Chapter 2(read pp. 29-37)The Scientific Method and

Units of MeasurementTest is Friday Aug 31st

Scientific Method A logical approach to solving problems

by observing and collecting data, formulating hypotheses, testing hypotheses, and formulating theories supported by data.

Observations-using senses to obtain information

Descriptive data – qualitative Numerical data – quantitative Examples of qualitative and quantitative:

Scientific Method Inferences – interpretations or

explanations Experiment- carrying out a procedure

under controlled conditions to make observations and collect data

Chemists study systems – a specific portion of matter in a given region of space that has been selected for study during an experiment

Examples of a system:

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Scientific Method• Scientists use generalizations about

data collected to formulate hypotheses.• Hypothesis- a testable statement,

serves as a basis for further experimenting

• only two possible answers

•hypothesis is right

•hypothesis is wrong• Modify hypothesis - repeat the cycle

Observations

Hypothesis

Experiment

Cycle repeats many times.

The hypothesis gets more and more certain.

Becomes a theory - A broad generalization that explains a body of facts or phenomenon

A model may be developed to support the theory

Theories are useful because they predict results of new experiments

Help us form mental pictures of processes (models)

Observations

Hypothesis

Experiment

Another outcome is that certain behavior is repeated many times

Scientific Law is developed

Description of how things behave

Law - how Theory- why

Observations

Hypothesis

Experiment

Law

Theory(Model)

Prediction

Experiment

Modify

Observations

Hypothesis

Experiment

The Metric System/SI System

An easy way to measure

Measurements are quantitative information

Units Matter

Measuring – number + unit The numbers are only half of a

measurement Recipe: 1 salt, 3 sugar, 2 flour ??? Numbers without units are meaningless. How many feet in a yard A mile A rod

The Metric System Easier to use because it is a decimal

system Every conversion is by some power of

10. A metric unit has two parts A prefix and a base unit. prefix tells you how many times to

divide or multiply by 10.

Prefixes Tera- T 1,000,000,000,000 1012

giga- G 1,000,000,000 109

mega - M 1,000,000 106

kilo - k 1,000 103

deci- d 0.1 10-1

centi- c 0.01 10-2

milli- m 0.001 10-3

micro- 0.000001 10-6

nano-n 0.000000001 10-9

pico- p 0.000000000001 10-12

Base Units Length - meter - m Mass - gram – g Time - second - s Temperature – Kelvin - K

– Celsius º C Energy - Joules- J Volume - Liter - L Amount of substance - mole – mol

Mass is the amount of matter in an

object. Tool - balance scale Standard SI unit – kilogram Base unit - gram Common units = g,mg, g, kg Weight – pull of gravity on matter

LengthThe distance between two

pointsTool – metric rulerStandard unit - meterCommon units – mm, cm, m,

km

Derived UnitsMany SI units are combinations

of base units called derived units

Examples we will use at this time are volume and density

Volume The amount of space an object

occupies V = L x W x H Tools – metric ruler, graduated cylinder,

buret, volumetric flask SI unit - m3

1 Liter = 1 dm3 1 mL = 1 cm3 = 1 cc

Using Scientific Measurements (pp. 44-52)

All measurements have a certain degree of uncertainty

Uncertainty can result in limitations that depend on the instrument or the experimenter

Scientists use two word to describe how good the measurements are

How good are the measurements?

Accuracy- how close the measurement is to the actual value

Precision- how closely the numerical values of a set of measurements agree with each other

Differences Accuracy can be true of an individual

measurement or the average of several Precision requires several

measurements before anything can be said about it

There can be precision without accuracy

There can be no accuracy without precision

Let’s use a golf anaolgy

Accurate? No

Precise? Yes

Accurate? Yes

Precise? Yes

Precise? No

Accurate? No

In terms of measurement Three students measure

the room to be 10.2 m, 10.3 m and 10.4 m across.

Were they precise? Were they accurate?

Percent ErrorAccuracy is judged using percent error.

The formula is:

Actual Value – Experimental Value x 100

Actual Value

Significant figures (sig figs)Scientists record

measurements in significant figures.

