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UN IVERSITY INDONESIAOF

Introduction:Matter and Measurement

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Introduction: Matterand Measurement

The Study of ChemistryClassification of Matter

Properties of MatterUnits of MeasurementUncertainty in Measurement

Dimensional Analysis

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UN IVERSITY INDONESIAOFTh e Study of

Ch emistry

Ch emistry is the study of matter and thechanges that matter undergoes.

observation p hypothesis p theory

o q

n n n n n n n

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Ch emistry

A. Scientific Method - systematic approach to research.1. Experimentation.2. Hypothesis - an interpretation that explains the results of many

experiments.3. Theory - consistent explanation of known observations; logical

interpretations of experimental results.

observation phypothesis

ptheory

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n n n n n n n

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Ch emistry

Matter is made up of almost infinitesimallysmall building blocks called atoms .

Atoms can combine together to formmolecules .Element - a fundamental substance that can'tbe chemically changed or broken down intoanything simpler.

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Ch emistry

Chemical symbol - used to represent specificelements

capitalize the first letter; if second letter ispresent, uselower case

Periodic Table - a tabular organization of all115 elements.

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Ch emistry

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Ch emistry

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Ch emistry

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Ch emistry

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Ch emistry

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Classification of Matter

Matter can exist in one of three states of matter : a gas, a liquid, or a solid. A gas is highly compressible and will assume both

the shape and the volume of its container. A liquid is not compressible and will assume the

shape but not the volume of its container.

A solid also is not compressible, and it has a fixedvolume and shape of its own.

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Matter can also be classified according to itscomposition.

Most of the matter that we encounter exists inmixtures , which are combinations of two ormore substances.

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Mixtures can be h omo geneous orh etero geneous .

Mixtures can be separated into p ure substances , and pure substances can beeither

com p ounds or

elements .

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A familiar example of a mixture is salt water.A sample of salt water has the same

composition throughout.It can be separated into pure substanceswater and ordinary table salt by a physicalprocess, such as distillation.

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Pure water is collected in the flask on the right.When all of the water has been distilled from the

mixture, pure salt NaCl will remain in the flaskon the left.

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B oth water and salt are pure substances.They cannot be further separated into simpler substances

by any physical process.Each, however, can be decomposed into other substances

by a chemical process, namely electrolysis.

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Electrolysis

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The substances produced by the electrolysis of water cannot be further separated by anyphysical or chemical means.Oxygen and hydrogen are elements .When water is separated into its constituentelements, the relative amounts of thoseelements are always the same.

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Water is 11 percent hydrogen and 89 percentoxygen by mass.

This is an example of the law of constantcom p osition , also known as the law of definite p ro p ortions .

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Salt can also be separated into its constituentelements, sodium and chlorine, byelectrolysis .Sodium chloride also has a constantcomposition, as do all pure substances. It is 39percent sodium and 61 percent chlorine bymass.

DEMO01-2.MOV

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Pro p erties of Matter

Different types of matter have differentdistinguishing characteristics that we can useto tell them apart.These characteristics are called p h ysicalp ro p erties and ch emical p ro p erties .Physical and chemical properties may beintensive or extensive .

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Intensive properties such as density, color, andboiling point do not depend on the size of thesample of matter and can be used to identifysubstances.Extensive properties such as mass and volumedo depend on the quantity of the sample.

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Physical properties are those that we candetermine without changing the identity of thesubstance we are studying.

For instance, we can observe or measure thephysical properties of sodium metal. It is a soft, lustrous, silver-colored metal with a

relatively low melting point and low density. Hardness, color, melting point and density are all

physical properties. Figure 7.15 shows a chunk of metallic sodium, which issoft enough to be cut with a knife.

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Chemical properties describe the way asubstance can change or react to form othersubstances.These properties, then, must be determinedusing a process that changes the identity of the substance of interest.

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One of the chemical properties of alkali metalssuch as sodium and potassium is that theyreact with water.To determine this, though, we would have tocombine an alkali metal with water andobserve what happens.

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Section 1.3

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Sodium metal (Na) reacts rather vigorouslywith water to produce sodium hydroxide(NaOH) and hydrogen gas (H 2).After the reaction has occurred, although wenow have evidence of one of sodium metal'schemical properties, we no longer havesodium metal.

