What Do You Think? Investigate GOALS

Transcription

1 Activity 1 Elements and Compounds GOALS In this activity you will: Decompose water by electrolysis into the two elements from which it is composed. Test the two elements to determine their identities. Learn one way to determine the chemical formula of a material. Compare characteristic properties of a material to those of its constituent elements. Represent materials with chemical formulas using numbers and the symbols of elements. Practice safe laboratory techniques with flames and explosions. What Do You Think? Matter is the name for all the stuff in the universe. Anything that has mass and occupies space is called matter. How many kinds of matter are there in the universe: 1, 10, 100, 1000, 10000, or ? What makes up matter? The What Do You Think? questions are meant to get you thinking about what you already know or think you know. Don t worry about being right or wrong. Discussing what you think you know is an important step in learning. Record your ideas in your log. Be prepared to discuss your responses with your small group and the class. Investigate 1. Look around your classroom at the kinds of matter that make up the things you see. a) Make a list of 10 kinds of materials that make up the objects you see. For example, you might list the wood in your pencil, or the glass in the windows. b) To understand the nature of matter, it helps to know if it is simple or complex. Is the matter made from only one kind of 101

2 Movie Special Effects Safety goggles and a lab apron must be worn at all times in a chemistry lab. material or is it a mixture of various materials? Classify each of the 10 materials you listed as pure or mixtures. c) For each material you thought was a mixture, write your best guess about what materials make it up. 2. Sometimes the materials that make up a substance are not obvious. Early scientists thought that water was an element. In other words, they thought that there was only one kind of material in water. They had not discovered a way of breaking it down further. Water, however, can be broken down further. a) Carefully observe the characteristic properties of water. Record at least three observations of water in your log. 3. Assemble the apparatus for decomposing water as shown in the diagram. Fill two test tubes with water. Submerge them in the water in the beaker and invert them. Make sure you do not allow any air to enter the test tubes. The ends of the wires should be stripped. Polish them with steel wool. Insert the ends of the wires into the test tubes. Add about 1 to 2 ml of sodium sulfate solution to the water in the beaker. 4. Plug in the set of three 9-V batteries in series or use a power supply. Let the reaction run until a test tube is half-full of gas. (Your teacher may decide to have you stop the reaction sooner.) a) Note what happens when the power is turned on. Record your observations in your Active Chemistry log. b) How do the relative amounts of gas formed in the test tubes compare? 5. Disconnect the batteries or power source. While inverted, place stoppers in the test tubes and remove them from the water. a) What gas do you think is contained in each test tube? (Hint: You ve probably heard that water is H-2-O, written H 2 O.) Record your prediction, and give reasons for your prediction. b) Observe the physical properties of each gas. Record these properties in your log. 6. Remove the stopper while the test tube is inverted. Drain the water and replace the stopper. You are going to use a lighted wooden splint to identify the gas in each test tube. First, examine the test tube with the smaller volume of gas. Light a wooden splint. Blow out the flame, but leave the splint glowing. Hold the test tube with its mouth up. Remove the stopper. Quickly bring the glowing splint to the mouth of the test tube. 102

3 Activity 1 Elements and Compounds a) Observe what happens to the splint and record your observations. b) What gas do you think was produced in this test tube? c) In your log record any additional properties of the gas that you discovered. 7. Next, examine the test tube that contains the larger volume of gas. Light a wooden splint. Keep the test tube inverted. Remove the stopper. Quickly bring the burning splint to the mouth of the test tube. a) Record your observations. b) What gas do you think was produced in this test tube? c) In your log record any additional properties of the gas that you discovered. 8. Return the splints and equipment as directed by your teacher. Clean up your workstation. 9. You may have deduced that the water (H 2 O) decomposed into hydrogen gas and oxygen gas. a) How would you write an equation to show this decomposition? 10. You may think of ways of using the tests (glowing splint and burning splint) as special effects in your show. Your teacher will demonstrate an additional way you can use what you investigated to produce a special effect. Your teacher will set up a gas generator, similar to the one shown in the diagram. The test tube contains 10 ml of 6 M HCl (hydrochloric acid is a compound of hydrogen and chlorine) and 3 g of zinc. a) What gas do you think is being produced in the test tube? Give a reason for your answer. 11. An egg has been emptied out. There is a small hole in the top and another in the bottom of the egg. The top hole is taped. The bottom hole in the egg will be placed over the gas generator, and gas will be collected in the egg for several minutes. 12. The egg will then be mounted in an egg holder behind a shield. If you have a video camera available, be prepared to start recording. Your teacher will remove the tape, and light the top of the egg with a burning splint. Begin recording or observing the reaction until it is over. a) Record your observations in your log. b) If the gas in the egg was hydrogen and air contains oxygen, what substance do you think may have been created in this process? c) Write an equation that shows how the two gases produced this new substance. Be certain that the mouth of the test tube is pointed away from everyone. Clean up spills immediately. Wash your hands and arms thoroughly after the activity. Both the teacher and the students must be shielded during this part of the activity. 103

