PreAP ChemistryChapter 2 Notes

2.1 Scientific Method

2.1 Scientific MethodThe Scientific Method is a logical approach to solving problems by observing and collecting data, formulating hypotheses, testing hypotheses, and formulating theories that are supported by data.

2.1 Scientific MethodThere are two kinds of data that can be observed and collected: Qualitative and Quantitative.

2.1 Scientific MethodThere are two kinds of data that can be observed and collected: Qualitative and Quantitative.•Qualitative data is data about qualities, like appearance and behavior.

2.1 Scientific MethodThere are two kinds of data that can be observed and collected: Qualitative and Quantitative.•Qualitative data is data about qualities, like appearance and behavior.

What qualitative data can be made about this apple?

2.1 Scientific MethodThere are two kinds of data that can be observed and collected: Qualitative and Quantitative.•Qualitative data is data about qualities, like appearance and behavior.•Quantitative data is data about quantities, like mass, density, and other numerical amounts.

2.1 Scientific MethodThere are two kinds of data that can be observed and collected: Qualitative and Quantitative.•Qualitative data is data about qualities, like appearance and behavior.•Quantitative data is data about quantities, like mass, density, and other numerical amounts.

What quantitative data can be made about these apples?

2.1 Scientific MethodA hypothesis is based on previously collected data and is an attempt to explain the data, as a testable prediction; if A, then B. (It may not necessarily contain the words “if” and “then”). A hypothesis is tested with an experiment.

2.1 Scientific MethodA hypothesis is based on previously collected data and is an attempt to explain the data, as a testable prediction; if A, then B. (It may not necessarily contain the words “if” and “then”). A hypothesis is tested with an experiment.If you go to http://www1.moe.edu.sg/learn@/quest/winners/primary/yu_neng_pri_light_nc/teexp.htm they ask:“Do different colours of light affect the growth of a green bean plant?”Is this a hypothesis? Is it testable? Is it a prediction?

2.1 Scientific MethodIn an experiment, usually the affect of one variable on another is tested. The variable that is being controlled directly by the experimenter is the independent variable. The independent variable should then have an affect on the variable being tested, called the dependent variable.

2.1 Scientific MethodIn an experiment, usually the affect of one variable on another is tested. The variable that is being controlled directly by the experimenter is the independent variable. The independent variable should then have an affect on the variable being tested, called the dependent variable.

What is the I.V.?What is the D.V.?

2.1 Scientific MethodOften there are additional variables that can be involved in an experiment, so care should be taken to be sure that these are held constant. In addition, to judge if the independent variable actually did affect the dependent variable and nothing else, a control situation should be used. This could be a separate specimen to which the independent variable is purposely held constant or is in the ‘usual’ state, or could be a separate trial of the same experiment, during which the independent variable is held constant or in the ‘usual’ state.

What needs to be constant?What is the control?

2.1 Scientific MethodIf a hypothesis is not disproved after many experiments to test it, then the hypothesis is considered a theory, like gravity or evolution.

2.1 Scientific MethodIf a hypothesis is not disproved after many experiments to test it, then the hypothesis is considered a theory, like gravity or evolution.

Sorry guys, they’re still just theories

2.2 Units of Measurement

2.2 Units of MeasurementHorses are measured in hands. This used to literally mean how many hand widths to go down the side of the horse. For someone with small hands their horses will seem bigger than someone with big hands.

2.2 Units of MeasurementHorses are measured in hands. This used to literally mean how many hand widths to go down the side of the horse. For someone with small hands their horses will seem bigger than someone with big hands.To avoid these issues in science, scientists have adopted SI Units. (SI = Système International, French for International System.) These base units have been agreed on by the scientific community and will be used in this class.

Quantity Quanitity Symbol

Base Unit Name

Abbreviation

Length l meter m

Mass m kilogram kg

Volume V (uppercase) liter L (uppercase)

Time t second s

Temperature

T (uppercase) Kelvin K (uppercase, no “°”)

Amount of Substance

n mole mol

Electric Current

I (uppercase) ampere A (uppercase)

2.2 Units of MeasurementMass is a quantity of matter and is constant. Do not confuse it with weight! Astronauts on the moon have less weight, but not less mass (less mass would mean they were shrinking!)

