1、Chapter 1 Matter and Measurement Chemistry is the science of matter and the changes it undergoes. Chemists study the composition, structure, and properties of matter. They observe the changes that matter undergoes and measure the energy that is produced or consumed during these changes. Chemistry p
2、rovides an understanding of many natural events and has led to the synthesis of new forms of matter that have greatly affected the way we live. Disciplines within chemistry are traditionally grouped by the type of matter being studied or the kind of study. These include inorganic chemistry, organ
3、ic chemistry, physical chemistry, analytical chemistry, polymer chemistry, biochemistry, and many more specialized disciplines, e.g. radiochemistry, theoretical chemistry. Chemistry is often called "the central science" because it connects the other natural sciences such as astronomy, physics, mate
4、rial science, biology and geology. 1.1. Classification of Matter Matter is usually defined as anything that has mass and occupies space. Mass is the amount of matter in an object. The mass of an object does not change. The volume of an object is how much space the object takes up. All the diffe
5、rent forms of matter in our world fall into two principal categories: (1) pure substances and (2) mixtures. A pure substance can also be defined as a form of matter that has both definite composition and distinct properties. Pure substances are subdivided into two groups: elements and compounds. An
6、element is the simplest kind of material with unique physical and chemical properties; it can not be broken down into anything simpler by either physical or chemical means. A compound is a pure substance that consists of two or more elements linked together in characteristic and definite proportions
7、 it can be decomposed by a chemical change into simpler substances with a fixed mass ratio. Mixtures contain two or more chemical substances in variable proportions in which the pure substances retain their chemical identities. In principle, they can be separated into the component substances by ph
8、ysical means, involving physical changes. A sample is homogeneous if it always has the same composition, no matter what part of the sample is examined. Pure elements and pure chemical compounds are homogeneous. Mixtures can be homogeneous, too; in a homogeneous mixture the constituents are distribut
9、ed uniformly and the composition and appearance of the mixture are uniform throughout. A solutions is a special type of homogeneous mixture. A heterogeneous mixture has physically distinct parts with different properties. The classification of matter is summarized in the diagram below: Matter Pure
10、 Substances Mixtures Elements Compounds Homogeneous mixtures Heterogeneous mixtures Matter can also be categorized into four distinct phases: solid, liquid, gas, and plasma. The solid phase of matter has the atoms packed closely together. An object that is solid has a definite shape and vo
11、lume that cannot be changed easily. The liquid phase of matter has the atoms packed closely together, but they flow freely around each other. Matter that is liquid has a definite volume but changes shape quite easily. Solids and liquids are termed condensed phases because of their well-defined volum
12、es. The gas phase of matter has the atoms loosely arranged so they can travel in and out easily. A gas has neither specific shape nor constant volume. The plasma phase of matter has the atoms existing in an excited state. 1.2. Properties of Matter All substances have properties, the characteristic
13、s that give each substance its unique identity. We learn about matter by observing its properties. To identify a substance, chemists observe two distinct types of properties, physical and chemical, which are closely related to two types of change that matter undergoes. Physical properties are thos
14、e that a substance shows by itself, without changing into or interacting with another substance. Some physical properties are color, smell, temperature, boiling point, electrical conductivity, and density. A physical change is a change that does not alter the chemical identity of the matter. A physi
15、cal change results in different physical properties. For example, when ice melts, several physical properties have changed, such as hardness, density, and ability to flow. But the sample has not changed its composition: it is still water. Chemical properties are those that do change the chemical n
16、ature of matter. A chemical change, also called a chemical reaction, is a change that does alter the chemical identity of the substance. It occurs when a substance (or substances) is converted into a different substance (or substances). For example, when hydrogen burns in air, it undergoes a chemica