4 Movie Special Effects Chem Words element: any material that cannot be broken down into simpler materials. THE STRUCTURE OF MATTER In this investigation you used electricity to decompose water into two gases. You learned that the gases were different because they reacted differently to the burning and glowing splints. Since water is referred to as H 2 O, a first guess would be that hydrogen (H) and oxygen (O) were produced in the experiment. The test for hydrogen is a small explosion when exposed to a burning splint. The test for oxygen is the re-ignition of a glowing splint. If you look back on the results of the experiment, you find that the volume of the hydrogen gas was twice as much as the oxygen gas. There was twice as much hydrogen as oxygen; that s where the 2 comes from in the chemical formula H 2 O. Hydrogen and oxygen are elements. An element is any material that cannot be broken down into simpler materials by chemical means. You are probably familiar with many elements like hydrogen, oxygen, zinc, gold, or helium. Other elements like strontium and beryllium are more exotic and less likely to be familiar to you. Every kind of matter you observe in your everyday life is made up of the chemical elements. There are only a little more than a hundred different kinds of chemical elements. This is an amazing discovery of chemistry everything you observe in the world is made of different combinations of a hundred elements. Chemistry is the study of the properties of these elements, how these elements combine, the new properties of these combinations, and the energy changes that result from these combinations. Elements are represented by symbols. The periodic table shows that the symbol used for each element is one or two letters to represent the name. It s easier to write O than to write oxygen. It s easier to write H than to write hydrogen. The symbols come from many different sources. However, the same symbols are used for each element in all countries of the world. 104

5 Activity 1 Elements and Compounds When elements combine, they form new substances called compounds. These compounds have entirely new characteristics. It is like combining the letters of the alphabet to make words. Twenty-six letters can be combined to make thousands of different words. Water is an example of a compound. A water molecule, H 2 O, is composed of two atoms of hydrogen and one atom of oxygen. (For now, think of an atom as the smallest particle of an element and a molecule as the smallest unit of a compound.) In this activity you used electricity to decompose water into its elements, hydrogen and oxygen. This process is called electrolysis. You observed that oxygen gas made a glowing splint burst into flame, and that hydrogen gas was explosive. However, to extinguish a burning splint, you could use liquid water. The compound has very different characteristics from the elements from which it is made. Symbols for Some Elements Name of element Symbol aluminum Al bromine Br calcium Ca carbon C chlorine Cl copper Cu gold Au helium He hydrogen H iodine I iron Fe lead Pb magnesium Mg mercury Hg neon Ne nickel Ni nitrogen N oxygen O phosphorus P potassium K silicon Si sodium Na sulfur S tin Sn zinc Zn Chem Words compound: a substance that consists of two or more elements bonded together in definite proportion. electrolysis: the conduction of electricity through a solution (that contains ions) or a molten (ionic) substance that results in a chemical change. chemical formula: the combination of the symbols of the elements in a definite numerical proportion used to represent molecules, compounds, radicals, ions, etc. Compounds are represented by chemical formulas. A chemical formula shows the symbols of the elements that are combined to make the compound. If there is more than one atom of an element, a subscript is added after the symbol indicating how many atoms of that element there are. For example, as you discovered in this activity, the chemical 105

6 Movie Special Effects Examples of Some Chemical Formulas Compound Common name Chemical formula calcium carbonate chalk CaCO 3 carbon dioxide dry ice CO 2 hydrochloric acid muriatic acid HCl hydrogen sulfide rotten-egg gas H 2 S sodium hydrogen carbonate (or sodium bicarbonate) baking soda NaHCO 3 sodium chloride table salt NaCl sodium nitrate fertilizer NaNO 3 sulfuric acid battery acid H 2 SO 4 formula for water is H 2 O. Two hydrogen atoms combine with one oxygen atom to produce one molecule of water. You could call H 2 O dihydrogen monoxide but you use the common name water. In chemistry, there are common names for some familiar chemical compounds. From the table of chemical formulas, you can see that carbon dioxide (CO 2 ) is a compound of carbon and oxygen. There are two atoms of oxygen for every atom of carbon. Sodium hydrogen carbonate or sodium bicarbonate (NaHCO 3 ) is a compound of sodium, hydrogen, carbon, and oxygen. There are three atoms of oxygen for every atom of the other elements. Also, there are a total of three atoms in the carbon dioxide formula and a total of six atoms in sodium hydrogen carbonate. Many elements appear as single atoms in nature. There are seven common elements that exist in diatomic form. They are: Hydrogen (H 2 ), Oxygen (O 2 ), Nitrogen (N 2 ), Fluorine (F 2 ), Chlorine (Cl 2 ), Bromine (Br 2 ), and Iodine (I 2 ). For example, hydrogen may be found as a single atom in a compound like HCl and oxygen may be found as a single atom in H 2 O. When hydrogen is not combined with any other element, it always appears as H 2, two hydrogen atoms combined together. You can write an equation that summarizes the electrolysis of water: H 2 O(I) + energy H 2 (g) + O 2 (g) Chem Words conservation of mass: states that matter can neither be destroyed or created in a chemical reaction. Notice that the letters g and l are used. The g means gaseous state and the l means liquid state. Also notice that the elements are written in their diatomic form. Conservation of mass and elements states that the number of hydrogen atoms must be equal before and after the reaction. In addition, the number of oxygen atoms must be 106

