Chapter 2 (read pp ) The Scientific Method and Units of Measurement Test is Friday Aug 31st

Scientific Method l A logical approach to solving problems by observing and collecting data, formulating hypotheses, testing hypotheses, and formulating theories supported by data. l Observations-using senses to obtain information l Descriptive data – qualitative l Numerical data – quantitative l Examples of qualitative and quantitative:

Scientific Method l Inferences – interpretations or explanations l Experiment- carrying out a procedure under controlled conditions to make observations and collect data l Chemists study systems – a specific portion of matter in a given region of space that has been selected for study during an experiment l 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 l Cycle repeats many times. l The hypothesis gets more and more certain. l Becomes a theory - A broad generalization that explains a body of facts or phenomenon l A model may be developed to support the theory

l Theories are useful because they predict results of new experiments l Help us form mental pictures of processes (models) Observations Hypothesis Experiment

l Another outcome is that certain behavior is repeated many times l Scientific Law is developed l Description of how things behave l Law - how l 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 l The numbers are only half of a measurement l Recipe: 1 salt, 3 sugar, 2 flour ??? l Numbers without units are meaningless. l How many feet in a yard l A mile l A rod

The Metric System l Easier to use because it is a decimal system l Every conversion is by some power of 10. l A metric unit has two parts l A prefix and a base unit. l prefix tells you how many times to divide or multiply by 10.

Prefixes l Tera-T1,000,000,000, l giga- G 1,000,000, l mega - M 1,000, l kilo - k 1, l deci-d l centi-c l milli-m micro-  l nano-n l pico-p

Base Units l Length - meter - m l Mass - gram – g l Time - second - s l Temperature – Kelvin - K –Celsius º C l Energy - Joules- J l Volume - Liter - L l Amount of substance - mole – mol

Mass l is the amount of matter in an object. l Tool - balance scale l Standard SI unit – kilogram l Base unit - gram Common units =  g,mg, g, kg l Weight – pull of gravity on matter

Length l The distance between two points l Tool – metric ruler l Standard unit - meter l Common units – mm, cm, m, km

Derived Units l Many SI units are combinations of base units called derived units l Examples we will use at this time are volume and density

Volume l The amount of space an object occupies l V = L x W x H l Tools – metric ruler, graduated cylinder, buret, volumetric flask l SI unit - m 3 l 1 Liter = 1 dm 3 l 1 mL = 1 cm 3 = 1 cc

Using Scientific Measurements (pp ) l All measurements have a certain degree of uncertainty l Uncertainty can result in limitations that depend on the instrument or the experimenter l Scientists use two word to describe how good the measurements are

How good are the measurements? l Accuracy- how close the measurement is to the actual value l Precision- how closely the numerical values of a set of measurements agree with each other

Differences l Accuracy can be true of an individual measurement or the average of several l Precision requires several measurements before anything can be said about it l There can be precision without accuracy l 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

Accurate?Yes Precise?Somewhat

In terms of measurement l Three students measure the room to be 10.2 m, 10.3 m and 10.4 m across. l Were they precise? l Were they accurate?

Percent Error Accuracy is judged using percent error. The formula is: Actual Value – Experimental Value x 100 Actual Value

Significant figures (sig figs) l Scientists record measurements in significant figures. l Sig figs consist of all the digits known with certainty plus a final digit that is estimated.

Significant figures (sig figs) l When using measuring devices, the location of the estimated digit depends on the smallest division on the scale 21345

Significant figures (sig figs) l The more marks the better we can estimate. l Scientist always understand that the last number recorded is actually an estimate 21345

Rules for Determining Sig Figs l All nonzero digits are significant l Exact numbers (from counting or definitions) do not limit sig figs l All zeros between nonzero digits are significant

Rules for Determining Sig Figs l All zeros to the right of a decimal point and after a nonzero digit are significant l Zeros used for placing the decimal point are not significant

Atlantic/Pacific Rule for Determining Sig Figs l If a decimal point is Present, count from the Pacific side l If a decimal point is Absent, count from the Atlantic Side l Begin counting with the first nonzero digit you come to and then keep counting

Sig figs. l How many sig figs in the following measurements? l 458 g3500 g l 4085 g m l 4850 g l g l g l g

Sig Figs. l g l 4050 g l g l g l g l Next we learn the rules for calculations

Adding and subtracting with sig figs l Round the answer so that the estimated digit is in the same place value as the least precise measurement

For example l First line up the decimal places Then do the adding Find the estimated numbers in the problem This answer must be rounded to the tenths place

Rounding rules l look at the number behind the one you’re rounding. l If it is 0 to 4 don’t change it l If it is 5 to 9 make it one bigger l round to four sig figs l to three sig figs l to two sig figs l to one sig fig

Multiplication and Division l The answer should have the same number of significant figures as the measurement with the least number of sig figs l 3.6 x 653 l l 3.6 has 2 s.f. 653 has 3 s.f. l answer can only have 2 s.f. l 2400

Practice l l l l l l l l

Multiplication and Division l 4.5 / l 4.5 x l x.043 l / 1983 l / 714

43 Homework l Workbook – p. 25 – 26 l # 1,2,3,4,8,10,16

Scientific Notation l Shorthand technique used by scientists to write extremely small or large numbers The form is: M x 10 n 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 l 7400 m l g l kg l cm l m l 6.3 x 10 4 cm l 5.42 x 10 5 g l x 10 2 cm l 6.2 x g

Dimensional Analysis lA problem solving method that treats units in calculations as algebraic factors lUnits common to both numerators and denominators are cancelled and removed from the expressions lA conversion factors is used to convert from one unit to the other lExact conversions do not limit significant figures

Density l D = M / V l An intensive property (it is unaffected by the size of the sample) l Density is often used to identify substances. l Common units - g/ cm 3, g/mL, g/L l Tools? -

Density l As the mass of the substance increases the volume increases proportionately and the ratio of mass to volume (density) is constant l This is a direct proportion therefore the graph is a straight line that passes through the origin. (See p. 55)

Density l Because most substances expand with an increase in temperature (increasing the volume), density usually decreases with increasing volume. l Density varies with temperature

Density of water l 1 g of water is 1 mL of water. l density of water is 1 g/mL (at 4ºC) l Specific gravity - the density of an object compared to the density of water l Specific gravity of water is 1.0

Measuring Temperature l The average kinetic energy of the particles in a sample of matter l Celsius scale l water freezes at 0ºC l water boils at 100ºC l body temperature 37ºC l room temperature ºC 0ºC

Measuring Temperature l Kelvin starts at absolute zero (-273 º C) l degrees are the same size l C = K -273 l K = C l Kelvin is always bigger. l Kelvin can never be negative. 273 K

53 Classwork Grade l textbook page 42 #5, l p. 57 # 7-9 l P. 60 # 28-30