Chapter 6 - Classification of Matter PDF

Classification of Matter BIG IDEA: Matter exists in different forms Forming new substances give new advancement in our way of living ESSENTIAL QUESTION: How can we compare the properties of solids, liquids, and gases in terms of how particles move and are arrange? How do making compounds make our li...

Classification of Matter BIG IDEA: Matter exists in different forms Forming new substances give new advancement in our way of living ESSENTIAL QUESTION: How can we compare the properties of solids, liquids, and gases in terms of how particles move and are arrange? How do making compounds make our lives easier? INTRODUCTION: Chemistry research is often full of surprises. One such surprise came to Stephanie Kwolek of the DuPont chemical company. She was working on a type of material known as polymers. These chemicals had been around for a while and were being used for new types of textiles. Kwolek was looking for a strong and rigid petroleum product. She came up with a material that did not look like your average polymer. But she played a hunch and had it made into threads. This new material had stiffness about nine times that of any of the known polymers of the time. Further research and development led to the production of Kevlar, a material now widely used in body armor. In addition, Kevlar has found wide application in racing sails, car tires, brakes, and fire-resistant clothing worn by firefighters. 6.1 Phases of Matter Matter is defined as anything that occupies space and has mass, and it is all around us. Solids and liquids are more obviously matter: We can see that they take up space, and their weight tells us that they have mass. Gases are also matter; if gases did not take up space, a balloon would stay collapsed rather than inflate when filled with gas. Solids, liquids, and gases are the three states of matter commonly found on earth. A solid is rigid and possesses a definite shape. A liquid flows and takes the shape of a container, except that it forms a flat or slightly curved upper surface when acted upon by gravity. (In zero gravity, liquids assume a spherical shape.) Both liquid and solid samples have volumes that are very nearly independent of pressure. A gas takes both the shape and volume of its container. A fourth state of matter, plasma, occurs naturally in the interiors of stars. A plasma is a gaseous state of matter that contains appreciable numbers of electrically charged particle. The presence of these charged particles imparts unique properties to plasmas that justify their classification as a state of matter distinct from gases. In addition to stars, plasmas are found in some other high-temperature environments (both natural and manmade), such as lightning strikes, certain television screens, and specialized analytical instruments used to detect trace amounts of metals. A plasma torch can be used to cut metal. (credit: “Hypertherm”/Wikimedia Commons) Matter can also have properties of more than one state when it is a mixture, such as with clouds. Clouds appear to behave somewhat like gases, but they are actually mixtures of air (gas) and tiny particles of water (liquid or solid). The mass of an object is a measure of the amount of matter in it. One way to measure an object’s mass is to measure the force it takes to accelerate the object. It takes much more force to accelerate a car than a bicycle because the car has much more mass. A more common way to determine the mass of an object is to use a balance to compare its mass with a standard mass. Although weight is related to mass, it is not the same thing. Weight refers to the force that gravity exerts on an object. This force is directly proportional to the mass of the object. The weight of an object changes as the force of gravity changes, but its mass does not. An astronaut’s mass does not change just because she goes to the moon. But her weight on the moon is only one-sixth her earth-bound weight because the moon’s gravity is only one-sixth that of the earth’s. She may feel “weightless” during her trip when she experiences negligible external forces (gravitational or any other), although she is, of course, never “massless.” The law of conservation of matter summarizes many scientific observations about matter: It states that there is no detectable change in the total quantity of matter present when matter converts from one type to another (a chemical change) or changes among solid, liquid, or gaseous states (a physical change). Brewing beer and the operation of batteries provide examples of the conservation of matter. During the brewing of beer, the ingredients (water, yeast, grains, malt, hops, and sugar) are converted into beer (water, alcohol, carbonation, and flavoring substances) with no actual loss of substance. This is most clearly seen during the bottling process, when glucose turns into ethanol and carbon dioxide, and the total mass of the substances does not change. This can also be seen in a lead-acid car battery: The original substances (lead, lead oxide, and sulfuric acid), which are capable of producing electricity, are changed into other substances (lead sulfate and water) that do not produce electricity, with no change in the actual amount of matter. Although this conservation law holds true for all conversions of matter, convincing