7 Activity 1 Elements and Compounds equal before and after the reaction. In the previous equation, you have one oxygen atom before the reaction and two oxygen atoms after the equation. You can fix this problem by balancing the equation. 2H 2 O(I) + energy 2H 2 (g) + O 2 (g) You now have 2 water molecules before the reaction (4 hydrogens and 2 oxygens). You also have 4 hydrogens and 2 oxygens after the reaction. You balanced the equation because you know that mass and elements must be conserved. By balancing the equation, you found that in electrolysis 2 water molecules decompose into 2 hydrogen molecules and 1 oxygen molecule. This is why you saw that there was twice as much hydrogen in one test tube than oxygen in the other test tube. To generate the gas to fill the empty eggshell in this activity (the teacher demonstration), zinc was placed in hydrochloric acid. Zinc is an element. Hydrochloric acid (HCl) is a compound of hydrogen and chlorine. The reaction of the zinc and hydrochloric acid created a gas. Given the explosion you observed, you can guess that the gas produced was hydrogen. The hydrogen gas came from the hydrogen in the hydrochloric acid. You can write this reaction as an equation: HCl(aq) + Zn(s) ZnCl 2 (aq) + H 2 (g) The sub letter s means that the substance is in the solid state. The sub letters aq stands for aqueous, meaning that the substance (HCl) is dissolved in water. The zinc combined with the chlorine and hydrogen was released. Once again, conservation of mass and elements requires you to balance the equation: 2HCl(aq) + Zn(s) ZnCl 2 (aq) + H 2 (g) There s much more to the structure of matter than you can discover in just one activity. However, this activity may have raised some new questions in your mind. For example: Can all compounds be decomposed into their elements? What techniques can be used to decompose compounds? What are elements made of? What are atoms? What are molecules? These questions and many more are answered in other activities. Checking Up 1. In your own words, explain the difference between an element and a compound. 2. Why are symbols useful in describing chemical elements? 3. What are the symbols for the following elements: carbon, copper, gold, and helium? 4. What information does a chemical formula of a compound provide? 107

8 Movie Special Effects What Do You Think Now? At the beginning of the activity you were asked: How many kinds of matter are there in the universe: 1, 10, 100, 1000, 10000, or ? What makes up matter? Matter is all of the stuff in the universe and it can either be a compound or an element. How many kinds of elements are there? How many kinds of compounds are there? Give an example of each. What does it mean? Chemistry explains a macroscopic phenomenon (what you observe) with a description of what happens at the nanoscopic level (atoms and molecules) using symbolic structures as a way to communicate. Complete the chart below in your log. MACRO NANO SYMBOLIC Describe what you observed in the decomposition of water and while generating hydrogen. In words, describe what is happening to the compounds during the two separate experiments (electrolysis of water and zinc reacting with hydrochloric acid) in this activity. How do you know? Water is often referred to as H 2 O. What evidence do you have that water is composed of hydrogen and oxygen? Why do you believe? Individual elements will behave one way when they are alone and another way when they are combined with other elements. Create an analogy that is true from your life to show how a compound can behave differently from its parts. Why should you care? Draw a picture that represents the decomposition of water (H 2 O) into hydrogen (H 2 ) and oxygen (O 2 ). Beneath the picture, write the chemical formulas that are used to symbolically represent the elements. You will be writing a movie scene for your challenge in this unit. Write a short scene using an egg explosion and describe how the hydrogen was generated. 108

9 Activity 1 Elements and Compounds Reflecting on the Activity and the Challenge Part of the problem you are facing in creating a special effect is understanding what matter is made of and how it can change. In this activity you broke a chemical compound down into its component elements using electrolysis. In another part of the activity a compound was made from chemical elements through a fast and noisy reaction. There are only about one hundred elements, but there are far more than a million compounds. You should begin thinking of ways in which some of the reactions you observe could be made to appear more dramatic on screen, without making them any larger in real life. You can now use the concepts of elements and compounds to provide the chemistry description of what is occurring. 1. The table shown contains several common compounds that are probably familiar to you. For each compound: a) List the names of the elements present. b) State the number of atoms of each element present. c) Give the total number of atoms present in each compound. 2. Write a chemical formula for nitrous oxide (laughing gas) that is made up of two atoms of nitrogen and one atom of oxygen. 3. Choose one compound from the table in Question 1. a) Investigate and record the properties of each element in the compound. b) Explain how the property of the compound is different from the property of each element. 4. Preparing for the Chapter Challenge In a short paragraph, summarize the difference between an element and a compound and describe how the properties of a compound can be very different from the properties of the elements that make it up. Explain why knowing these differences is important when designing special effects for a movie set. Common name Formula table sugar C 12 H 22 O 11 marble CaCO 3 natural gas CH 4 rubbing alcohol C 3 H 8 O glass SiO 2 Inquiring Further How is electrolysis used in industry? Use the reference materials available to you to explore how electrolysis is used in industry to produce hydrogen gas and other elements from compounds. 109