preap chemistry chapter 2 notes. 2.1 scientific method

PreAP ChemistryChapter 2 Notes

2.1 Scientific Method


The 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.


There 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?




•Quantitative data is data about quantities, like mass, density, and other numerical amounts.




•Quantitative data is data about quantities, like mass, density, and other numerical amounts.

What quantitative data can be made about these apples?


A 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.


A 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?

http://www1.moe.edu.sg/learn@/quest/winners/primary/yu_neng_pri_light_nc/teexp.htm

http://www1.moe.edu.sg/learn@/quest/winners/primary/yu_neng_pri_light_nc/teexp.htm


In 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.


In 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.?


Often 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?


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


If 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


Horses 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.


Horses 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)


Mass 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)


Length is the distance between two points.

•1 inch = 2.54 centimeters (exactly)


Temperature is the measurement of heat intensity (more about this later).

•1 Kelvin is the same magnitude as 1 °C

•0 ºC = 273.15 K


A 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


The 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) 0.1

one-tenth 10-1

Centi (c) 0.01

one-hundredth 10-2

Milli (m) 0.001

one-thousandth 10-3

Micro (µ) 0.000001

one-millionth 10-6

Nano (n) 0.000000001

one-billionth 10-9

Pico (p) 0.000000000001

one-trillionth 10-12


Derived 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

Volumelength × width ×

heightL or m3 or cm3

Pressure Force per unit area Pascals (Pa)


Conversion 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


Conversion 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


For example:

How many quarters are in 6.53 dollars?

6.53 dollars 4 quarters = 26.12 quarters 1 dollar


How 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


When 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.


Accuracy 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.


To 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.


Then use this equation:

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

Percent error =|Accepted value – Experimental value|

× 100Accepted Value


For 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

preap chemistry chapter 2 notes. 2.1 scientific method

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