examples are few and far between because, outside of the controlled conditions in a laboratory, we seldom collect all of the material that is produced during a particular conversion. For example, when you eat, digest, and assimilate food, all of the matter in the original food is preserved. But because some of the matter is incorporated into your body, and much is excreted as various types of waste, it is challenging to verify by measurement. SOLID Solids are defined characteristics: by the following  definite shape (rigid)  definite volume  particles vibrate around fixed axes LIQUID Liquids have the following characteristics:  no definite shape (takes the shape of its container)  has definite volume  particles are free to move over each other, but are still attracted to each other GAS Gases have the following characteristics:  no definite shape (takes the shape of its container)  no definite volume  particles move in random motion with little or no attraction to each other  highly compressible 6.2 Atoms and Molecules An atom is the smallest particle of an element that has the properties of that element and can enter into a chemical combination. Consider the element gold, for example. Imagine cutting a gold nugget in half, then cutting one of the halves in half, and repeating this process until a piece of gold remained that was so small that it could not be cut in half (regardless of how tiny your knife may be). This minimally sized piece of gold is an atom (from the Greek atomos, meaning “indivisible”). This atom would no longer be gold if it were divided any further. (a) This photograph shows a gold nugget. (b) A scanning-tunneling microscope (STM) can generate views of the surfaces of solids, such as this image of a gold crystal. Each sphere represents one gold atom. (credit a: modification of work by United States Geological Survey; credit b: modification of work by “Erwinrossen”/Wikimedia Commons) The first suggestion that matter is composed of atoms is attributed to the Greek philosophers Leucippus and Democritus, who developed their ideas in the 5th century BCE. However, it was not until the early nineteenth century that John Dalton (1766–1844), a British schoolteacher with a keen interest in science, supported this hypothesis with quantitative measurements. Since that time, repeated experiments have confirmed many aspects of this hypothesis, and it has become one of the central theories of chemistry. Other aspects of Dalton’s atomic theory are still used but with minor revisions (details of Dalton’s theory are provided in the chapter on atoms and molecules). One of the smallest things we can see with our unaided eye is a single thread of a spider web: These strands are about 1/10,000 of a cm (0.0001 cm) in diameter. Although the cross-section of one strand is almost impossible to see without a microscope, it is huge on an atomic scale. A single carbon atom in the web has a diameter of about 0.000000015 cm, and it would take about 7000 carbon atoms to span the diameter of the strand. To put this in perspective, if a carbon atom were the size of a dime, the cross-section of one strand would be larger than a football field, which would require about 150 million carbon atom “dimes” to cover it. It shows increasingly close microscopic and atomic-level views of ordinary cotton. These images provide an increasingly closer view: (a) a cotton bowl, (b) a single cotton fiber viewed under an optical microscope (magnified 40 times), (c) an image of a cotton fiber obtained with an electron microscope (much higher magnification than with the optical microscope); and (d and e) atomic-level models of the fiber (spheres of different colors represent atoms of different elements). (Credit c: modification of work by “Featheredtar”/Wikimedia Commons) 6.3 Classification of Matter We can classify matter into several categories. Two broad categories are mixtures and pure substances. A pure substance has a constant composition. All specimens of a pure substance have exactly the same makeup and properties. Any sample of sucrose also has the same physical properties, such as melting point, color, and sweetness, regardless of the source from which it is isolated. Pure substances can be divided into two classes: elements and compounds. Pure substances that cannot be broken down into simpler substances by chemical changes are called elements. Iron, silver, gold, aluminum, sulfur, oxygen, and copper are familiar examples of the more than 100 known elements, of which about 90 occur naturally on the earth, and two dozen or so have been created in laboratories. Pure substances that can be broken down by chemical changes are called compounds. This breakdown may produce either elements or other compounds, or both. Mercury (II) oxide, an orange, crystalline solid, can be broken down by heat into the elements mercury and oxygen. When heated in the absence of air, the compound sucrose is broken down into the element carbon and the compound water. (a)The compound mercury (II) oxide, (b)when heated, (c) decomposes into silvery droplets of liquid mercury and invisible oxygen gas. (Credit: modification of work by Paul Flowers) A mixture is composed of two or more types of matter that can be present in varying amounts and can be separated by physical changes, such as evaporation (you will learn more