6.1. How Compounds Form. Chemicals Everywhere. Hydrogen peroxide. Compounds

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6.1 How Compounds Form Here is a summary of what you will learn in this section: Compounds are composed of two or more elements that combine in a specific ratio.

1 6.1 How Compounds Form Here is a summary of what you will learn in this section: Compounds are composed of two or more elements that combine in a specific ratio. Ionic compounds form when metallic and non-metallic elements combine chemically. Molecular compounds form when non-metallic elements combine chemically. Figure 6.1 Hydrogen peroxide is a chemical compound that may be used to create highlights in hair. Hydrogen peroxide Strong oxidizer. Keep out of contact with finely divided reducing agents and metals. Can only be stored for prolonged periods of time if stabilized with a little phosphoric acid and stored in amber bottles. OXIDIZER 4 Health CORROSIVE 0 Flammability Special Stability: 5 Reactivity 3 Storage: Figure 6.2 People using a chemical such as hydrogen peroxide in their workplace must, by law, be trained in the meaning of all the safety symbols on the label. 7 Chemicals Everywhere We live in a chemical world. Every kind of substance that you can think of is made of a type of chemical or mixture of chemicals. Water is a chemical, and the air you breathe is a mixture of chemicals. The ink in your pen, hair dyes and bleaches, the lead-lined cover that protects you during a dental X-ray, and life-saving medicines are all made of chemicals, too (Figure 6.1). A quick look around your home will reveal an amazing variety of chemicals in your cupboards and on your shelves. In the bathroom, you will find water, soap, shampoo, deodorant, and toothpaste all chemicals. In the storage area, you might find cleaning products, such as ammonia and bleach, and perhaps painting and gardening products. In your kitchen, you might find table salt, baking soda, and baking powder. Each of these products contains one or more chemical compounds. Compounds A compound is a pure substance made up of two or more elements that are chemically combined. For example, water is a compound consisting of hydrogen and oxygen. Hydrogen peroxide is also a 210 UNIT B Atoms, Elements, and Compounds

2 compound of hydrogen and oxygen but with completely different properties than water (Figure 6.2 on the previous page). Some of the differences between water and hydrogen peroxide are listed in Table 6.1. Hydrogen peroxide is a blue liquid that can dissolve in water to form a solution, which is commonly available in pharmacies. In certain concentrations, it can be used on skin to kill bacteria or in hair as a bleaching agent. All compounds have properties that make them potentially useful as well as hazardous. If a highly concentrated solution of hydrogen peroxide gets on the skin, it will cause a chemical burn. Even water, if consumed in huge volumes, can make a person sick. Understanding the properties of compounds gives us the knowledge to make use of compounds safely and responsibly. Table 6.1 Properties of Water and Hydrogen Peroxide Water (H 2 O) colourless liquid boils at 100 C stable in strong sunlight dissolves the chemical potassium iodide does not bleach pigments Hydrogen peroxide (H 2 O 2 ) blue liquid boils at C breaks down in light reacts with the chemical potassium iodide strong bleaching agent B20 Quick Lab Water and Hydrogen Peroxide (Teacher Demonstration) Purpose To observe properties of water and hydrogen peroxide Materials & Equipment blue litmus paper two 400-mL beakers water hydrogen peroxide solution cobalt chloride paper scoopula stirring rod CAUTION: Hydrogen peroxide is corrosive to skin. Potassium iodide will stain skin and clothing. Procedure potassium iodide crystals 250-mL graduated cylinder basin dish soap matches wooden splint 2. Your teacher will dip one piece of cobalt chloride paper into water and another into hydrogen peroxide solution. Observe the cobalt chloride paper. 3. Observe as your teacher pours water into a beaker and then stirs in some potassium iodide crystals. 4. Your teacher will place the graduated cylinder in the basin and then pour 20 ml of hydrogen peroxide solution and 3 drops of dish soap into the graduated cylinder. Observe as your teacher adds a small scoop of potassium iodide crystals. 5. Step 4 will produce bubbles of gas. Observe as your teacher places a glowing splint into the gas. Question 6. Describe three differences in the properties of water and hydrogen peroxide that you observed. 1. Your teacher will dip one piece of blue litmus paper into water and another into hydrogen peroxide solution. Observe the litmus paper. Elements combine to form ionic compounds and molecular compounds. 211

During Writing Staying on Topic Writers support their topic sentence by adding sentences with details that relate to the topic.

Notice how the paragraph about salt seems unified because all the sentences support our understanding of the topic.

Although millions of compounds have been discovered, almost all of them can be classified as one of two types: ionic or molecular.

