensive. Extensive properties depend on the amount of matter that is present. Such properties include volume, mass, and the amount of energy in a substance. In contrast, intensive properties do not depend on the amount of matter present. Such properties include the melting point, boiling point, density, and ability to conduct electricity and to transfer energy as heat. Intensive properties are the same for a given substance regardless of how much of the substance is present. For example, iron melts at 1538 C regardless of whether or not you have 20 g or 20 kg of it. Properties can also be grouped into two general types: physical properties and chemical properties. Physical Properties and Physical Changes A physical property is a characteristic that can be observed or measured without changing the identity of the substance. We commonly use physical properties to describe a substance. Examples of physical properties are melting point and boiling point. For example, water melts from ice to liquid at 0 C (273 K or 32 F). Liquid water, as shown in Figure 2.2, boils to vapor at 100 C (373 K or 212 F). Density is also another physical property. Water s density at 4 C (277 K or 39 F) is about 1000 kg/m 3. Unlike most substances, the density of water decreases when it freezes to become ice. As a result, a pond or lake that freezes in the winter does so from the top down, enabling some fish to survive in the water at the bottom. Below Level Read students the following paragraph: Amy was making a salad for dinner. First, she cut up the carrots and tore the lettuce. The dressing she was making required her to mix oil and vinegar in a jar. She was unsure what to do with the food scraps. She thought about grinding them in the garbage disposal. But then she remembered she could put them in the compost pile to decompose. Students should look at each underlined phrase and state whether it is an example of a physical FIGURE 2.2 Physical Properties Water boils at 100 C. This is an example of a physical property. CRITICAL THINKING Classify Is the boiling point of water an extensive or an intensive property? Explain. Matter and Change or a chemical change. Physical change: cut carrots, tore lettuce, mixed oil and vinegar, ground scraps; chemical change: decompose. Challenge students to write their own short paragraphs involving physical and chemical changes. 7 6/29/2011 6:41:18 AM Students may be confused regarding the use of the term molecule, because all compounds are not made of molecules. It is not necessary to get into the details now, as this will be discussed later. Demonstration CONDUCTIVITY OF WATER Using two clear drinking glasses, fill one with tap water and the other with completely dissolved salt water. The two look the same, but are different. Tell students that a difference in properties may not be immediately obvious, so we use instruments to measure differences that we may not be able to see. Show students the difference in electrical conductivity between the two liquids by using a handheld ohmmeter. Set the ohmmeter to 200K range and insert both electrodes in the tap water about an inch apart from each other, noting the measurement. Then do the same with the salt water. Explain that the two different readings reveal that the liquids have different electrical conductivities. This demonstration shows students that differences in properties are not always obvious without using an instrument to measure the properties. Reading Toolkit Discussion Have students read about physical and chemical properties and changes. Then have them choose items in the classroom and classify the properties of each item, giving justification for each classification. Critical Thinking Answer: Since water boils at the same temperature regardless of how much water is in the sample, the boiling point of water is an intensive property. Matter and Change 7

3 Teach continued Present different properties to students and have them determine whether they are intensive or extensive properties. Density, conductivity, and melting and boiling points are examples of intensive properties. Energy absorbed, volume, and mass are extensive properties. Compare the two types of properties by heating 100 ml and 400 ml of water. Both will boil at the same temperature (intensive), but the 400-mL sample will absorb more energy (extensive) and will therefore take longer to reach the boiling point. Ask students to predict which beaker will return to room temperature first. FIGURE 2.3 Help students recognize the differences between the states of matter at the macroscopic and microscopic levels. They will see illustrations throughout this book showing micromodels for systems that explain the macrophenomena they observe. Point out that ice floats on water because it is less dense. The model for ice shows that its structure is less dense than that of the liquid. Demonstration FIGURE 2.3 Modeling States of Matter Models for water in three states. The molecules are close together in the solid and liquid states but far apart in the gas state. The molecules in the solid state are relatively fixed in position, but those in the liquid and gas states can flow around each other. Liquid A change in a substance that does not involve a change in the identity of the substance is called a physical change. Examples of physical changes include grinding, cutting, melting, and boiling a material. These