about this later). A mixture with a composition that varies from point to point is called a heterogeneous mixture. Italian dressing is an example of a heterogeneous mixture. Its composition can vary because we can make it from varying amounts of oil, vinegar, and herbs. It is not the same from point to point throughout the mixture—one drop may be mostly vinegar, whereas a different drop may be mostly oil or herbs because the oil and vinegar separate and the herbs settle. Other examples of heterogeneous mixtures are chocolate chip cookies (we can see the separate bits of chocolate, nuts, and cookie dough) and granite (we can see the quartz, mica, feldspar, and more). A homogeneous mixture, also called a solution, exhibits a uniform composition and appears visually the same throughout. An example of a solution is a sports drink, consisting of water, sugar, coloring, flavoring, and electrolytes mixed together uniformly. Each drop of a sports drink tastes the same because each drop contains the same amounts of water, sugar, and other components. Note that the composition of a sports drink can vary—it could be made with somewhat more or less sugar, flavoring, or other components, and still be a sports drink. Other examples of homogeneous mixtures include air, maple syrup, gasoline, and a solution of salt in water. (a) Oil and vinegar salad dressing is a heterogeneous mixture because its composition is not uniform throughout. (b) A commercial sports drink is a homogeneous mixture because its composition is uniform throughout. (Credit a “left”: modification of work by John Mayer; credit a “right”: modification of work by Umberto Salvagnin; credit b “left: modification of work by Jeff Bedford) Although there are just over 100 elements, tens of millions of chemical compounds result from different combinations of these elements. Each compound has a specific composition and possesses definite chemical and physical properties by which we can distinguish it from all other compounds. And, of course, there are innumerable ways to combine elements and compounds to form different mixtures. A summary of how to distinguish between the various major classifications of matter is shown in. Physical and Chemical Properties A physical property is a characteristic of matter that is not associated with a change in its chemical composition. Familiar examples of physical properties include density, color, hardness, melting and boiling points, and electrical conductivity. We can observe some physical properties, such as density and color, without changing the physical state of the matter observed. Other physical properties, such as the melting temperature of iron or the freezing temperature of water, can only be observed as matter undergoes a physical change. A physical change is a change in the state or properties of matter without any accompanying change in its chemical composition (the identities of the substances contained in the matter). We observe a physical change when wax melts, when sugar dissolves in coffee, and when steam condenses into liquid water. Other examples of physical changes include magnetizing and demagnetizing metals (as is done with common anti-theft security tags) and grinding solids into powders (which can sometimes yield noticeable changes in color). In each of these examples, there is a change in the physical state, form, or properties of the substance, but no change in its chemical composition. Figure (a) Wax undergoes a physical change when solid wax is heated and forms liquid wax. (b) Steam condensing inside a cooking pot is a physical change, as water vapor is changed into liquid water. The change of one type of matter into another type (or the inability to change) is a chemical property. Examples of chemical properties include flammability, toxicity, acidity, reactivity (many types), and heat of combustion. Iron, for example, combines with oxygen in the presence of water to form rust; chromium does not oxidize. Nitroglycerin is very dangerous because it explodes easily; neon poses almost no hazard because it is very unreactive. Figure (a) One of the chemical properties of iron is that it rusts; (b) one of the chemical properties of chromium is that it does not. (credit a: modification of work by Tony Hisgett; credit b: modification of work by “Atoma”/Wikimedia Commons) To identify a chemical property, we look for a chemical change. A chemical change always produces one or more types of matter that differ from the matter present before the change. The formation of rust is a chemical change because rust is a different kind of matter than the iron, oxygen, and water present before the rust formed. The explosion of nitroglycerin is a chemical change because the gases produced are very different kinds of matter from the original substance. Other examples of chemical changes include reactions that are performed in a lab (such as copper reacting with nitric acid), all forms of combustion (burning), and food being cooked, digested, or rotting 6.4 Kinetic Particle Theory of Matter The kinetic-molecular theory is a theory that explains the states of matter and is based on the idea that matter is composed of tiny particles that are always in motion. The theory helps explain observable properties and behaviors of solids, liquids, and gases. However, the theory is most easily understood as it applies to gases and it is with gases that we will begin our detailed study. The theory applies specifically to a model of a gas called an ideal gas. An ideal gas is an imaginary gas whose behavior perfectly fits all the assumptions of the kinetic-molecular theory. In reality, gases are not ideal, but are very close to being so under most everyday conditions. The kinetic-molecular theory as it applies to gases has five basic assumptions. 