3 During Writing Staying on Topic Writers support their topic sentence by adding sentences with details that relate to the topic. Each new sentence seems to flow logically from the words and ideas in the previous one. Notice how the paragraph about salt seems unified because all the sentences support our understanding of the topic. Suggested Activity B21 Quick Lab on page 215 Two Types of Compounds A small change in the way the atoms combine can make a big difference in the chemical and physical properties of compounds. Although millions of compounds have been discovered, almost all of them can be classified as one of two types: ionic or molecular. Ionic Compounds Common table salt is familiar to most people as a white substance composed of tiny crystals. As discussed in Chapter 5, sodium chloride forms when a very reactive metal sodium is placed in a container with a poisonous, yellow-green non-metal chlorine gas (Figure 6.3). When these two chemical elements are combined, the sodium metal explodes in a bright orange flame. As the sodium burns, a white, coarse-grained powder is produced. This new substance has properties that are very different from the properties of sodium and chlorine. The substance is table salt, or sodium chloride (NaCl). Sodium chloride is called an ionic compound. Ionic compounds are pure substances usually consisting of at least one metal and one non-metal. Most ionic compounds share the following properties: have high melting points form crystals, which are very regular arrangements of particles dissolve in water to form solutions that conduct electricity (a) (b) (c) Figure 6.3 Sodium (a), is a metal. Sodium combines with chlorine gas in a violent reaction (b). The compound that forms is sodium chloride, NaCl (c). 212 UNIT B Atoms, Elements, and Compounds

4 ist9_ch06.qxd 7/21/09 2:55 PM Page 213 Home Quit All ionic compounds are solids at room temperature. In fact, table salt will not melt until it is heated to 800 C. When sodium chloride is dissolved in water or melted, it will conduct electricity. Investigations of this property led to the study of electrochemical cells, which can convert chemical energy into electricity. W O R D S M AT T E R The word ion comes from a Greek word meaning to go or to wander. Forming Ionic Compounds While combining, each atom changes into an ion. Ions form when one or more electrons move from a metal atom over to a nonmetal atom. This process is shown in Figure Na 1- Na+ Cl Cl- Figure 6.4 A sodium atom becomes a positive ion when it loses an electron to chlorine. By gaining an electron, chlorine becomes chloride, a negative ion. Together, they form an ionic compound. As the figure shows, a sodium atom loses an electron by giving it to a chlorine atom. This produces a positive sodium ion and a negative chloride ion. Positive and negative ions attract each other, so in an ionic compound, all the positive ions are attracted to all the negative ions. A connection between atoms or ions is known as a bond. The attractions between ions are called ionic bonds. The ionic bonds cause the ions to group together in an alternating pattern called a crystal arrangement (Figure 6.5). Na+ CI- Figure 6.5 The crystals in this table salt are held together by ionic bonds. Learning Checkpoint 1. What is the main difference between a compound and an element? 2. What two kinds of elements join together to form an ionic compound? 3. What is an ionic bond? 4. How must atoms change so that they can join to form an ionic compound? 5. What physical property or properties could you use to identify an ionic compound? Elements combine to form ionic compounds and molecular compounds. 213

5 Suggested Activity B23 Quick Lab on page 216 Take It Further Hydrogen peroxide is considered to be a more environmentally friendly alternative to chlorine bleaches. It is currently used in many paper-processing facilities to produce white paper, instead of using the more harmful chlorine. The chemical activity of the hydrogen peroxide changes the colour of the fibre in paper to white. With a partner, create a pamphlet to advertise one of the many other uses of hydrogen peroxide. Begin your research at ScienceSource. Molecular Compounds When non-metals combine, a pure substance called a molecular compound is formed. In molecular compounds, the atoms share electrons to form small groups, called molecules. Most molecular compounds share the following properties: can be solids, liquids, or gases at room temperature usually good insulators but poor conductors of electricity have relatively low boiling points Examples of molecular compounds include table sugar, hydrogen peroxide, and water. How can the same two elements (hydrogen and oxygen) combine to form compounds as different as water and hydrogen peroxide? Water is made from joining two atoms of hydrogen to one atom of oxygen. This forms a water molecule, and this is the smallest possible amount of water that can exist. We often represent this as H 2 O, where the subscript 2 indicates that two atoms of hydrogen are included. Hydrogen peroxide is formed when four atoms two each of hydrogen and oxygen join to form a single molecule. This molecule is represented as H 2 O 2. There are a number of ways to represent the atoms in a molecule. In every representation of water in Figure 6.6, two hydrogen atoms are connected to an oxygen atom in the middle. The Bohr diagram of water shows that each hydrogen atom shares a pair of electrons with the oxygen atom (a). In the balland-stick model, a stick represents each bond between atoms (b). In the third diagram, the areas in which the atoms overlap represents the connections that holds them together (c). (a) (b) (c) Figure 6.6 Representations of a water molecule: (a) a Bohr diagram; (b) a ball-and-stick model, in which the sticks represent connections between atoms; (c) a diagram with overlap between atoms to show how they are connected 214 UNIT B Atoms, Elements, and Compounds

B21 Quick Lab Salt and Sugar Adding sugar to your tea, or salt to your soup, not only changes the way your food tastes, it will dissolve to form a solution.