17、l change because it combines with oxygen to form water. Separation of Mixtures The separation of mixtures into its constituents in a pure state is an important process in chemistry. The constituents of any mixture can be separated on the basis of their differences in their physical and chemical pr
18、operties, e.g., particle size, solubility, effect of heat, acidity or basicity etc. Some of the methods for separation of mixtures are: (1) Sedimentation or decantation. To separate the mixture of coarse particles of a solid from a liquid e.g., muddy river water. (2) Filtration. To separate the
19、insoluble solid component of a mixture from the liquid completely i.e. separating the precipitate (solid phase) from any solution. (3) Evaporation. To separate a non-volatile soluble salt from a liquid or recover the soluble solid solute from the solution. (4) Crystallization. To separate a solid
20、compound in pure and geometrical form. (5) Sublimation. To separate volatile solids, from a non-volatile solid. (6) Distillation. To separate the constituents of a liquid mixture, which differ in their boiling points. (7) Solvent extraction method. Organic compounds, which are easily soluble in o
21、rganic solvents but insoluble or immiscible with water forming two separate layers can be easily separated. 1.3 Atoms, Molecules and Compounds The fundamental unit of a chemical substance is called an atom. The word is derived from the Greek atomos, meaning “undivisible” or “uncuttable”. An atom i
22、s the smallest possible particle of a substance. Molecule is the smallest particle of a substance that retains the chemical and physical properties of the substance and is composed of two or more atoms; a group of like or different atoms held together by chemical forces. A molecule may consist of a
23、toms of a single chemical element, as with oxygen (O2), or of different elements, as with water (H2O). A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. The term is also used to refer to
24、a pure chemical substance composed of atoms with the same number of protons. Until March 2010, 118 elements have been observed. 94 elements occur naturally on earth, either as the pure element or more commonly as a component in compounds. 80 elements have stable isotopes, namely all elements with at
25、omic numbers 1 to 82, except elements 43 and 61 (technetium and promethium). Elements with atomic numbers 83 or higher (bismuth and above) are inherently unstable, and undergo radioactive decay. The elements from atomic number 83 to 94 have no stable nuclei, but are nevertheless found in nature, eit
26、her surviving as remnants of the primordial stellar nucleosynthesis that produced the elements in the solar system, or else produced as short-lived daughter-isotopes through the natural decay of uranium and thorium. The remaining 24 elements so are artificial, or synthetic, elements, which are produ
27、cts of man-induced processes. These synthetic elements are all characteristically unstable. Although they have not been found in nature, it is conceivable that in the early history of the earth, these and possibly other unknown elements may have been present. Their unstable nature could have resulte
28、d in their disappearance from the natural components of the earth, however. The naturally occurring elements were not all discovered at the same time. Some, such as gold, silver, iron, lead, and copper, have been known since the days of earliest civilizations. Others, such as helium, radium, alumin
29、ium, and bromine, were discovered in the nineteenth century. The most abundant elements found in the earth’s crust, in order of decreasing percentage, are oxygen, silicon, aluminium, and iron. Others present in amounts of 1% or more are calcium, sodium, potassium, and magnesium. Together, these repr
30、esent about 98.5% of the earth’s crust. The nomenclature and their origins of all known elements will be described in Chapter 2. A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reaction
31、s. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together in a defined spatial arrangement by chemical bonds. Compounds that exist as molecules are called molecular compounds. An ionic compound is a chemical compound in which io
32、ns are held together in a lattice structure by ionic bonds. Usually, the positively charged portion consists of metal cations and the negatively charged portion is an anion or polyatomic ion. The relative amounts of the elements in a particular compound do not change: Every molecule of a particular
33、 chemical substance contains a characteristic number of atoms of its constituent elements. For example, every water molecule contains two hydrogen atoms and one oxygen atom. To describe this atomic composition, chemists write the chemical formula for water as H2O. The chemical formula for water sho
34、ws how formulas are constructed. The formula lists the symbols of all elements found in the compound, in this case H (hydrogen) and O (oxygen). A subscript number after an element's symbol denotes how many atoms of that element are present in the molecule. The subscript 2 in the formula for water in