types of changes do not change the identity of the substance present. States of Matter Melting and boiling are part of an important class of physical changes called changes of state. As the name suggests, a change of state is a physical change of a substance from one state to another. The three common states of matter are solid, liquid, and gas. Figure 2.3 shows the differences between the three states of matter at the molecular level. Matter in the solid state has definite volume and definite shape. For example, a piece of quartz or coal keeps its size and its shape, regardless of the container it is in. Solids have this characteristic because the particles in them are packed together in relatively fixed positions. The particles are held close together by the strong attractive forces between them, and only vibrate about fixed points. The amount of attraction varies with different solids. This accounts for some solids being more easily compressible. Solid Gas Particle Energy You can easily model the amount of energy a substance can have by putting marbles in a plastic cup or plastic beaker. The intensity with which you shake the beaker represents the energy of the particles and is demonstrated by how much the marbles move. With very little shaking, the marbles merely vibrate in their positions, like the particles of a solid. With moderate shaking, the marbles move relative to one another but stay inside the container (as in a liquid). With vigorous shaking, the marbles have enough energy to escape the container (as with a gas). 8 Chapter 1 Differentiated Instruction Pre-AP Plasma is often called the fourth state of matter. Have students choose one aspect of plasma to research on the Internet. Aspects to cover could include (but are not limited to): the scientific discovery of plasma, examples of plasma in space, examples of plasma on Earth (in the lab or natural), or implications of the designation of plasma as a different state of matter. Students should create a slide presentation or a simulation about their plasma topic and present it to the class. Untitled /29/2011 6:41:19 8 Chapter 1

4 Matter in the liquid state has a definite volume but an indefinite shape. A liquid assumes the shape of its container. For example, a given quantity of liquid water takes up a definite amount of space, but the water takes the shape of its container. Liquids have this characteristic because the particles in them are close together but can move past one another. The particles in a liquid move more rapidly than those in a solid. This causes them to overcome the strong attractive forces between them and flow. Matter in the gas state has neither definite volume nor definite shape. For example, a given quantity of helium expands to fill any size container and takes the shape of the container. All gases have this characteristic because they are composed of particles that move very rapidly and are at great distances from one another compared with the particles of liquids and solids. At these great distances, the attractive forces between gas particles have a lesser effect than they do at the small distances between particles of liquids and solids. An important fourth state of matter is plasma. Plasma is a high-temperature physical state of matter in which atoms lose most of their electrons, particles that make up atoms. Plasma is found in fluorescent bulbs. Melting, the change from solid to liquid, is an example of a change of state. Boiling is a change of state from liquid to gas. Freezing, the opposite of melting, is the change from a liquid to a solid. A change of state does not affect the identity of the substance. For example, when ice melts to liquid water or when liquid water boils to form water vapor, the same substance, water, is still present. The water has simply changed state, but it has not turned into a different compound. Only the distances and interactions between the particles that make up water have changed. Chemical Properties and Chemical Changes Physical properties can be observed without changing the identity of the substance, but properties of the second type chemical properties cannot. A chemical property relates to a substance s ability to undergo changes that transform it into different substances. Chemical properties are easiest to see when substances react to form new substances. For example, the ability of charcoal (carbon) to burn in air is a chemical property. When charcoal burns, it combines with oxygen in air to become carbon dioxide gas. After the chemical change, the amounts of the original substances, carbon and oxygen, are not less than before. They simply have recombined to form different substances with different properties. Figure 2.4 shows how a chemical property of a substance known as Benedict s solution is used to test for sugars in urine. A change in which one or more substances are converted into different substances is called a chemical change or chemical reaction. The substances that react in a chemical change are called the reactants. The substances that are formed by the chemical change are called the products. In the case of burning charcoal, carbon and oxygen are the reactants in a combustion, or burning, reaction. Carbon dioxide, a gas, and ashes are the products, which are clearly different from the reactants. FIGURE 2.4 Chemical Properties Because Benedict s solution possesses certain