Sig figs consist of all the digits known with certainty plus a final digit that is estimated.

Significant figures (sig figs) When using measuring devices, the

location of the estimated digit depends on the smallest division on the scale

21 3 4 5

Significant figures (sig figs) The more marks the better we can

estimate. Scientist always understand that the

last number recorded is actually an estimate

21 3 4 5

Rules for Determining Sig Figs

All nonzero digits are significantExact numbers (from counting

or definitions) do not limit sig figs

All zeros between nonzero digits are significant

Rules for Determining Sig Figs

All zeros to the right of a decimal point and after a nonzero digit are significant

Zeros used for placing the decimal point are not significant

Atlantic/Pacific Rule for Determining Sig Figs

If a decimal point is Present, count from the Pacific side

If a decimal point is Absent, count from the Atlantic Side

Begin counting with the first nonzero digit you come to and then keep counting

Sig figs. How many sig figs in the following

measurements? 458 g 3500 g 4085 g 0.057010 m 4850 g 0.0485 g 0.004085 g 40.004085 g

Sig Figs. 405.0 g 4050 g 0.450 g 4050.05 g 0.0500060 g Next we learn the rules for calculations

Adding and subtracting with sig figs

Round the answer so that the estimated digit is in the same place value as the least precise measurement

For example

27.93 6.4+ First line up the decimal places

27.936.4+

Then do the adding

34.33Find the estimated numbers in the problem

27.93 6.4

This answer must be rounded to the tenths place

Rounding rules look at the number behind the one

you’re rounding. If it is 0 to 4 don’t change it If it is 5 to 9 make it one bigger round 45.462 to four sig figs to three sig figs to two sig figs to one sig fig

Multiplication and Division The answer should have the same

number of significant figures as the measurement with the least number of sig figs

3.6 x 653 2350.8 3.6 has 2 s.f. 653 has 3 s.f. answer can only have 2 s.f. 2400

Practice 4.8 + 6.8765 520 + 94.98 0.0045 + 2.113 6.0 - 3.82 5.4 - 3.28 6.7 - .542 500 -126 6.01 - 3.8

Multiplication and Division

4.5 / 6.245 4.5 x 6.245 9.8764 x .043 3.876 / 1983 16547 / 714

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HomeworkWorkbook – p. 25 – 26# 1,2,3,4,8,10,16

Scientific Notation Shorthand technique used by scientists

to write extremely small or large numbers

The form is:

M x 10n

M is a number greater than or equal to 1 but less than 10. The exponent, n, is a positive or negative integer

Examples and Practice 7400 m 328 500 g 0.00900 kg .00705 cm 0.002 m 6.3 x 104 cm 5.42 x 105 g 12.25 x 102 cm 6.2 x 10-2 g

Dimensional AnalysisA problem solving method that treats units in calculations as algebraic factorsUnits common to both numerators and denominators are cancelled and removed from the expressionsA conversion factors is used to convert from one unit to the otherExact conversions do not limit significant figures

Density D = M / V An intensive property (it is

unaffected by the size of the sample)

Density is often used to identify substances.

Common units - g/ cm3, g/mL, g/L Tools? -

Density As the mass of the substance

increases the volume increases proportionately and the ratio of mass to volume (density) is constant

This is a direct proportion therefore the graph is a straight line that passes through the origin. (See p. 55)

Density Because most substances expand

with an increase in temperature (increasing the volume), density usually decreases with increasing volume.

Density varies with temperature

Density of water1 g of water is 1 mL of water.density of water is 1 g/mL (at

4ºC)Specific gravity - the density of

an object compared to the density of water

Specific gravity of water is 1.0

Measuring Temperature

The average kinetic energy of the particles in a sample of matter

Celsius scale water freezes at 0ºC water boils at 100ºC body temperature 37ºC room temperature 20 - 25ºC

0ºC

Measuring Temperature Kelvin starts at absolute zero (-273 º

C) degrees are the same size C = K -273 K = C + 273 Kelvin is always bigger. Kelvin can never be negative.

273 K

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Classwork Gradetextbook page 42 #5, p. 57 # 7-9P. 60 # 28-30