So yesterday we So yesterday we talked about talked about

experiments, but…experiments, but…

Experiments Don’t Always Go Experiments Don’t Always Go According to PlanAccording to Plan

►Accidental DiscoveriesAccidental Discoveries TeflonTeflon (page 48) (page 48) Mauve clothing dye Mauve clothing dye (Page 49)(Page 49)

Theories and LawsTheories and Laws

►When a hypothesis withstands When a hypothesis withstands repeated testing, it may become a repeated testing, it may become a theorytheory – a well-tested explanation of – a well-tested explanation of observationsobservations Theories are Theories are explanationsexplanations, not facts, not facts They can be They can be disproveddisproved, but can , but can

never be completely never be completely provenproven..►Law is a Law is a statementstatement or mathematical or mathematical

expressionexpression that reliably that reliably describesdescribes a a behavior in the natural worldbehavior in the natural world

The main differenceThe main difference

While a theory is an While a theory is an attempt to attempt to explainexplain the the

cause of certain events in cause of certain events in the natural world, a the natural world, a

scientific law scientific law describesdescribes the the eventsevents

ModelsModels

►A model is a A model is a representationrepresentation of an of an object, a system, a process, or an idea.object, a system, a process, or an idea.

►A model is used to help A model is used to help describedescribe or or communicatecommunicate information about ideas information about ideas or theories that are difficult to observe or theories that are difficult to observe or explain otherwise.or explain otherwise.

Dimensional AnalysisDimensional Analysis

Using Using ConversionConversion Factors Factors

SystSystèème Internationale d’Unitme Internationale d’Unitèès s UnitsUnits

► There are There are 77 SI Base Units SI Base Units

QuantityQuantity SymboSymboll

UnitUnit Abbr.Abbr.

LengthLength ll metermeter mm

MassMass mm kilogramkilogram kgkg

TimeTime tt secondsecond ss

ThermodynamicThermodynamic

temperaturetemperatureTT kelvinkelvin KK

Amount of a Amount of a substancesubstance

nn molemole molmol

Electric currentElectric current II ampereampere AA

Luminous intensityLuminous intensity IIvv candelacandela cdcd

Derived UnitsDerived Units

►These units are derived by These units are derived by multiplyingmultiplying or or dividingdividing the base units. the base units.

►ExamplesExamples Speed – Speed – m/sm/s Area – Area – mm22

Volume – Volume – mm33, but scientist tend to , but scientist tend to use the use the LL or or mLmL *1cm *1cm3 3 = = 11mLmL

Density = Density = g/mLg/mL (D=m/V) (D=m/V)

Example Density ProblemsExample Density Problems(from page 858)(from page 858)

16) What is the density of an 84.7 g sample of an 16) What is the density of an 84.7 g sample of an unknown substance if the sample occupies 49.6 unknown substance if the sample occupies 49.6 cmcm33??

17) What volume would be occupied by 7.75 g of 17) What volume would be occupied by 7.75 g of a substance with a density of 1.70766 g/cma substance with a density of 1.70766 g/cm33??

Somethings…Somethings…

►These base units may be too These base units may be too largelarge or or smallsmall depending on the situation depending on the situation

►Therefore we need to use Therefore we need to use dimensional dimensional analysisanalysis – conversion factors – conversion factors

►For this we need:For this we need: Prefixes (page 13; Table 3) (page 13; Table 3)

Helpful Chart to rememberHelpful Chart to remember

SmallerSmaller BiggerBigger

decideci (d)(d) 1010 deca (da)deca (da)

centicenti (c) (c) 100100 (hecto, h)(hecto, h)

millimilli (m)(m) 10001000 kilo (k)kilo (k)

micro (micro ()) 10000001000000 mega (M)mega (M)

nanonano (n) (n) 10000000001000000000 giga (G)giga (G)

EqualitiesEqualitiesSometimes more than Sometimes more than

oneone

Let us look at some Let us look at some examples examples

0.765 g

kg

0.765 g to kg

xkgg

1) Rewrite what is given2) Set up the equalities so that the unit you start with is canceled out and the new one is left.3) Fill in the numerical values

4) Multiply by the top numbers; divide by the bottom numbers

1000

1= 0.00076

5

1000

Sometimes 2 steps are Sometimes 2 steps are neededneeded

23745 kg

xkg

g

1=

23745000000 mg

xmg

g

1000

1

23745 kg to milligrams

Answers to ExamplesAnswers to Examples

►0.950 kg0.950 kg►4.5 m4.5 m►3.5 dL3.5 dL►60000 MB60000 MB►0.15 L (remember 1cm0.15 L (remember 1cm33 = 1mL) = 1mL)