chemical properties, a test strip containing it is used to test for the presence of sugar in urine. The test strip is dipped into the sample. The test strip is then matched to a color scale to determine the sugar level in the urine. ChemisTrivia Along with the three common states of matter, there are several more. The ultra-dense matter in a neutron star is unlike a solid, a liquid, or a gas. Other forms of matter, such as the Bose-Einstein condensate, can be created at low temperatures when several atoms occupy the same quantum state. Classroom Discussion Have students brainstorm about chemical changes with which they are familiar. Food digestion and rusting are two common examples. Matter and Change 9 led-82 9 Pre-AP Tell students that the image on the first page of the chapter depicts crystals of tartaric acid. In addition to the uses listed in the caption, tartaric acid is sometimes added to foods as an antioxidant. Have students find out what the term oxidation means in relation to foods. As an antioxidant food additive, what function does tartaric acid serve? Tartaric acid acts as an antioxidant by preventing foods such as fruits and some vegetables from turning brown (oxidizing). Ask students to name at least one other chemical change that is a form of oxidation, 6/29/2011 6:41:20 AM from their research. Rusting iron is the most common example of oxidation. Matter and Change 9

5 Teach continued Make students aware that most physical and chemical changes involve some transfer of energy. The body uses energy from the chemical reactions that break down food. Weather involves a transfer of energy to evaporate or condense water in the atmosphere. Therefore, the study of matter also involves the study of energy. FIGURE 2.5 This figure gives students the opportunity to read descriptions of the physical and chemical properties of substances found in the chemical literature. Have students identify aspects of this figure that signify a chemical change. Ask students to verify that the substances produced have properties different from those of the reactants. Tell students that mercury is both volatile and poisonous. The decomposition of mercury oxide should never be attempted outside the classroom or without teacher supervision. FIGURE 2.5 Chemical Changes When mercury (II) oxide is heated, it decomposes to form oxygen gas and mercury (which can be seen on the side of the test tube). Decomposition is a chemical change that can be observed by comparing the properties of mercury (II) oxide, mercury, and oxygen. Mercury (II) oxide Physical properties: bright red or orange - red, odorless crystalline solid, almost insoluble in water Chemical properties: decomposes when exposed to light or at 500ºC to form mercury and oxygen gas Mercury Physical properties: silver - white, liquid metal; in the solid state, mercury is ductile and malleable and can be cut with a knife Chemical properties: forms alloys with most metals except iron; combines readily with sulfur at normal temperatures; reacts with nitric acid and hot sulfuric acid; oxidizes to form mercury (II) oxide upon heating Chemical reactions are normally written with arrows and plus signs. These stand for the words yields and plus, respectively. For example, to describe the decomposition of the mercury compound shown in Figure 2.5, we d write it as follows: mercury (II) oxide mercury + o x y g e n Oxygen Physical properties: colorless, odorless gas, soluble in water Chemical properties: supports combustion; reacts with many metals In other words, mercury (II) oxide yields mercury plus oxygen. Although chemical reactions form products whose properties can differ greatly from those of the reactants, they do not affect the total amount of matter present before and after a reaction. The law of conservation of mass is always followed in chemical reactions. Check for Understanding Answer: When a substance dissolves, that is a physical change. When a gas is produced, however, a chemical change is occurring. This action involves both a physical AND a chemical change. CHECK FOR UNDERSTANDING Explain An antacid tablet is dropped into a glass of water and dissolves. The tablet fizzes, and bubbles of gas rise to the surface. Is this a physical change or a chemical change? Explain your answer. 10 Chapter 1 Differentiated Instruction Energy and Changes in Matter When physical or chemical changes occur, energy is always involved. The energy can take several different forms, such as heat or light. Sometimes heat provides enough energy to cause a physical change, as in the melting of ice, and sometimes heat provides enough energy to cause a chemical change, as in the decomposition of water vapor to form oxygen gas and hydrogen gas. But the boundary between physical and chemical changes isn t always so clear. For example, although most chemists would consider the dissolving of sucrose in water to be a physical change, many chemists would consider the dissolving of table salt in water to be a chemical change. The boundaries can sometimes be confusing! Charles D. Winters English learners Have students create a chart that lists examples of chemical changes and physical changes. Students can draw pictures that represent each example. Ask the students to label each picture with each type of change and the identity of each substance before and after the change has taken place. Students may use the textbook, reference books, and the Internet to complete their charts. Inclusion Ask students to brainstorm examples of physical or chemical changes. Answers may include change in color, melting, freezing, change in shape, boiling, emission of heat, flames, change in odor. Write their answers on the board. Now ask students which changes indicate that energy is somehow involved. Boiling, emission of heat, flames. Circle these items on the board. Explain to students that while energy is most obviously involved in these examples, energy is actually involved in all chemical or physical changes in some way. Untitled /29/2011 6:41:22 10 Chapter 1