35、dicates that each molecule contains two hydrogen atoms. No subscript is used when only one atom is present, as is the case for the oxygen atom in a water molecule. Atoms are indivisible, so molecules always contain whole numbers of atoms. Consequently, the subscripts in chemical formulas of molecula
36、r substances are always integers. We explore chemical formulas in greater detail in Chapter 2. The simple formula that gives the simplest whole number ratio between the atoms of the various elements present in the compound is called its empirical formula. The simplest formula that gives the actual
37、number of atoms of the various elements present in a molecule of any compound is called its molecular formula. Elemental analysis is an experiment that determines the amount (typically a weight percent) of an element in a compound. The elemental analysis permits determination of the empirical formul
38、a, and the molecular weight and elemental analysis permit determination of the molecular formula. 1.4. Numbers in Physical Quantities 1.4.1. Measurement 1. Physical Quantities Physical properties such as height, volume, and temperature that can be measured are called physical quantity. A numbe
39、r and a unit of defined size are required to describe physical quantity, for example, 10 meters, 9 kilograms. 2. Exact Numbers Exact Numbers are numbers known with certainty. They have unlimited number of significant figures. They arise by directly counting numbers, for example, the number of side
40、s on a square, or by definition: 1 m = 100 cm, 1 kg = 1000 g 1 L = 1000 mL, 1 minute = 60 seconds 3. Uncertainty in Measurement Numbers that result from measurements are never exact. Every experimental measurement, no matter how precise, has a degree of uncertainty to it because there is a limit
41、 to the number of digits that can be determined. There is always some degree of uncertainty due to experimental errors: limitations of the measuring instrument, variations in how each individual makes measurements, or other conditions of the experiment. Precision and Accuracy In the fields of engi
42、neering, industry and statistics, the accuracy of a measurement system is the degree of closeness of measurements results to its actual (true) value. The precision of a measurement system, also called reproducibility or repeatability, is the degree to which repeated measurements under unchanged cond
43、itions show the same results. Although the two words can be synonymous in colloquial use, they are deliberately contrasted in the context of the scientific method. A measurement system can be accurate but not precise, precise but not accurate, neither, or both. A measurement system is called valid
44、if it is both accurate and precise. Related terms are bias (non-random or directed effects caused by a factor or factors unrelated by the independent variable) and error (random variability), respectively. Random errors result from uncontrolled variables in an experiment and affect precision; system
45、atic errors can be assigned to definite causes and affect accuracy. For example, if an experiment contains a systematic error, then increasing the sample size generally increases precision but does not improve accuracy. Eliminating the systematic error improves accuracy but does not change precision
46、 1.4.2 Significant Figures The number of digits reported in a measurement reflects the accuracy of the measurement and the precision of the measuring device. Significant figures in a number include all of the digits that are known with certainty, plus the first digit to the right that has an unc
47、ertain value. For example, the uncertainty in the mass of a powder sample, i.e., 3.1267g as read from an “analytical balance” is ± 0.0001g. In any calculation, the results are reported to the fewest significant figures (for multiplication and division) or fewest decimal places (addition and subtrac
48、tion). 1. Rules for deciding the number of significant figures in a measured quantity: The number of significant figures is found by counting from left to right, beginning with the first nonzero digit and ending with the digit that has the uncertain value, e.g., 459 (3) 0.206 (3) 2.17(3) 0
49、00693 (3) 25.6 (3) 7390 (3) 7390. (4) (1) All nonzero digits are significant, e.g., 1.234 g has 4 significant figures, 1.2 g has 2 significant figures. (2) Zeroes between nonzero digits are significant: e.g., 1002 kg has 4 significant figures, 3.07 mL has 3 significant figures. (3) Leadin
50、g zeros to the left of the first nonzero digits are not significant; such zeroes merely indicate the position of the decimal point: e.g., 0.001 m has only 1 significant figure, 0.012 g has 2 significant figures. (4) Trailing zeroes that are also to the right of a decimal point in a number are sign
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