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Potassium reacts even more vigorously withwater to produce potassium hydroxide (KOH)and hydrogen gas.

As with sodium, once we have determined achemical property of potassium metal, we nolonger have potassium metal.

To determine the chemical properties of asubstance, it is necessary to change thesubstance's chemical identity.

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The changes undergone by sodium andpotassium when they react with water arech emical c h an ges , also known as ch emicalreactions .Matter can also undergo p h ysical c h an ges inwhich the chemical identity of the matterdoes not change.

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One example of a physical change is themelting of a solid.

When ice melts, it changes from a solid stateto a liquid state, but its chemical identity(H2O) is unchanged.Allch an ges of state are physical changes.

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Elements and Th ePeriodic Table

Periodic table grid of the elements arranged in 7horizontal rows and 18 vertical columns.Periods seven horizontal rows in the periodic table.

Groups - 18 vertical columns in the periodic table. Groups numbered 1A p 8A and 1 B p 8B (or 1 p 18). Actually have 32 groups

lanthanides (14 elements after lanthanum) and actinides (14elements after actinium) are not included in the group numbers.

The elements in a given group have similar chemicalproperties.

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The periodic table of the elements is the mostimportant organizing principle of chemistry. Regular progression in the size of the seven periods.

reflects a similar regularity in atomic structure Main Group (or Representative) Elements - Groups 1A- 8A; (two larger groups on the left and the six largergroups on the right of the table).

Transition-metal Elements - Groups 1 B - 8B ; (the 10smaller groups in the middle of the table).

Inner transition-metal (or Rare Earth) - the 14 groupsshown separately at the bottom of the table.

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Property - any characteristic that can be used todescribe or identify matter. Physical properties - characteristics that can be

determined without changing the chemical makeup of the sample. Chemical properties - properties that do change the

chemical makeup of the sample. Intensive properties - properties that do not depend

on the size of the sample. Extensive properties - properties that depend on the

size of the sample.

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Groups of elements show similarities inchemical properties. Group 1A - Alkali metals; lustrous, silvery metals;

react rapidly with water to form highly alkalineproducts.

Group 2A - Alkaline earth metals; lustrous, silverymetals; less reactive than alkali metals.

Group 7A - Halogens; corrosive, nonmetallicelements; salt formers.

Group 8A - Noble gases; gases with low reactivity.

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Three major classes of elements in the periodictable.1. Metals - largest category of elements; found on the

left side of the periodic table (left of the heavyzigzag line).

2. Nonmetals - found on the right side of the periodictable (right of the heavy zigzag line).

3. Semimetals (metalloids) - elements adjacent to thezigzag boundary between metals and nonmetals.

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Metals solids (except mercury) Malleable ductile - can be drawn into thin wires without

breaking conduct heat and electricity

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Nonmetals gases, liquids or solids brightly colored brittle solids poor conductors of heat and electricity

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Semimetals (metalloids) properties fall between metals and nonmetals brittle poor conductors of heat and electricity

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Units of Measurement

The scientific community uses SI units formeasurement of such properties as mass,length, and temperature.There are seven SI base units from which allother necessary units are derived.

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Although the meter is the base SI unit used forlength, it may not be convenient to report thelength of an extremely small object or an

extremely large object in units of meters.Decimal prefixes allow us to choose a unit that isappropriate to the quantity being measured.Thus, a very small object might best be measuredin millimeters (1 millimeter = 0.001 meters),while a large distance might best be measured inkilometers (1 kilometer = 1000 meters).

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The SI unit of temperature is the kelvin,although the Celsius scale is also commonlyused.The Kelvin scale is known as the absolutetemperature scale, with 0 K being the lowesttheoretically attainable temperature.

K =oC + 273.15

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Note that there are no units of volume inTable 1.4.For measurements of volume, density, andother properties, we must derive the desiredunits from SI base units.In the case of volume, which has units of length cubed, (length) 3, the basic SI unit forvolume is the cubic meter (m 3).

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This is an extremely large volume, though, andmore often you will see volumes reported inliters, L (1 cubic decimeter, or 1 dm 3), or

milliliters, mL (which are the same as cubiccentimeters: 1 mL = 1 cm 3).Density has units of mass per unit volume and isoften reported as grams per cubic centimeter,

g/cm3.

