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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:
4
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
43
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
53
Classwork Gradetextbook page 42 #5, p. 57 # 7-9P. 60 # 28-30