1. Gases consist of very large numbers of tiny spherical particles that are far apart from one another compared to their size. The distance between the particles of a gas is much, much greater than the distances between the particles of a liquid or a solid. Most of the volume of a gas, therefore, is composed of the empty space between the particles. In fact, the volume of the particles themselves is considered to be insignificant compared to the volume of the empty space. 2. Gas particles are in constant rapid motion in random directions. The fast motion of gas particles gives them a relatively large amount of kinetic energy. Recall that kinetic energy is the energy that an object possesses because of its motion. The particles of a gas move in straight-line motion until they collide with another particle or with one of the walls of its container. 3. Collisions between gas particles and between particles and the container walls are elastic collisions. An elastic collision is one in which there is no overall loss of kinetic energy. Kinetic energy may be transferred from one particle to another during an elastic collision, but there is no change in the total energy of the colliding particles. 4. There are no forces of attraction or repulsion between gas particles. Attractive forces are responsible for particles of a real gas condensing together to form a liquid. It is assumed that the particles of an ideal gas have no such attractive forces. The motion of each particle is completely independent of the motion of all other particles. 5. The average kinetic energy of gas particles is dependent upon the temperature of the gas. As the temperature of a sample of gas is increased, the speeds of the particles are increased. This results in an increase in the kinetic energy of the particles. Not all particles of gas in a sample have the same speed and so they do not have the same kinetic energy. The temperature of a gas is proportional to the average kinetic energy of the gas particles. 6.5 Elements and Compounds If you pull a flower petal from a plant and break it in half, and then take that piece and break it in half again, and take the next piece and break it half, and so on, and so on, until you cannot even see the flower anymore, what do you think you will find? We know that the flower petal is made of cells, but what are cells made of? Scientists have broken down matter, or anything that takes up space and has mass—like a cell—into the smallest pieces that cannot be broken down anymore. Every physical object, including rocks, animals, flowers, and your body, are all made up of matter. Matter is made up of a mixture of things called elements. Elements are substances that cannot be broken down into simpler substances. There are more than 100 known elements, and 92 occur naturally around us. The others have been made only in the laboratory. Inside of elements, you will find identical atoms. An atom is the simplest and smallest particle of matter that still has chemical properties of the element. Atoms are the building block of all of the elements that make up the matter in your body or any other living or non-living thing. Atoms are so small that only the most powerful microscopes can see them. Atoms themselves are composed of even smaller particles, including positively charged protons, uncharged neutrons, and negatively charged electrons. Protons and neutrons are located in the center of the atom, or the nucleus, and the electrons move around the nucleus. For example, hydrogen (H) has just one proton, helium (He) always has two protons, while sodium (Na) always has 11. All the atoms of a particular element have the exact same number of protons, and the number of protons is that element's atomic number. An atom usually has the same number of protons and electrons, but sometimes an atom may gain or lose an electron, giving the atom a positive or negative charge. These atoms are known as ions and are depicted with a "+" or "-" sign. Ions, such as H+, Na+, K+, or Cl- have significant biological roles. PERIODIC TABLE In 1869, a Russian scientist named Dmitri Mendeleev created the periodic table, which is a way of organizing elements according to their unique characteristics, like atomic number, density, boiling point, and other values. Each element is represented by a one or two letter symbol. For example, H stands for hydrogen, and Au stands for gold. The vertical columns in the periodic table are known as groups, and elements in groups tend to have very similar properties. The table is also divided into rows, known as periods. Compounds The truck is spreading crystals of the salt calcium chloride on a snowy road to prevent ice from forming. The crystals lower the freezing point of water so when the snow melts, it won’t turn to ice. Do you know why calcium chloride lowers the freezing point of water? The answer has to do