6 B21 Quick Lab Salt and Sugar Adding sugar to your tea, or salt to your soup, not only changes the way your food tastes, it will dissolve to form a solution. That solution may or may not be able to conduct an electric current. Purpose To compare the conductivity of different compounds: sodium chloride (table salt) and sucrose (table sugar) Materials & Equipment three 100-mL beakers marker tap water 2 small spoons sucrose (table sugar) sodium chloride (table salt) battery-operated conductivity tester 4. Use a clean spoon to add one small spoonful of sodium chloride to the third beaker. Use the spoon to stir until the sodium chloride is dissolved. 5. When your solutions are prepared, use a batteryoperated conductivity tester to determine whether any of the three samples conduct electricity. Questions 6. Sodium chloride and sucrose are both shiny white crystals, and both dissolve in water. What evidence shows which of these crystals is an ionic compound? 7. Suggest why it is often unsafe to have high voltage electricity near water. Refer to the results of your experiment in your answer. Procedure 1. Use the marker to label three 100-mL beakers as water, sucrose, and sodium chloride. 2. Fill each beaker with 50 ml of tap water. Set the beaker labelled water aside. 3. Add one small spoonful of sucrose to the second beaker. Use the spoon to stir until the sucrose is dissolved. Figure 6.7 A conductivity tester B22 Skill Builder Activity Molecular Model Kits In this activity, you will examine the components of a molecular model kit to prepare you to build simple models. In the kit, balls represent atoms and sticks represent connections between the atoms. Carbon is represented by a black ball, oxygen by a red ball, chlorine by a green ball, and hydrogen by a white ball. The number of holes in each ball represents the number of connections the atom can make with other atoms. 1. Examine the model atoms of carbon, oxygen, chlorine, and hydrogen. Build as many connections as you can between the different model atoms. 2. Describe what you observe about the different models you built. Elements combine to form ionic compounds and molecular compounds. 215

DI Key Activity B23 Quick Lab Building Molecular Models Chemists use models to gain information about the shape of a molecule. The shape of a molecule is a good predictor of its properties.

7 DI Key Activity B23 Quick Lab Building Molecular Models Chemists use models to gain information about the shape of a molecule. The shape of a molecule is a good predictor of its properties. In this activity, you will build ball-and-stick models of simple molecules using a molecular model kit. Purpose To represent the molecules of some common substances Materials & Equipment molecular model kit Figure 6.8 A ball-and-stick model of glucose, a type of sugar. Glucose can be straight, as shown here, or can twist into a ring structure. Procedure 1. Work in a small group to use the molecular model kit. Your teacher will provide specific information about how to use your particular kit. 2. There are two guidelines that you must follow when building molecular models: Each molecule is complete when all the balls are connected in such a way that all the holes are filled and every connector ends in a hole. It is possible in some cases for more than one connection to exist between the same two atoms. 3. For each of the following, build the model and then make a sketch of it. (a) H 2 O (water) (b) H 2 (hydrogen gas) (c) O 2 (oxygen gas) (d) CH 4 (methane, also called natural gas) (e) CH 2 Cl 2 (a solvent used as a degreaser) (f) C 2 H 4 (starter material for making polyethylene plastic) (g) C 3 H 8 (propane, a camp fuel) (h) HCl (hydrogen chloride, present in stomach acids) (i) H 2 O 2 (hydrogen peroxide) (j) CO 2 (carbon dioxide) 4. The following molecules can each be assembled in two different ways. Build and sketch each: (a) C 2 H 6 0 (b) C 3 H 7 Cl Questions 5. Identify the two molecules that represented elements rather than compounds. 6. How do the positions of the holes in each kind of model atom produce a molecule that has a three-dimensional shape? 216 UNIT B Atoms, Elements, and Compounds

8 6.1 CHECK and REFLECT Key Concept Review 1. What is the difference between a compound and a mixture? 2. What are the two major types of compounds? How do the bonds differ in each type? 3. Give an example of a molecular compound. 9. Use the diagram below to answer the following questions Give an example of a substance held together by ionic bonds. 5. What types of elements join to form molecular compounds? Name three such elements. 6. (a) What is the total number of atoms in a water molecule? (b) How many elements are in a water molecule? Connect Your Understanding 7. How is it possible for two different compounds, such as water and hydrogen peroxide, to both be made of the same two elements? 8. Examine the following table of properties for two unknown compounds. Which is most likely an ionic compound and which is most likely a molecular compound? Explain your reasoning. Properties of Unknown Compounds Property Compound X Compound Y Boiling point ( C) Melting point ( C) (a) What type of diagram is shown? (b) What type of compound is shown? (c) Which elements does this compound contain? 10. Describe how a salt crystal holds together. 11. Is it possible to wash dishes without using chemicals? Explain. 12. Can compounds have different properties than their elements have? Explain, using an example. Reflection Question Describe three types of models used in this section to represent compounds. Which model did you find most useful and why? 14. Name two compounds that you were familiar with before reading this section. Now that you have completed this section, what have you learned about these two compounds? For more questions, go to ScienceSource. Conductivity in solution poor good Elements combine to form ionic compounds and molecular compounds. 217

Na Na + +e - Cl+e - Cl -

Na Na + +e - Cl+e - Cl - LAB-Ionic vs. Covalent Bonding Have you ever accidentally used salt instead of sugar? Drinking tea that has been sweetened with salt or eating vegetables that have been salted with sugar tastes awful!