Apr 18, 2016

Classification of Matter

Matter is made up of very tiny units called atoms. Each different type of atom is the building block of a different chemical element. Presently, the International Union of Pure and Applied Chemistry (IUPAC) recognizes 112 elements, and all matter is made up of just these types! The known elements range from common substances, such as carbon, iron, and silver, to uncommon ones, such as lutetium and thulium. About 90 of the elements can be obtained from natural sources. The remainder do not occur naturally and have been created only in laboratories. On the inside front cover you will find a complete listing of the elements and also a special tabular arrangement of the elements known as the periodic table. The periodic table is the chemist s directory of the elements. We will describe it in Chapter 2and use it throughout most of the text.


Chemical compounds are substances comprising atoms of two or more elements joined together. Scientists have identified millions of different chemical compounds. In some cases, we can isolate a molecule of a compound. A molecule is the smallest entity having the same proportions of the constituent atoms as does the compound as a whole. A molecule of water consists of three atoms: two hydrogen atoms joined to a single oxygen atom. A molecule of hydrogen peroxide has two hydrogen atoms and two oxygen atoms; the two oxygen atoms are joined together and one hydrogen atom is attached to each oxygen atom. By contrast, a molecule of the blood protein gamma globulin is made up of 19,996 atoms, but they are of just four types: carbon, hydrogen, oxygen, and nitrogen.


The composition and properties of an element or a compound are uniform throughout a given sample and from one sample to another. Elements and compounds are called substances. (In the chemical sense, the term substance should be used only for elements and compounds.) A mixture of substances can vary in composition and properties from one sample to another. One that is uniform in composition and properties throughout is said to be a homogeneous mixture or a solution. A given solution of sucrose (cane sugar) in water is uniformly sweet throughout the solution, but the sweetness of another sucrose solution may be rather different if the sugar and water are present in different proportions. Ordinary air is a homogeneous mixture of several gases, principally the elements nitrogen and oxygen. Seawater is a solution of the compounds water, sodium chloride (salt), and a host of others. Gasoline is a homogeneous mixture or solution of dozens of compounds.

Is it homogeneous or heterogeneous? When viewed through a microscope, homogenized milk is seen to consist of globules of fat dispersed in a watery medium. Homogenized milk is a heterogeneous mixture.

In heterogeneous mixtures sand and water, for example the components separate into distinct regions. Thus, the composition and physical properties vary from one part of the mixture to another. Salad dressing, a slab of concrete, and the leaf of a plant are all heterogeneous. It is usually easy to distinguish heterogeneous from homogeneous mixtures. A scheme for classifying matter into elements and compounds and homogeneous and heterogeneous mixtures is summarized in Figure below. 

Every sample of matter is either a single substance (an element or compound) or a mixture of substances. At the molecular level, an element consists of atoms of a single type and a compound consists of two or more different types of atoms, usually joined into molecules. In a homogeneous mixture, atoms or molecules are randomly mixed at the molecular level. In heterogeneous mixtures, the components are physically separated, as in a layer of octane molecules (a constituent of gasoline) floating on a layer of water molecules.

Separating Mixtures
A mixture can be separated into its components by appropriate physical means. Consider again the heterogeneous mixture of sand in water. When we pour this mixture into a funnel lined with porous filter paper, the water passes through and sand is retained on the paper. This process of separating a solid from the liquid in which it is suspended is called filtration (Figure). You will probably use this procedure in the laboratory. Conversely, we cannot separate a homogeneous mixture (solution) of copper (II) sulfate in water by filtration because all components pass through the paper. We can, however, boil the solution of copper (II) sulfate and water. In the process of distillation, a pure liquid is condensed from the vapor given off by a boiling solution. When all the water has been removed by boiling a solution of copper (II) sulfate in water, solid copper (II) sulfate remains behind.

Another method of separation available to modern chemists depends on the differing abilities of compounds to adhere to the surfaces of various solid substances, such as paper and starch. The technique of chromatography relies on this principle. The dramatic results that can be obtained with chromatography are illustrated by the separation of ink on a filter paper.

(a) Separation of a heterogeneous mixture by filtration: Solid copper(II) sulfate is retained on the filter paper, while liquid hexane passes through. (b) Separation of a homogeneous mixture by distillation: Copper(II) sulfate remains in the flask on the left as water passes to the flask on the right, by first evaporating and then condensing back to a liquid. (c) Separation of the components of ink using chromatography: A dark spot of black ink can be seen just above the water line as water moves up the paper. (d) Water has dissolved the colored components of the ink, and these components are retained in different regions on the paper according to their differing tendencies to adhere to the paper.