with the type of compound that calcium chloride is. Calcium chloride is an ionic compound. All compounds form when atoms of different elements share or transfer electrons. Compounds in which electrons are transferred from one atom to another are called ionic compounds. In this type of compound, electrons actually move between the atoms, rather than being shared between them. When atoms give up or accept electrons in this way, they become charged particles called ions. The ions are held together by ionic bonds, which form an ionic compound. Ionic compounds generally form between elements that are metals and elements that are non-metals. For example, the metal calcium (Ca) and the non-metal chlorine (Cl) form the ionic compound calcium chloride (CaCl2). In this compound, there are two negative chloride ions for each positive calcium ion. Because the positive and negative charges cancel out, an ionic compound is neutral in charge. THINKING TIME: Q: Now can you explain why calcium chloride prevents ice from forming on a snowy road? A: If calcium chloride dissolves in water, it breaks down into its ions (Ca2+ and Cl-). When water has ions dissolved in it, it has a lower freezing point. Pure water freezes at 0°C. With calcium and chloride ions dissolved in it, it won’t freeze unless the temperature reaches -29°C or lower. Many compounds form molecules, but ionic compounds form crystals instead. A crystal consists of many alternating positive and negative ions bonded together in a matrix. Look at the crystal of sodium chloride (NaCl), the sodium and chloride ions are attracted to each other because they are oppositely charged, so they form ionic bonds. NAMING IONIC COMPOUNDS Ionic compounds are named for their positive and negative ions. The name of the positive ion always comes first, followed by the name of the negative ion. For example, positive sodium ions and negative chloride ions form the compound named sodium chloride. Similarly, positive calcium ions and negative chloride ions form the compound named calcium chloride. THINKING TIME: Q: What is the name of the ionic compound that is composed of positive barium ions and negative iodide ions? A: The compound is named barium iodide. PROPERTIES OF IONIC COMPOUNDS The crystal structure of ionic compounds is strong and rigid. It takes a lot of energy to break all those ionic bonds. As a result, ionic compounds are solids with high melting and boiling points. You can see the melting and boiling points of several different ionic compounds in the Table below. To appreciate how high they are, consider that the melting and boiling points of water, which is not an ionic compound, are 0°C and 100°C, respectively. Ionic Compound Melting Point (°C) Boiling Point (°C) Sodium chloride (NaCl) 801 1413 Calcium chloride (CaCl2) 772 1935 Barium oxide (BaO) 1923 2000 Iron bromide (FeBr3) 684 934 Solid ionic compounds are poor conductors of electricity. The strong bonds between their oppositely charged ions lock them into place in the crystal. Therefore, the charged particles cannot move freely and carry electric current, which is a flow of charge. But all that changes when ionic compounds dissolve in water. When they dissolve, they separate into individual ions. The ions can move freely, so they can carry current. Dissolved ionic compounds are called electrolytes. The rigid crystals of ionic compounds are brittle. They are more likely to break than bend when struck. As a result, ionic crystals tend to shatter easily. Try striking salt crystals with a hammer and you’ll find that they readily break into smaller pieces. USES OF IONIC COMPOUNDS Ionic compounds have many uses. Some are shown in the Figure below. Many ionic compounds are used in industry. The human body needs several ions for good health. Having low levels of the ions can endanger important functions such as heartbeat. Solutions of ionic compounds can be used to restore the ions. COVALENT COMPOUNDS Compounds that form from two or more non-metallic elements, such as carbon and hydrogen, are called covalent compounds. In a covalent compound, atoms of the different elements are held together in molecules by covalent bonds. These are chemical bonds in which atoms share valence electrons. The force of attraction between the shared electrons and the positive nuclei of both atoms holds the atoms together in the molecule. A molecule is the smallest particle of a covalent compound that still has the properties of the compound. The largest, most complex covalent molecules have thousands of atoms. Examples include proteins and carbohydrates, which are compounds in living things. The smallest, simplest covalent compounds have molecules with just two atoms. An example is hydrogen chloride (HCl). It consists of one hydrogen atom and one chlorine atom, as you can see in the Figure below. NAMING COVALENT COMPOUNDS 1. Start with the name of the element closer to the left side of the periodic table. 