6 Accounting for all the energy present before and after a change is not a simple process. But scientists who have done such experimentation are confident that the total amount of energy remains the same. Although energy can be absorbed or released in a change, it is not destroyed or created. It simply assumes a different form. This is the law of conservation of energy. MAIN IDEA Matter can be a pure substance or a mixture. Matter exists in an enormous variety of forms. Any sample of matter, however, can be classified either as a pure substance or as a mixture. The composition of a pure substance is the same throughout and does not vary from sample to sample. A pure substance can be an element or a compound. Mixtures, in contrast, contain more than one substance. They can vary in composition and properties from sample to sample and sometimes from one part of a sample to another part of the same sample. All matter, whether it is a pure substance or a mixture, can be classified in terms of uniformity of composition and properties of a given sample. Figure 2.6 illustrates the overall classification of matter into elements, compounds, and mixtures. Mixtures You deal with mixtures every day. Nearly every object around you, including most things you eat and drink and even the air you breathe, is a mixture. Mixtures can be very simple or very complex, and they can have some unique properties. Demonstration Homogeneous vs. Heterogeneous Mixtures Have students bring in examples of matter that represent heterogeneous and homogeneous mixtures. Granite and milk are two common heterogeneous mixtures; different components of these mixtures have different physical properties. A solution such as a soft drink and a metallic alloy are both examples of a homogeneous mixture. The physical properties in a homogeneous mixture are the same regardless of how the mixture is divided. Students might enjoy a study of frosted cupcakes (a heterogeneous mixture) and soft drinks (a homogeneous mixture). FIGURE 2.6 Classification of Matter This classification scheme for matter shows the relationships among mixtures, compounds, and elements. Mixtures Yes Matter Can it be separated? No Pure substances FIGURE 2.6 Have students carefully examine this chart. Have them use the chart by asking them to classify examples of matter they have brought to class. Ask students where atoms and molecules would fit into this scheme. You may also want pairs of students to brainstorm about other examples for each type of matter listed on the chart. Is the composition uniform? Can it be decomposed by ordinary chemical means? Yes No Yes No Homogeneous mixtures (air, sugar in water, stainless steel) Heterogeneous mixtures (granite, wood, blood) Compounds (water, sodium chloride, sucrose) Elements (gold, aluminum, oxygen, chlorine) Matter and Change 11 titled Below Level Have students create flashcards for compounds, elements, heterogeneous mixtures, and homogeneous mixtures. Each card should include information and examples for that category. For example, a flashcard for Homogeneous Mixtures would have the answers shown in blue: Category: Homogeneous Mixture Type of matter: Mixture Properties of the category: Uniform composition Examples: air, sugar in water, salt water Encourage students to think of other 6/29/2011 examples 6:41:23 AM from their real lives, as well as examples from the text, and then add them to their flashcards. Matter and Change 11

7 Teach continued FIGURE 2.7 Either physical or chemical properties can be used to separate components in a mixture. Ask students whether the examples in this figure are being separated based on their physical properties or their chemical properties. All of the methods in this figure are using physical means to separate mixtures. FIGURE 2.7 Separating Mixtures CRITICAL THINKING Classify Would the mixtures shown in the photos be defined as homogeneous or heterogeneous? Explain. Reading Toolkit Vocabulary Building Point out the Greek prefixes in the words homogeneous and heterogeneous: homo- means same, and heteromeans different. Help students use this information to develop definitions such as the following: A homogeneous mixture is one that has the same proportion of components throughout; a heterogeneous mixture is one that is not uniform throughout. Invite students to list word pairs that have these prefixes, such as homonym and heteronym, and look up their meanings. This is a good time to review with students the information found in Safety in the Chemistry Laboratory at the front of the Student Edition. You may also wish to review the Merck Index, the CRC Handbook of Chemistry and Physics, Lange s Handbook of Chemistry, or other reference materials that you have available in the laboratory. It is important that students take responsibility for knowing the hazards of chemicals in the classroom. Critical Thinking Answer: The mixtures are heterogenous because they are not uniform throughout, as demonstrated by the separation of the mixtures into different components using the methods shown in the photos. (a) Filtration Barium chromate can be separated from the solution in the beaker using filtration. 