“The world is full of obvious things. Which nobody by any chance ever observe.”-- Sherlock Holmes • Careful observation is the foundation of chemistry as an experimental science, leading us to question what we have observed – how, what why? • The answers to these questions are sought in experiments, which may be described as observations made under controlled conditions • Observation and experimentation – the twin pillars of the scientific method
Scientific Method… • The scientific way of knowing – often called the scientific method – is sometimes presented as a rigid sequence of events • It is not however a rigid path – it is a process of discovery! • Discovery begins when we make observations and then try to understand what we have observed by asking key questions and proposing possible answers • This process of discovery begins as we design and conduct experiments to test whether our answers to these questions are valid!!
What are the steps of the Scientific Method? • Make an observation(s) • Propose a hypothesis • Design and conduct a controlled experiment • Analyze the results • Form conclusions
Controlled Experiment… • Experiments should be designed so that the effects of different variables on the behavior of a substance can be studied independently • A controlled experiment is when only one variable at a time is changed • There are two types of variables in an experiment: • Independent variable – the one that is deliberately changed • Dependent variable – the thing that changes because of the independent variable
An Example… • Students were trying to determine if the amount of a Sodium chloride added to calcium carbonate effected the amount of heat given off • Give 3 possible variables for this experiment • Give the independent and dependent variables
What is chemistry? • Chemistry is the study and investigation of the structure, composition and property of matter and the changes it undergoes • The properties of materials are always related to their structure • Hence, structure determines properties
Measuring and Calculating in Science… • Chemistry is a quantitative science because it involves measuring and calculating • A measurement must have a number and a scale (called a unit) to be meaningful • It can also be a qualitative science because it can involve describing what is happening in a reaction
What makes a measurement? • In order to make measurements, we must meet three requirements… • Know what we are trying to measure • Have a standard with which to compare whatever we are measuring • Have a method for making comparisons
Exact Numbers… • A number that is the result from a definition or an exact count • For example – there are 12 apples or π = 3.14 • All are significant • Do not limit the number of sig figs
Uncertainty in Measurement… • A measurement always has some degree of uncertainty • The amount of uncertainty depends on the precision of the measuring device • In science it is customary to report a measurement by recording all certain digits plus the first uncertain (estimated) digit – these numbers are called the significant figures of a measurement
Estimating Uncertainty in a measurement… • Remember all measurements are a result of known values and one estimated number • When finding the uncertainty in a measurement, we look at the estimated number • For example: 0.023 • The 3 is the estimated number • It is in the 1000th place • The estimated uncertainty is written +0.001 • This measurement has a very small uncertainty
Rules for Counting Significant Figures… • Nonzero integers are always significant figures • Zeros – there are three classes of zeros • Leading zeros precede all the nonzero digits and are not significant figures • Captive zeros are between nonzero digits and always count as significant figures • Trailing zeros are at the right end of the number and are only significant if the number contains a decimal point
Try a few… • Tell how many sig figs are in each measurement and tell the uncertainty in each measurement • 1508 cm • 300.0 ft • 20.003 g • 0.00705 L
Bell Ringer… • Tell the number of sig fig • 94300 • 0.000400670 • 100000. • 56.00 • Tell the uncertainty of each measurement • 3.45 • 6.0 • 12 • 4.725
Bell Ringer… • Each of the following are statements from different labs – tell if they are quantitative or qualitative • Bubbling • Heat given off • 23.6 cm wide • A strong odor • pH of 5.4 • 273 K
Sig figs in Mathematical Operations… • To this point we have learned to count the significant figures in a given number, but we must also consider how uncertainty accumulates as calculations are carried out
Rules for Sig figs in Mathematical Operations… • For multiplication or division the number of sig figs in the answer is the same as the number in the least precise measurement used in the calculation • 4.56 x 1.4 = 6.38 → 6.4 (correct answer) • For addition or subtraction the number of sig figs in the answer has the same number of decimal places as the least precise measurement used in the calculation • 12.11 + 18.0 + 1.013 = 31.123 → 31.1 (correct answer)
Bell Ringer… • Carry out the following mathematical operations and give each result with the correct number of significant figures • 1.05 x 10-3 / 6.135 • 21 – 13.8 • 20 X 23.00 • 14.75 + 34.25
Bell Ringer… • The actual length of a certain plank is 26.782 cm. Which of the following measurements is the most accurate? Are the measurements precise? • 26.5 cm • 26.8 cm • 26.202 cm • 26.98 cm
Rules for Rounding… • In most calculations you will need to round numbers to obtain the correct number of sig figs • When rounding, use only the first number to the right of the last significant figure • In a series of calculations, carry the extra digits through to the final result, then round • If the digit to be removed • Is less than 5, then the preceding digit stays the same 1.33 → 1.3 • Is equal to or greater than 5, the preceding digit is increased by 1 1.36 → 1.4
Precision and Accuracy… • Two terms often used to describe the reliability of measurements are precision and accuracy • Precision – the degree of agreement among several measurements of the same quantity. It also is known as the degree of reproducibility of the measurement • Accuracy – the agreement of a particular value with the true value
Bell Ringer… • Decide if the following lab data is accurate or precise or both • 13.2 mL, 13.3 mL, 13.1 mL, 13.2 mL • The actual value is 13.0 mL • Make each of the following have 3 sig figs • 34098 • 0.0003219 • 7154 • 76.78
Types of errors… • There are two types of errors in measurements • Random error (indeterminate error) – a measurement that has an equal probability of being high or low. This type of error occurs in estimating the value of the last digit of a measurement • Systemic error (determinate error) – occurs in the same directions each time. The measurement is either always too high or too low
In groups… • There are 365 days/year, 24 hours/day, 12 months/year and 60 minutes/hr. Use this data to determine how many minutes are in a month. • Now use the following data to calculate the number of minutes in a month: 24 hours/day, 60 minutes/hour, 7 days/week, and 4 weeks/month. • Why are these answers different? Which, if any, is more correct and why?
