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The study of chemistry is the study of matter. Matter is defined as any substance that has mass and takes up space. Therefore the study of chemistry includes any thing. One of the few examples of nonmatter is energy, such as heat and light. However, most changes in matter include changes in energy, and chemists also study the energy that accompanies the changes in matter.

Normally, the first steps in studying something are to describe and categorize it. Chemistry is no exception. Matter can be described as having either physical properties or chemical properties. Physical properties can be observed without changing the identity of the substance. Physical properties include such qualities as color, size, and luster. Chemical properties are observed when matter actually changes its identity. For example, a chemical property of water is that electricity can transform it into oxygen gas and hydrogen gas.

Chemical and physical properties are used to categorize matter. One category of matter is as either a mixture or a pure substance. Mixtures can be separated physically, that is, the components can be separated without changing their identity. Physical separations can be as simple as sorting M&Ms by color, or they can be more complicated, such as the process of distillation, which requires turning one component into a gas while leaving another in its liquid state. One characteristic of mixtures that is useful to help identify them, is that their composition is variable. For example, a mixture of sugar and water might be very sweet or only slightly sweet. The sugar and the water retain their identities whether they are mixed or not. If you heat the sugar water until all the water is boiled off, the sugar will remain. If you collect the gas as you boil, you can get the pure water. The process involves physical separation of the sugar and water. On the other hand, pure substances cannot be separated physically. If a pure substance is separated—and this is not always possible—it is separated chemically, yielding the same ratio of the components every time; these components have a different identity than when they were first combined.

Mixtures can be divided into two categories. Homogeneous mixtures have a uniform composition. Sugar and water provide an example of this. Whatever portion of the mixture is sampled will have the same characteristics. The composition of heterogeneous mixtures depends on the location within the mixture. For example, variations in color across the surface of this page show that it is a heterogeneous mixture.

Pure substances can also be divided into two categories, elements and compounds. Compounds can be separated chemically, divided into two or more components. However, these components will differ in identity from the compound and from each other. The identity and mass ratio of these components will be the same in a specific compound. For example, water can be chemically divided into hydrogen and oxygen, and the oxygen produced will always weigh eight times more than the resulting hydrogen. Elements cannot be chemically separated. All known elements are listed, usually by symbol, in the periodic table. Elements that are chemically combined create compounds.

The smallest fraction of an element that still retains the identity of the element is called an atom. Atoms are made of even smaller particles called protons, neutrons and electrons. Protons are positively charged particles with a mass of about 1 atomic mass unit (amu). The identity of an element is defined as its number of protons. The number of protons is also called an element's atomic number (Z) and is listed with the element in the periodic table. Atoms are electrically neutral, so there is also a negatively charged particle called an electron. To maintain the zero charge, the number of electrons and protons must be the same. If protons and electrons are not equal, the atom will have a charge and be called an ion. Electrons are very lightweight, even compared with a proton. In fact, their mass is approximately 0 amu. Like the proton, the neutron has a mass of 1 amu. However, it does not have a charge. Therefore the sum of the protons and neutrons is called the mass number (A). It is possible for atoms to have the same number of protons but different numbers of neutrons; these atoms are called isotopes. Elements prefer some proton/neutron combinations above others. The percent of each isotope in an element is called its isotopic abundance. The average atomic mass takes into account not only all the naturally occurring isotopes, but also the relative abundance of each. The average atomic mass is also listed in the periodic table. To represent a type of atom, its symbol (an uppercase letter sometimes followed by a lowercase letter) is used. The symbol and the atomic number are two ways of expressing the same idea. To also include the number of neutrons, the mass number is included as a superscript before the symbol. The symbol together with the mass number is called isotopic notation. Radioactive elements will disintegrate. The rate at which they do this is normally expressed as half-life, which is the time required for half of the initial quantity to disintegrate.

The periodic table not only lists all the elements, their symbols, atomic numbers, and average atomic mass, but does so in such a way as to group them by properties. Many periodic tables, including the one on the inside front cover of the text, have a staircase-like division that runs between aluminum (Al) and silicon (Si) and continues between germanium (Ge) and arsenic (As) to the bottom of the table. This "staircase" provides a useful way to categorize elements. Those to the left of the staircase are metals. Properties of metals include a metallic luster (they are shiny), malleability (it dents), and ductility (they can be pulled into a wire). Elements to the right of the staircase are nonmetals, which are neither malleable nor ductile. Elements that actually touch the staircase are metalloids. Metalloids tend to have the physical properties of metals, such as luster, and the chemical properties of nonmetals.