Decomposing Compounds
A chemical compound retains its identity during physical changes, but it can be decomposed into its constituent elements by chemical changes. The decomposition of compounds into their constituent elements is a more difficult matter than the mere physical separation of mixtures. The extraction of iron from iron oxide ores requires a blast furnace. The industrial production of pure magnesium from magnesium chloride requires electricity. It is generally easier to convert a compound into other compounds by a chemical reaction than it is to separate a compound into its constituent elements. For example, when heated, ammonium dichromate decomposes into the substances chromium (III) oxide, nitrogen, and water. This reaction, once used in movies to simulate a volcano, is illustrated in Figure below.

A chemical change: decomposition of ammonium dichromate

States of Matter
Matter is generally found in one of three states: solid, liquid, or gas. In a solid, atoms or molecules are in close contact, sometimes in a highly organized arrangement called a crystal. A solid has a definite shape. In a liquid, the atoms or molecules are usually separated by somewhat greater distances than in a solid. Movement of these atoms or molecules gives a liquid it’s most distinctive property the ability to flow, covering the bottom and assuming the shape of its container. In a gas, distances between atoms or molecules are much greater than in a liquid. A gas always expands to fill its container. Depending on conditions, a substance may exist in only one state of matter, or it may be present in two or three states. Thus, as the ice in a small pond begins to melt in the spring, water is in two states: solid and liquid (actually, three states if we also consider water vapor in the air above the pond). The three states of water are illustrated at two levels in Figure.

The picture shows a block of ice on a heated surface and the three states of water. The circular insets show how chemists conceive of these states microscopically, in terms of molecules with two hydrogen atoms joined to one of oxygen. In ice (a), the molecules are arranged in a regular pattern in a rigid framework. In liquid water (b), the molecules are rather closely packed but move freely. In gaseous water (c), the molecules are widely separated.

The macroscopic level refers to how we perceive matter with our eyes, through the outward appearance of objects. The microscopic level describes matter as chemists conceive of it in terms of atoms and molecules and their behavior. In this text, we will describe many macroscopic, observable properties of matter, but to explain these properties, we will often shift our view to the atomic or molecular level the microscopic level.

Apr 15, 2016

Properties of Matter

Dictionary definitions of chemistry usually include the terms matter, composition, and properties, as in the statement that chemistry is the science that deals with the composition and properties of matter. In this and the next section, we will consider some basic ideas relating to these three terms in hopes of gaining a better understanding of what chemistry is all about.

Matter is anything that occupies space and displays the properties of mass and inertia. Every human being is a collection of matter. We all occupy space, and we describe our mass in terms of weight, a related property. (Mass and weight are described in more detail in Section 1-4. Inertia is described in Appendix B.) All the objects that we see around us consist of matter. The gases of the atmosphere, even though they are invisible, are matter they occupy space and have mass. Sunlight is not matter; rather, it is a form of energy. Energy is discussed in later chapters.

Composition refers to the parts or components of a sample of matter and their relative proportions. Ordinary water is made up of two simpler substances hydrogen and oxygen present in certain fixed proportions. A chemist would say that the composition of water is 11.19% hydrogen and 88.81% oxygen by mass. Hydrogen peroxide, a substance used in bleaches and antiseptics, is also made up of hydrogen and oxygen, but it has a different composition. Hydrogen peroxide is 5.93% hydrogen and 94.07% oxygen by mass.

Properties are those qualities or attributes that we can use to distinguish one sample of matter from others; and, as we consider next, the properties of matter are generally grouped into two broad categories: physical and chemical.

Physical Properties and Physical Changes
A physical property is one that a sample of matter displays without changing its composition. Thus, we can distinguish between the reddish brown solid, copper, and the yellow solid, sulfur, by the physical property of color (as shown in the figure).

Another physical property of copper is that it can be hammered into a thin sheet of foil (as seen in the figure). Solids having this ability are said to be malleable. Sulfur is not malleable. If we strike a chunk of sulfur with a hammer, it crumbles into a powder. Sulfur is brittle. Another physical property of copper that sulfur does not share is the ability to be drawn into a fine wire (ductility). Also, sulfur is a far poorer conductor of heat and electricity than is copper.

Sometimes a sample of matter undergoes a change in its physical appearance. In such a physical change, some of the physical properties of the sample may change, but its composition remains unchanged. When liquid water freezes into solid water (ice), it certainly looks different and, in many ways, it is different. Yet, the water remains 11.19% hydrogen and 88.81% oxygen by mass.

A lump of sulfur (left) crumbles into a yellow powder when hammered. Copper (right) can be obtained as large lumps of native copper, formed into pellets, hammered into a thin foil, or drawn into a wire.


Chemical Properties and Chemical Changes
In a chemical change, or chemical reaction, one or more kinds of matter are converted to new kinds of matter with different compositions. The key to identifying chemical change, then, comes in observing a change in composition. The burning of paper involves a chemical change. Paper is a complex material, but its principal constituents are carbon, hydrogen, and oxygen. The chief products of the combustion are two gases, one consisting of carbon and oxygen (carbon dioxide) and the other consisting of hydrogen and oxygen (water, as steam). The ability of paper to burn is an example of a chemical property. A chemical property is the ability (or inability) of a sample of matter to undergo a change in composition under stated conditions.