Dimensional Analysis… • It is often necessary to convert a given result from one system of units to another • The best way to do this is by a method called unit factor method OR dimensional analysis
Converting from One Unit to Another… • To convert from one unit to another, use the equivalence statement that relates the two units • Derive the appropriate unit factor by looking at the direction of the required change (to cancel unwanted units) • Multiply the quantity to be converted by the unit factor to give the quantity with desired units
Bell Ringer… • A marathon race is 26 miles and 385 yards. • What is the distance in rods • What is the distance in meters • What is the distance in furlongs? 5.5 yards = 1 rod 40 rods = 1 furlong 8 furlongs = 1 mile 1 meter = 39.37 inches 1 yard = 36 inches
What if there is more than one unit present? • When more than one unit is present, decide which unit you want to convert first • Convert it first • Then convert the second unit ***do not get confused!!! EX: How fast is a car going 35 miles/hour going in yards/second? 1 mile = 1760 yards;1 hour = 60 minutes; 1 minute = 60 seconds
A few problems… • How many doughnuts can one purchase for $123 if doughnuts cost $3.25/doz? • Convert 9.85 L to gal. 1.06 qt = 1.00 L and 4 qt = 1 gal • A certain size of nail cost $1.25/lb. What is the cost of 3.25 kg of these nails? 1kg = 2.2 lb
Metric System Review… • Scientists recognized that long ago a standard system of units had to be adopted if measurements were to be useful • The system agreed upon in 1960 was the International System or le Systeme International (SI system) • The SI system is based on the metric system and units derived from the metric system • Because fundamental units are not always convenient, the SI system employs prefixes to change the size of the unit
Derived units… • Many SI units are combinations of quantities • These units are produced by multiplying or dividing standard units
Dimensional Analysis with metric units… When converting with metric, always use that value of the unit as compared to the base unit • Convert 35.4 mm to m • Convert 2327.9 cg to kg • How many grams are in 53.24 dg?
Bell Ringer… • Why do we use the metric system? • Convert 35.4 mm to m • Convert 2327.9 cg to kg • How many grams are in 53.24 dg? • Convert the following: • How many inches are in 3.0 meters? • A baby weighs 8.5 lbs. How many grams is that? • How many gallons of Coke would you drink if you drank entire 2 liter?
Science fiction often uses nautical analogies to describe space travel. If the starship U.S.S. Enterprise is traveling at warp factor 1.71, what is its speed in knots? Warp 1.71 = 5.00 times the speed of light The speed of light = 3.00 x 108 m/s 1 knot = 2000 yd/hr
Mass • The measure of the resistance of an object to a change in its state of motion OR the amount of “stuff” in an object • A scale is used to mass an object
Mass vs. weight… • An important point concerning measurements is the relationship between mass and weight • Weight is the force gravity exerts on mass, therefore weight varies with the strength of the gravitational field • Therefore if you went to the moon your weight would change but not your mass • Many times the terms mass and weight are sometimes used interchangeably, although this is incorrect!
Volume… • The derived SI unit of volume is cubic meters (m3) • Many times this unit is way too large to be a practical way of expressing volume in a chemistry lab • Instead, a smaller unit cubic centimeters (cm3) is used • When dealing with the volumes of liquids and gases, the non-SI unit liter(L) is often used • Again the liter is often too large so the unit milliliter (mL) is used • This means 1 cm3 = 1mL
Review… • Round the following to 3 sig figs • 96747210 • 91 • 0.0006589 • How many sig figs are in each in #1? • What is the uncertainty of each measurement? • 34.09 • 6.0222 • 12 • What is the difference between precision and accuracy?
Review… • Convert the following: • How many grams are in 548.9 mg? • How many feet are in 34.2 m? • How many liters are in 2 gallon and 3.4 quarts?
It can be tricky with volume conversions… • How many mL are in 14.65 kL? • How many L are in 48.6 cm3? • How many dm3 are in 29100 mL?
Bell Ringer… • A piece of metal has the mass of 3.45 kg. What is its mass in g? • A container has 2.3 L of gas in it? What is its volume in mL? • A container has 750.00 mL of liquid in it. What is its volume in m3?
What is Temperature? • A measure of the AVERAGE kinetic energy • When looking at the different temperature scales, all are talking about the same height of mercury
Temperature Conversions… • There are three systems used to measure temperature • Degrees Fahrenheit (°F) • Degrees Celsius (°C) • Kelvin (K) • Each has a different way of converting between the values
Notice -- 0°C = 32°F and 100°C = 212°F If we subtract these values then… 100°C = 180°F * Find the value of 1°C 1°C = (180/100) °F 1°C = 9/5 °F How the equation for °F to °C was derived…
Converting… 1. Converting from °C to Kelvin TC = TK – 273.15 TK = TC + 273.15 • Converting from °C to °F TF = TC x 9°F + 32°F 5°C
More Converting… • Converting °F to °C TC = (TF - 32°F)5°C 9°F
Try these… 1. Normal body temperature is 98.6°F. Convert this to the Celsius and Kelvin scales. 2. Liquid nitrogen, which is often used as a coolant for low-temperature experiments has a boiling point of 77 K. What is this temperature on the Fahrenheit scale?