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Uncertainty inMeasurement

Even the most carefully taken measurementsare always inexact.This can be a consequence of inaccuratelycalibrated instruments, human error, or anynumber of other factors.

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Two terms are used to describe the quality of measurements: p recision and accuracy .Precision is a measure of how closely individualmeasurements agree with one another.Accuracy refers to how closely individuallymeasured numbers agree with the correct or

"true" value.

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Whatever the source, all measurementscontain error.Thus, all measured numbers containuncertainty.It is important that these numbers bereported in such a way as to convey themagnitude of this uncertainty.

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Consider a fourth-grade student who, whenasked by his teacher how old the Earth is,replies "Four billion and three years old." (The student had been told by a first-grade

teacher three years earlier that the Earth was fourbillion years old.)

Obviously, we don't know the age of Earth tothe year, so it is not appropriate to report anumber that suggests we do.

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In order to convey the appropriate uncertaintyin a reported number, we must report it to thecorrect number of significant fi gures .The number 83.4 has three digits. All three digits are significant. The 8 and the 3 are "certain digits" while the 4 is

the "uncertain digit.

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As written, this number implies uncertainty of plus or minus 0.1, or error of 1 part in 834.Thus, measured quantities are generallyreported in such a way that only the last digitis uncertain.All digits, including the uncertain one, arecalled significant figures.

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G uidelines Nonzero digits are always significant 457 cm (3

significant figures); 2.5 g (2 significant figures).

Zeros between nonzero digits are alwayssignificant 1005 kg (4 significant figures); 1.03 cm(3 significant figures).

Zeros at the beginning of a number are never

significant; they merely indicate the position of the decimal point 0.02 g (one significant figure);0.0026 cm (2 significant figures).

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G uidelines Zeros that fall at the end of a number or after the

decimal point are always significant 0.0200 g (3significant figures); 3.0 cm (2 significant figures).

When a number ends in zeros but contains nodecimal point, the zeros may or may not besignificant 130 cm (2 or 3 significant figures);10,300 g (3, 4, or 5 significant figures).

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To avoid ambiguity with regard to the number of significant figures in a number with tailing zerosbut no decimal point, such as 700, we usescientific (or exponential) notation to express thenumber.If we are reporting the number 700 to threesignificant figures, we can leave it written as it is,or we can express it as 7.00 10 2.

There is no ambiguity in the latter regarding thenumber of significant figures, because zeros aftera decimal point are always significant.

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However, if there really should be only two significantfigures, we can express this number as 7.0 x10 2.Likewise, if there should be only one significant figure,we can write 7 x10 2.Scientific notation is convenient for expressing theappropriate number of significant figures.It is also useful to report extremely large and extremelysmall numbers.It would be most inconvenient for us to have to writeall of the zeros in the number 1.91 10 -24(0.00000000000000000000000191).


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When measured numbers are used in a calculation, thefinal answer cannot have any greater certainty than themeasured numbers that went into the calculation.In other words, the precision of the result is limited bythe precision of the measurements used to obtain thatresult.For example: If we measure the length of one side of acube and find it to be 1.35 cm; and we then calculatethe volume of the cube using this measured length, weget an answer of 2.460375 cm 3.


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Our original measurement had three significantfigures.The implied uncertainty in 1.35 is 1 part in 135.

If we report the volume of the cube to sevensignificant figures, we are implying an uncertaintyof 1 part in over two million!We can't do that.

In order to report results of calculations so as toimply a realistic degree of uncertainty, we mustfollow the following rules.


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When multiplying or dividing measurednumbers, the answer must have the samenumber of significant figures as the measured

number with the fewest significant figures.When adding or subtracting, the answer canhave only as many places to the right of thedecimal point as the measured number withthe smallest number of places to the right of the decimal point.


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Using these rules, we would report thevolume of the cube in the example above as2.46 cm 3.


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Solving problems in chemistry requires carefulmanipulation of numbers and their associatedunits, a method known as dimensional

analysis .For example: What is the volume of a 5.25-gram sample of a liquid with density 1.23g/mL?The density of the liquid can be used as aconversion factor .


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For the liquid in the example, 1.23 grams areequal to 1 milliliter (1 mL).When the numerator and denominator of afraction are equal, the fraction has a value of 1, meaning that we can multiply by it for thepurpose of changing units.


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The density conversion factor can beexpressed in either of the following two ways.


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