2. Follow this with the name of element closer to the right of the periodic table. Give this second name the suffix –ide. 3. Use prefixes to represent the numbers of the different atoms in each molecule of the compound. The most commonly used prefixes are shown in the Table below. Number Prefix 1 mono- (or none) 2 di- 3 tri- 4 tetra- 5 penta- 6 hexa- THINKING TIME: Q: What is the name of the compound that contains three oxygen atoms and two nitrogen atoms? A: The compound is named dinitrogen trioxide. Nitrogen is named first because it is farther to the left in the periodic table than oxygen. Oxygen is given the -ide suffix because it is the second element named in the compound. The prefix di- is added to nitrogen to show that there are two atoms of nitrogen in each molecule of the compound. The prefix tri- is added to oxygen to show that there are three atoms of oxygen in each molecule. PROPERTIES OF COVALENT COMPOUNDS The covalent bonds of covalent compounds are responsible for many of the properties of the compounds. Because valence electrons are shared in covalent compounds, rather than transferred between atoms as they are in ionic compounds, covalent compounds have very different properties than ionic compounds.  Many covalent compounds, especially those containing carbon and hydrogen, burn easily. In contrast, many ionic compounds do not burn.  Many covalent compounds do not dissolve in water, whereas most ionic compounds dissolve well in water.  Unlike ionic compounds, covalent compounds do not have freely moving electrons, so they cannot conduct electricity.  The individual molecules of covalent compounds are more easily separated than the ions in a crystal, so most covalent compounds have relatively low boiling points. This explains why many of them are liquids or gases at room temperature. You can compare the boiling points of some covalent and ionic compounds in the Table below. Name of Compound (Chemical Formula) Type of Compound Boiling Point (°C) Methane (CH4) covalent -164 Nitrogen oxide (NO) covalent -152 Sodium chloride (NaCl) ionic 1413 Lithium fluoride (LiF) ionic 1676 THINKING TIME: Q: The two covalent compounds in the table are gases at room temperature, which is 20°C. For a compound to be a liquid at room temperature, what does its boiling point have to be? A: To be a liquid at room temperature, a covalent compound has to have a boiling point higher than 20°C. Water is an example of a covalent compound that is a liquid at room temperature. The boiling point of water is 100°C. How do chess players monitor their moves in a game? Suppose you were walking along and noticed a piece of paper on the ground with markings on it. You pick it up and see the paper in the picture above. To most people, these notes are meaningless (maybe they’re a secret spy code). But to a chess player, these symbols tell the story of a chess game. Each abbreviation describes a chess piece or a move during the game. The use of special symbols allows us to “see” the game without having to read a wordy and possibly incomplete description of what happened. CHEMICAL SYMBOLS AND FORMULA In order to illustrate chemical reactions and the elements and compounds involved in them, chemists use symbols and formulas. A chemical symbol is a one-or two-letter designation of an element. Some examples of chemical symbols are "O" for oxygen, "Zn" for zinc, and "Fe" for iron. The first letter of a symbol is always capitalized. If the symbol contains two letters, the second letter is lowercase. The majority of elements have symbols that are based on their English names. However, some of the elements that have been known since ancient times have maintained symbols that are based on their Latin names, as shown in table below. Chemical Symbol Name Latin Name Na sodium natrium K potassium kalium Fe iron ferrum Cu copper cuprum Ag silver argentum Sn tin stannum Sb antimony stibium Au gold aurum Pb lead plumbum Compounds are combinations of two or more elements. A chemical formula is an expression that shows the elements in a compound and the relative proportions of those elements. Water is composed of hydrogen and oxygen in a two to one ratio. The chemical formula for water is H2O. Sulfuric acid is one of the most widely produced chemicals in the Unites States and is composed of the elements hydrogen, sulfur, and oxygen. The chemical formula for sulfuric acid is H2SO4. CHEMICAL EQUATIONS Look at this rusty bike. It has been left outside in damp weather too many times, so the iron in the metal parts has rusted. Iron rusts when it combines with oxygen in the air. Iron rusting is an example of a chemical reaction. In a chemical reaction, substances change into entirely different substances. For example, the iron in the bike and the oxygen in the air have changed into rust. THINKING TIME: Q: How could you represent this reaction, besides just describing it in words? A: Scientists use a standard method to represent a chemical reaction, called a chemical equation. A chemical equation is a shorthand way to sum up what occurs in a chemical reaction. The general form of a chemical equation is: Reactants → Products The reactants in a chemical equation are the substances that begin the reaction, and the products are the substances that are produced in the reaction. The reactants are always written on the left side of the equation and the products on the right. The arrow pointing from left to right shows that the reactants change into the products during the reaction. This happens when chemical bonds break in the reactants and new bonds form in the products. As a result, the products are different chemical substances than the reactants that started the reaction. THINKING TIME: Q: What is the general