12 Chapter 1 Differentiated Instruction English Learner Many terms that students will encounter in this chapter and throughout the course may be especially difficult for students who are learning English as they are studying chemistry. In particular, the names of some of the items of equipment used in chemistry labs will be unfamiliar. Be sure to have photographs, diagrams, or examples of the equipment used in the labs that the students are doing. Illustrations of the equipment mentioned in the text will also give students a reference point. (b) Using a Centrifuge A centrifuge can be used to separate certain solid components. The centrifuge spins rapidly, which causes the solids to settle to the bottom of the test tube. (c) Chromatography The components of an ink can be separated using paper chromatography. A mixture is a blend of two or more kinds of matter, each of which retains its own identity and properties. The parts, or components, of a mixture are simply mixed together physically and can usually be separated. As a result, the properties of a mixture are a combination of the properties of its components. Because mixtures can contain various amounts of different substances, a mixture s composition must be specified. This is often done in terms of percentage by mass or by volume. For example, a mixture might be 5% sodium chloride and 95% water by mass. Mixtures that are uniform in composition are said to be homogeneous. They have the same proportion of components throughout. Homogeneous mixtures are also called solutions. A salt - water solution is an example of such a mixture. Mixtures that are not uniform throughout are said to be heterogeneous. For example, in a mixture of clay and water, heavier clay particles concentrate near the bottom of the container. Some mixtures can be separated by filtration or vaporized to separate the different components. In Figure 2.7a, the yellow barium compound is trapped by the filter paper, but the solution passes through. If the solid in a liquid-solid mixture settles to the bottom of the container, the liquid can be carefully poured off (decanted). A centrifuge (Figure 2.7b) can be used to separate some solid - liquid mixtures, such as those in blood. Another technique, called paper chromatography, can be used to separate mixtures of dyes or pigments because the different substances move at different rates on the paper (Figure 2.7c). Below Level Give students small samples of table salt and sand. Have students examine the two samples with magnifiers. Ask them if salt crystals appear to be the same throughout the sample. Yes; they are little cubes. Explain that salt is a pure substance called a compound and can only be separated by chemical means. Ask students how sand appears when viewed through the magnifier. It looks like small particles of different colors, shapes, and sizes. Explain that sand is made up of many different substances. Therefore, sand is a mixture. Untitled /29/2011 6:41:25 12 Chapter 1

8 Pure Substances A pure substance has a fixed composition. Pure substances are always homogeneous. They differ from mixtures in the following ways: 1. Every sample of a given pure substance has exactly the same characteristic properties. All samples of a pure substance have the same characteristic physical and chemical properties. These properties are so specific that they can be used to identify the substance. In contrast, the properties of a mixture depend on the relative amounts of the mixture s components. 2. Every sample of a pure substance has exactly the same composition. All samples of a pure substance have the same makeup. For example, pure water is always 11.2% hydrogen and 88.8% oxygen by mass. Pure substances are either compounds or elements. A compound can be decomposed, or broken down, into two or more simpler compounds or elements by a chemical change. Water is made of hydrogen and oxygen chemically bonded to form a single substance. Water can be broken down into hydrogen and oxygen through electrolysis, as shown in Figure 2.8a. Sucrose is made of carbon, hydrogen, and oxygen. Sucrose breaks down under intense heating, to produce carbon and water (Figure 2.8b). This is of particular delight to anyone with a sweet tooth! FIGURE 2.8 Two very different reactions are shown, but they are both decomposition reactions. Ask students how they know that a chemical reaction has taken place in each photograph. Students should recognize that gases are produced during the electrolysis of water, while there is a color change in the oxidation of sugar. Point out to students that hydrogen gas differs chemically from oxygen gas even though they look the same. Use this illustration to reinforce the definitions of atoms, molecules, and compounds. Students should also note that two states of matter are represented in the electrolysis experiment. FIGURE 2.8 Decomposition of Compounds Hydrogen molecule, H 2 Oxygen molecule, O 2 Water molecule, H 2 O (bl) Tom Pantages Photography (a) Separation by Electrolysis Passing an electric current through water causes the compound to break down into the elements hydrogen and oxygen, which differ in composition from water. (b) Decomposition by Heating When sucrose is heated, it caramelizes. When it is heated to a high enough temperature, it breaks down completely into carbon and water. Matter and Change 13 led Pre-AP Think-Pair-Share: Fill some clear containers with samples of mixtures and other containers with samples of pure substances. Place each container at a different, numbered station around the room. Have students silently rotate through the stations, recording which samples they believe are mixtures and which are pure substances. When all students have rotated through all stations, have each student choose a partner with whom to share and compare their findings. Allow several minutes for them to share, and then ask partners to revisit those stations they decided were mixtures. This time, have them record which samples they believe are homogeneous and which 6/29/2011 6:41:26 AM are heterogeneous. When all partners have recorded their findings, write the numbers of the stations on the board. Have one student from each pair write their findings for each station beside the corresponding number. Go through the list one item at a time, and ask other students to raise their hands if they have different answers. Call on one or two students per item to explain the reasoning they used to arrive at their conclusion. Matter and Change 13

9 Teach continued FIGURE 2.9 Establishing standards for chemical purity is the mission of the American Chemical Society Committee on Analytical Reagents. This group compiles the data and publishes the tests to determine if a manufactured chemical meets specifications for purity. Show students labels from stockroom reagents of different grades. Allow students to look through your chemical supply catalog for the listing of reagent grades provided by that distributor. Have them compare prices for different grades of the same chemical. Note that the names of grades in this table can vary by chemical manufacturer. FIGURE 2.9 Increasing purity SOME GRADES OF CHEMICAL PURITY Primary standard reagents ACS (American Chemical Society specified reagents) USP (United States Pharmacopoeia standards) CP (chemically pure; purer than technical grade) NF (National Formulary specifications) FCC (Food Chemical Code specifications) Technical (industrial chemicals) Laboratory Chemicals and Purity The chemicals in laboratories are generally treated as if they are pure. However, all chemicals have some impurities. The purity ranking of the grades can vary with different agencies, as seen in Figure 2.9. For some chemicals, the USP grade may specify higher purity than the CP grade. For other chemicals, the opposite may be true. However, the primary standard reagent grade is always purer than the technical grade for the same chemical. Chemists need to be aware of the kinds of impurities in a reagent because these impurities could affect the results of a reaction. The chemical manufacturer must ensure that the standards set for that reagent by the American Chemical Society (A.C.S.) are met. Reading and understanding the labels placed on chemicals, like those shown in Figure 2.10, is a crucial skill for chemists. FIGURE 2.10 Chemical Purity The labeling on this bottle lists the grade of the reagent (a) and the percentages of impurities for that grade (b). What grade is this chemical? Assess and Reteach Assess Use the Formative Assessment on this page to evaluate student mastery of the section. Reteach For students who need additional instruction, download the Section Study Guide. Response to Intervention To reassess students mastery, use the Section Quiz, available to print or to take directly online at HMDScience.com. SECTION 2 FORMATIVE ASSESSMENT Reviewing Main Ideas 1. a. What is the main difference between physical properties and chemical properties? b. Give an example of each. 2. Classify each of the following as either a physical change or a chemical change. a. tearing a sheet of paper b. melting a piece of wax c. burning a log (a) (b) 3. How do you decide whether a sample of matter is a solid, a liquid, or a gas? 4. Contrast mixtures with pure substances. Critical Thinking 5. ANALYZING INFORMATION Compare the composition of sucrose purified from sugar cane with the composition of sucrose purified from sugar beets. Explain your answer. 14 Chapter 1 Answers to Section Assessment 1. a. Physical properties can be observed without changing the identity of the substance; chemical properties are observed by changing the substance into different substances. b. Physical: color, size, boiling point, etc.; chemical: burning of coal, rusting of iron, etc. 2. a. physical b. physical c. chemical 3. You can compare the sample with the characteristic properties of solids, liquids, and gases. If the sample has a fixed shape, it is a solid. If its volume changes to fill the volume of different containers, then it is a gas. If neither condition applies, the sample is a liquid. 4. Mixtures contain two or more substances blended together and generally can be separated. Pure substances have a fixed composition. 5. The compositions are identical. Sucrose is a compound, so it must have a definite composition no matter what its source is. Untitled /29/2011 6:41:27 14 Chapter 1