The zinc-plated (galvanized) nail reacts with hydrochloric acid, producing the bubbles of hydrogen gas seen on its surface. The gold bracelet is unaffected by hydrochloric acid. In this photograph, the zinc plating has been consumed, exposing the underlying iron nail. The reaction of iron with hydrochloric acid imparts some color to the acid solution.


Zinc reacts with hydrochloric acid solution to produce hydrogen gas and a solution of zinc chloride in water (as shown in the figure). This reaction is one of zinc s distinctive chemical properties, just as the inability of gold to react with hydrochloric acid is one of gold s chemical properties. Sodium reacts not only with hydrochloric acid but also with water. In some of their physical properties, zinc, gold, and sodium are similar. For example, each is malleable and a good conductor of heat and electricity. In most of their chemical properties, though, zinc, gold, and sodium are quite different. Knowing these differences helps us to understand why zinc, which does not react with water, is used in roofing nails, roof flashings, and rain gutters, and sodium is not. Also, we can appreciate why gold, because of its chemical inertness, is prized for jewelry and coins: It does not tarnish or rust. In our study of chemistry, we will see why substances differ in properties and how these differences determine the ways in which we use them.

Mar 5, 2016

The Scientific Method

Science differs from other fields of study in the method that scientists use to acquire knowledge and the special significance of this knowledge. Scientific knowledge can be used to explain natural phenomena and, at times, to predict future events.

The ancient Greeks developed some powerful methods of acquiring knowledge, particularly in mathematics. The Greek approach was to start with certain basic assumptions, or premises. Then, by the method known as deduction, certain conclusions must logically follow. For example, if a = b and b = c then a = c. Deduction alone is not enough for obtaining scientific knowledge, however. The Greek philosopher Aristotle assumed four fundamental substances: air, earth, water, and fire. All other materials, he believed, were formed by combinations of these four elements. Chemists of several centuries ago (more commonly referred to as alchemists) tried, in vain, to apply the four-element idea to turn lead into gold. They failed for many reasons, one being that the four-element assumption is false.

The Scientific Method

The scientific method originated in the seventeenth century with such people as Galileo, Francis Bacon, Robert Boyle, and Isaac Newton. The key to the method is to make no initial assumptions, but rather to make careful observations of natural phenomena. When enough observations have been made so that a pattern begins to emerge, a generalization or natural law can be formulated describing the phenomenon. Natural laws are concise statements, often in mathematical form, about natural phenomena. The form of reasoning in which a general statement or natural law is inferred from a set of observations is called induction. For example, early in the sixteenth century, the Polish astronomer Nicolas Copernicus (1473 1543), through careful study of astronomical observations, concluded that Earth revolves around the sun in a circular orbit, although the general teaching of the time, not based on scientific study, was that the sun and other heavenly bodies revolved around Earth. We can think of Copernicus s statement as a natural law. Another example of a natural law is the radioactive decay law, which dictates how long it takes for a radioactive substance to lose its radioactivity.


The success of a natural law depends on its ability to explain, or account for, observations and to predict new phenomena. Copernicus s work was a great success because he was able to predict future positions of the planets more accurately than his contemporaries. We should not think of a natural law as an absolute truth, however. Future experiments may require us to modify the law. For example, Copernicus s ideas were refined a half-century later by Johannes Kepler, who showed that planets travel in elliptical, not circular, orbits. To verify a natural law, a scientist designs experiments that show whether the conclusions deduced from the natural law are supported by experimental results.

A hypothesis is a tentative explanation of a natural law. If a hypothesis survives testing by experiments, it is often referred to as a theory. In a broader sense, a theory is a model or way of looking at nature that can be used to explain natural laws and make further predictions about natural phenomena. When differing or conflicting theories are proposed, the one that is most successful in its predictions is generally chosen. Also, the theory that involves the smallest number of assumptions the simplest theory is preferred. Over time, as new evidence accumulates, most scientific theories undergo modification, and some are discarded.

The scientific method is the combination of observation, experimentation, and the formulation of laws, hypotheses, and theories. The method is illustrated by the flow diagram in Figure 1-1. Scientists may develop a pattern of thinking about their field, known as a paradigm. Some paradigms may be successful at first but then become less so. When that happens, a new paradigm may be needed or, as is sometimes said, a paradigm shift occurs. In a way, the method of inquiry that we call the scientific method is itself a paradigm, and some people feel that it, too, is in need of change. That is, the varied activities of modern scientists are more complex than the simplified description of the scientific method presented here.* In any case, merely following a set of procedures, rather like using a cookbook, will not guarantee scientific success.