equation for the reaction in which iron rusts? A: Iron combines with oxygen to produce rust, which is the compound named iron oxide. This reaction could be represented by the general chemical equation below. Note that when there is more than one reactant, they are separated by plus signs (+). If more than one product were produced, plus signs would be used between them as well. Iron + Oxygen → Iron Oxide Using Chemical Symbols and Formulas When scientists write chemical equations, they use chemical symbols and chemical formulas instead of names to represent reactants and products. Look at the chemical reaction illustrated in the figure below. In this reaction, carbon reacts with oxygen to produce carbon dioxide. Carbon is represented by the chemical symbol C. The chemical symbol for oxygen is O, but pure oxygen exists as diatomic (“two-atom”) molecules, represented by the chemical formula O2. A molecule of the compound carbon dioxide consists of one atom of carbon and two atoms of oxygen, so carbon dioxide is represented by the chemical formula CO2. THINKING TIME: Q: How have the atoms of the reactants been rearranged in the products of the reaction? What bonds have been broken, and what new bonds have formed? A: Bonds between the oxygen atoms in the oxygen molecule have been broken, and new bonds have formed between the carbon atom and the two oxygen atoms. 6.6 Electrolysis In 1989, two scientists announced that they had achieved “cold fusion”, the process of fusing together elements at essentially room temperature to achieve energy production. The hypothesis was that the fusion would produce more energy than was required to cause the process to occur. Their process involved the electrolysis of heavy water (water molecules containing some deuterium instead of normal hydrogen) on a palladium electrode. The experiments could not be reproduced and their scientific reputations were pretty well shot. However, in more recent years, both industry and government researchers are taking another look at this process. The device illustrated above is part of a government project, and NASA is completing some studies on the topic as well. Cold fusion may not be so “cold” after all. A voltaic cell uses a spontaneous redox reaction to generate an electric current. It is also possible to do the opposite. When an external source of direct current is applied to an electrochemical cell, a reaction that is normally nonspontaneous can be made to proceed. Electrolysis is the process in which electrical energy is used to cause a nonspontaneous chemical reaction to occur. Electrolysis is responsible for the appearance of many everyday objects such as gold-plated or silver-plated jewelry and chrome-plated car bumpers. An electrolytic cell is the apparatus used for carrying out an electrolysis reaction. In an electrolytic cell, electric current is applied to provide a source of electrons for driving the reaction in a nonspontaneous direction. In a voltaic cell, the reaction goes in a direction that releases electrons spontaneously. In an electrolytic cell, the input of electrons from an external source forces the reaction to go in the opposite direction. The spontaneous direction for the reaction between Zn and Cu is for the Zn metal to be oxidized to Zn2+ ions, while the Cu2+ ions are reduced to Cu metal. This makes the zinc electrode the anode and the copper electrode the cathode. When the same half-cells are connected to a battery via the external wire, the reaction is forced to run in the opposite direction. The zinc electrode is now the cathode and the copper electrode is the anode. The standard cell potential is negative, indicating a non-spontaneous reaction. The battery must be capable of delivering at least 1.10 V of direct current in order for the reaction to occur. Another difference between a voltaic cell and an electrolytic cell is the signs of the electrodes. In a voltaic cell, the anode is negative and the cathode is positive. In an electrolytic cell, the anode is positive because it is connected to the positive terminal of the battery. The cathode is therefore negative. Electrons still flow through the cell form the anode to the cathode. 6.7 Mixtures and Separation Techniques HOW DID GOLD MINERS SEARCH FOR GOLD? Beginning in the late 1840s, thousands of prospectors rushed to California to search for gold. One of the approaches taken to isolate the gold from the soil was called “panning”. Dirt would be placed in the pan and covered with water. After thorough mixing, the pan is gently swirled to remove dissolved material while the heavier gold settles to the bottom of the pan. The gold is then separated from the mixture of soil and water. SEPARATION TECHNIQUES Not everyone is out searching for gold (and not many of those searchers is going to get much gold, either). In a chemical reaction, it is important to isolate the component(s) of interest from all the other materials so they can be further characterized. Studies of biochemical systems, environmental analysis, pharmaceutical research – these and many other areas of research require reliable separation methods. Here are a number of common separation techniques: CHROMATOGRAPHY Chromatography is the separation of a mixture by passing it in solution or suspension or as a vapor (as in gas chromatography) through a medium in which