•1 kilogram is the same mass as 2.204622621849 pounds (but we'll use 2.20 lbs for this course)

2.2 Units of MeasurementLength is the distance between two points. •1 inch = 2.54 centimeters (exactly)

2.2 Units of MeasurementTemperature is the measurement of heat intensity (more about this later).•1 Kelvin is the same magnitude as 1 °C •0 ºC = 273.15 K

2.2 Units of MeasurementA mole is a count of something, like a dozen. By definition this is an exact count and not a measurement, but of course no one can count six hundred two sextillion two hundred fourteen quintillion one hundred and fifty quadrillion, so the number is determined mathematically. The best calculation is currently 602,214,150,000,000,000,000,000. Most often for this course the rounded off number below will be used. •1 mole = 6.022 × 1023 “particles” or things

2.2 Units of MeasurementThe beauty of the SI units is that almost all of them can be scaled up or down by factors of ten with relative ease. Often once this is done a new prefix is added to base unit to indicate the new unit of measurement. You will need to understand and be able to use these prefixes for this course. Be sure to note which abbreviations use lowercase and uppercase letters! This distinction is very important. For example m and M are two different orders of magnitude.

Prefix and Symbol Meaning Exponetial Factor

Mega (M)  1,000,000 106

Kilo (k)         1,000 103

Hecto (h) 100 102

Deka (da or dk) 10 101

Deci (d)           0.1one-tenth 10-1

Centi (c)                0.01one-hundredth 10-2

Milli (m)                0.001one-thousandth 10-3

Micro (µ)                0.000001one-millionth 10-6

Nano (n)                0.000000001one-billionth 10-9

Pico (p)                0.000000000001one-trillionth 10-12

2.2 Units of MeasurementDerived units are ‘new’ units that are made by combining two or more of the standard units together. Below are several derived units, how they are derived, and the usual SI units used with each. There are many, many more derived units.

Derived Unit Origin Usual SI Units

Density

Mass per unit volumeg/m3 = g m-3 or

g/cm3 = g cm-3 org/L = g L-1 or

g/mL = g mL-3

Volume length × width × height L or m3 or cm3

Pressure Force per unit area Pascals (Pa)

2.2 Units of MeasurementConversion factors are a ratio derived from the equality between two different units that can be used to convert from one unit to the other. For example:

4 quartersor

1 dollaror even

0.25 dollars

1 dollar 4 quarters 1 quarter

2.2 Units of MeasurementConversion factors allow a quantity measured one way to be converted into a different way of measuring (although the overall amount should never change). Sometimes several conversion factors are used in the same calculation, but it is important to note that units in the numerator will cancel with identical units in the denominator. This process is called the factor label method or dimensional analysis

2.2 Units of MeasurementFor example:How many quarters are in 6.53 dollars?

6.53 dollars 4 quarters = 26.12 quarters 1 dollar

2.2 Units of MeasurementHow many centimeters are in 2.34 miles?

2.34 miles 5280 feet 12 inches 2.54 cm = 376,586.496 cm 1 mile 1 foot 1 inch

2.3 Using Scientific Measurements

2.3 Using Scientific MeasurementsWhen scientists make measurements, there is usually a little discrepancy in the measurements due to many reasons, often human error. For example, the same person could measure a string three different times and get three, slightly-different measurements. For this reason, when making measurements or reading someone else’s measurements it is important to consider the accuracy and precision of those measurements.

2.3 Using Scientific MeasurementsAccuracy refers to how close the measurement is to the real or accepted value. Precision is how close a group of measurements of the same thing are to each other. The precision of a series of measurements is usually easy to determine just by looking at them, but this is not the case with the accuracy.

2.3 Using Scientific MeasurementsTo find the level of accuracy of a measurement, the percent error is calculated. To make the calculation the accepted value (the value you were supposed to get) and the value you actually did get during the experiment are needed.

2.3 Using Scientific MeasurementsThen use this equation:

The label to your percent error is the “%” sign.

Percent error =|Accepted value – Experimental value|

× 100Accepted Value

2.3 Using Scientific MeasurementsFor example; An experiment finds the density of lead to be 10.95 g/cm3. The textbooks states the density should be 11.34 g/cm3. What was the percent error for this experiment?

Percent error =

11.34 g/cm3 – 10.95 g/cm3

× 100 = 3.439 %11.34 g/cm3