Compounds can also be divided into categories. Molecular compounds are usually combinations of nonmetal atoms that are chemically bonded into a group, called a molecule. Ionic compounds are held together by oppositely charged atoms or groups of atoms called ions. Ions with a positive charge are called cations and are usually formed from metal atoms. Ions with a negative charge are usually formed from nonmetals and are called anions. An ionic compound will have anions and cations in a ratio that has a net charge of zero.

One of the ways chemists study matter is by observing its interaction with light.

                              >> Explore: Light Diffraction Tutorial

A more general term for light is electromagnetic radiation, since light is a form of energy that has both electric and magnetic properties. Electromagnetic radiation also has particle-wave duality, properties of both particles and waves. When light is described as a wave, it usually is described in terms of amplitude, the height of the wave, and wavelength, the distance between waves. Sometimes it is convenient to use the term frequency, the rate at which waves move, instead of wavelength. Frequency and wavelength give the same information about a wave but do so in different ways. Frequency and wavelength are related by

= c       (Equation 1.1)

where is wavelength, is frequency, and c is the speed of light (2.998 x 10⁸ m/s).

                              >> Explore: Electromagnetic Radiation Tutorial

>When light is considered a particle, that particle is called a photon. The photon is characterized by its kinetic energy (the energy of movement) and the number of photons. The energy of light as a particle (E) is an equivalent idea to the wavelength or frequency of light. The two values are related by

E = h = hc/       (Equations 1.2 and 1.3)

where h is Planck's constant (6.626 x 10^–34 J•s). Therefore high frequencies and short wavelength represent higher-energy light. The number of photons is an equivalent idea to the amplitude of the wave, and both are often called intensity. The higher the amplitude and the more photons, the more intense the light. If the source of light or the observer of light (or both) is moving, there is a perceived change in frequency. This is called the Doppler effect.

                              >> Explore: Doppler Effect Tutorial

                              >> Explore: Big Bang Tutorial

As a science, chemistry tries to explain observations about the world. It tests its explanations through experiments. This is called the scientific method. Tentative explanations are called hypotheses; tested explanations are called theories. However, good hypotheses and theories first require very good observations. Therefore measurements in chemistry are of vital importance.

The ideal measurement is both accurate and precise. An accurate answer is a correct or true answer. A precise answer is one that can be repeated. Another way of looking at precision is to consider to what extent a measurement can be repeated. Each measurement made will have some values that are always the same and usually at least one that varies slightly from measurement to measurement. It is standard when recording a scientific measurement to always include all the digits you are certain of and one digit that you must estimate (or that varies a little). These digits are called significant figures and are a way of expressing the precision of the measurement.

                              >> Explore: Significant Figures Tutorial

Some of the measured values in chemistry are very large or very small (fractional numbers). It is generally convenient to express these numbers with a method called scientific notation, which has the form N x 10^x. Scientific notation moves the decimal so that the number can be expressed as a value between 1 and 10 (N). The second part (x) tells you how many spaces and in which direction the decimal was moved. If x is positive, the decimal was moved from right to left. If x is negative, the decimal was moved from left to right.

                              >> Explore: Scientific Notation Tutorial

Many values in chemistry are the result of calculations made from measurements. These calculated values should also express the precision of the values they came from. To do this, rules for rounding have been developed. The rule you use depends on the type of calculation. How you round depends on the precision (number of signifcant figures) of the numbers you use to do the calculation. The "weakest link" principle is used; the answer is only as precise as the value in the calculation that is the least precise.

                              >> Explore: Dimensional Analysis Tutorial

Numbers themselves are not a sufficient description of a measurement. All numbers should also include units. Units tell what type of measurement is made (length, mass, time, etc.) as well as general information on the magnitude of the value (miles are longer than inches). In this text, SI units (Système International d'Unités) are used. SI units have a base value for each common type of unit (Table 1.2). The base values can be modified for magnitude using prefixes (Table 1.1). A unit that is given particular attention is the kelvin, which is the SI unit for temperature. The Kelvin scale is a temperature scale designed so that absolute zero (the coldest possible temperature) is zero. However, the magnitude of each unit is the same as that for the Celsius scale. Therefore it is easy to convert between Celsius and Kelvin using the equation

K = °C + 273.15       (Equation 1.11)

This is a frequent conversion because most scientific measurements are made in Celsius and most scientific calculations are made in Kelvin.

                              >> Explore: Temperature Conversion Tutorial