the components move at different rates. Thin-layer chromatography is a special type of chromatography used for separating and identifying mixtures that are or can be colored, especially pigments. DISTILLATION Distillation is an effective method to separate mixtures comprised of two or more pure liquids. Distillation is a purification process where the components of a liquid mixture are vaporized and then condensed and isolated. In simple distillation, a mixture is heated and the most volatile component vaporizes at the lowest temperature. The vapor passes through a cooled tube (a condenser), where it condenses back into its liquid state. The condensate that is collected is called distillate. In the figure above, there is a heat source, a test tube with a one-hole stopper attached to a glass elbow and rubber tubing. The rubber tubing is placed into a collection tube which is submerged in cold water. There are other more complicated assemblies for distillation that can also be used, especially to separate mixtures, which are comprised of pure liquids with boiling points that are close to one another. EVAPORATION Evaporation is a technique used to separate out homogenous mixtures where there is one or more dissolved solids. This method drives off the liquid components from the solid components. The process typically involves heating the mixture until no more liquid remains, prior to using this method, the mixture should only contain one liquid component, unless it is not important to isolate the liquid components. This is because all liquid components will evaporate over time. This method is suitable to separate a soluble solid from a liquid. In many parts of the world, table salt is obtained from the evaporation of seawater. The heat for the process comes from the sun. FILTRATION Filtration is a separation method used to separate out pure substances in mixtures comprised of particles some of which are large enough in size to be captured with a porous material. Particle size can vary considerably, given the type of mixture. For instance, stream water is a mixture that contains naturally occurring biological organisms like bacteria, viruses, and protozoans. Some water filters can filter out bacteria, the length of which is on the order of 1 micron. Other mixtures, like soil, have relatively large particle sizes, which can be filtered through something like a coffee filter. 6.8 Solubility Rhonda wanted to see if salt or sugar dissolves faster in water. She added the same amount of salt and sugar to a half liter of room temperature (20 °C) water in separate glasses. Then she stirred both mixtures. All of the sugar dissolved in less than a minute, but after 5 minutes of stirring, some of the salt still hadn’t dissolved. Even if she had kept stirring the saltwater mixture all day, the remaining salt would not dissolve. Do you know why? The answer is their solubility. Solubility is the amount of solute that can dissolve in a given amount of solvent at a given temperature. In a solution, the solute is the substance that dissolves, and the solvent is the substance that does the dissolving. For a given solvent, some solutes have greater solubility than others. For example, sugar is much more soluble in water than is salt. But even sugar has an upper limit on how much can dissolve. In a half liter of 20 °C water, the maximum amount is 1000 grams. If you add more sugar than this, the extra sugar won’t dissolve. You can compare the solubility of sugar, salt, and some other solutes in the table below. Solute Grams of Solute that Will Dissolve in 0.5 L of Water (20 °C) Baking Soda 48 Epsom salt 125 Table salt 180 Table sugar 1000 THINKING TIME: Q: How much salt do you think Rhonda added to the half-liter of water in her experiment? A: The solubility of salt is 180 grams per half liter of water at 20 °C. If Rhonda had added less than 180 grams of salt to the half-liter of water, then all of it would have dissolved. Because some of the salt did not dissolve, she must have added more than 180 grams of salt to the water. FACTORS AFFECTING SOLUBILITY Certain factors can change the solubility of a solute. Temperature is one such factor. How temperature affects solubility depends on the state of the solute, as you can see in the figure below.  If a solute is a solid or liquid, increasing the temperature increases its solubility. For example, more sugar can dissolve in hot water than in cold water.  If a solute is a gas, increasing the temperature decreases its solubility. For example, less carbon dioxide can dissolve in warm water than in cold water. The solubility of gases is also affected by pressure. Pressure is the force pushing against a given area. Increasing the pressure on a gas increases its solubility. Did you ever open a can of soda and notice how it fizzes out of the can? Soda contains dissolved carbon dioxide. Opening the can reduces the pressure on the gas in solution, so it is less soluble. As a result, some of the carbon dioxide comes out of solution and rushes into the air. THINKING TIME: Q: Which do you think will fizz more when you open it, a can of warm soda or a can of cold soda? A: A can of warm soda will fizz more because increasing the temperature decreases the solubility of a gas. Therefore, less carbon dioxide can remain dissolved in warm soda than in cold soda.