Another factor in scientific discovery is chance, or serendipity. Many discoveries have been made by accident. For example, in 1839, the American inventor Charles Goodyear was searching for a treatment for natural rubber that would make it less brittle when cold and less tacky when warm. In the course of this work, he accidentally spilled a rubber sulfur mixture on a hot stove and found that the resulting product had exactly the properties he was seeking. Other chance discoveries include X-rays, radioactivity, and penicillin. So scientists and inventors always need to be alert to unexpected observations. Perhaps no one was more aware of this than Louis Pasteur, who wrote, “Chance favors the prepared mind”.



Oldest DNA and Human Evolution

In December 2013, the world’s oldest evidence of human development was discovered, a finding that raised a number of evolutionary questions. A femur (thigh bone) fossil was recovered from the “pit of bones,” an underground cave in northern Spain, from which scientists had recovered twenty-eight nearly complete human skeletons since the 1970s. From the powdered femur, Matthias Meyer and his colleagues at the Max Planck Institute in Leipzig, Germany, extracted mitochondrial DNA (mtDNA) that dated back some 400,000 years, 300,000 years older than the previous humanoid DNA sample. 

Upon preliminary examination, the anatomy of the femur resembled a Neanderthal, but a comparison of the DNA evidence showed a much closer relationship to the Denisovans, whose DNA had been previously analyzed from 80,000-year-old remains found in Siberia, 4,000 miles to the east. This finding challenged the narrative of human development based on previously discovered fossil remains and DNA analysis. Humans, Neanderthals, and Denisovans were generally believed to have had a common ancestor in Africa some 500,000 years ago. This ancestor diverged from humans, left Africa, and split once again, 300,000 years ago, into the Neanderthals and Denisovans. The Neanderthals traveled west, toward Europe, and the Denisovans, east. Our human ancestor remained in Africa, giving rise to Homo sapiens, who 60,000 years ago migrated to Europe and Asia, where they interbred with the Neanderthals and Denisovans, who became extinct. But the new DNA evidence raises the question: Why are Denisovan fossil remains in Spain? 

The jaws of Homo heidelbergensis, an extinct species that lived in Europe, Africa, and western Asia possibly as far back as 1.3 million years ago. It was the first human species to live in colder climates and may have been the first to bury its dead.

The new DNA findings were only made possible because of advances in retrieving ancient DNA. When a biological organism dies, its DNA breaks down into small fragments, which in time mix and become contaminated with DNA from other species—in particular, soil bacteria. In 1997, Svante Pääbo, a Swedish biologist specializing in evolutionary genetics, also working at the Max Planck Institute, discovered a new technique for retrieving DNA fragments, which he used to determine the genome sequence of the Neanderthal in 2010 and the femur in Spain. It is possible that advances such as this may rewrite our biological history.

De-Extinction

Extinction is the end of a species, marked by the death of the last member of that species. In 1796, the French naturalist Georges Cuvier presented convincing evidence that established extinction to be a fact, and it is estimated that more than 99 percent of all species that have ever existed are now extinct. Fossil evidence documents that over the past 500 million years, there have been five major extinction events—the most recent occurring during the Cretaceous period 65 million years ago, which eliminated more than half of all marine species as well as many families of terrestrial plants and animals. Its cause is thought to have been an asteroid or comet. In more recent times, extinctions have been attributed to climatic changes, genetic factors, and habitat destruction and pollution. Other identified causes are overexploitation by hunting and fishing, the introduction of invasive species, and disease. 

This illustration of a Pyrenean ibex appeared in the book Wild Oxen, Sheep & Goats of All Lands: Living and Extinct (1898) by English naturalist Richard Lydekker (1849–1915).

In more recent times, extinct species have included the woolly mammoth (3,000–10,000 years ago), passenger pigeon (1914), Tasmanian tiger (1930), and the Pyrenean or Spanish ibex (2,000). But not all scientists accept the premise that “extinct is forever,” and there have been active deextinction efforts to bring back extinct animal and plant species. The most widely proposed method has been by cloning, which was popularized in John Brosnan’s Carnosaur (1984) and Michael Crichton’s Jurassic Park (1990). In this de-extinction process, a viable DNA sample is taken from a species that has been extinct for no more than thousands of years—not millions of years, as in these novels—and gestated in a host animal. 

Thus far, limited de-extinction success has been achieved. In 2003, Spanish researchers used frozen tissue obtained from the last living Pyrenean ibex that had died three years earlier, which was implanted in a goat; this attempt was unsuccessful. In 2009, an ibex clone was born alive but died seven minutes later from an unrelated respiratory ailment. Renewed and enthusiastic interest in deextinction occurred in 2013, with active discussion and debate, as well as plans by Russian and South Korean scientists to clone a woolly mammoth from well-preserved remains found in Siberia.