UNIT 1.0 - THE BASICS
Chemistry is the study of matter and interactions. Chemistry overlaps with many other sciences.
Below is the contents for this sub unit. This will allow you to jump to any section you desire. If you have taken Chemistry 1, this should all be review but it is always good to brush up on the basics (significant figures especially).
SCIENTIFIC METHOD
Scientist observe the world around them and make theories. Then they test their theories with experiments. This is done through the scientific process.
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Scientific Process:
1. Someone makes an observation and comes up with a possible explanation.
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2. Hypothesis - a possible explanation of the observations
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3. The hypothesis is tested by an experiment.
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Experiments - highly controlled procedure to produce observations that confirm or refute hypotheses
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4. Step are repeated (adjustments made to hypotheses) until hypotheses is proven correct. If it is 100% accurate, it becomes a law.
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CLASSIFYING MATTER
We have several classifications for matter
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Matter - anything that occupies space and has mass
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Atoms - sub-microscopic particles that are the building blocks of matter
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Molecules - atoms that are bound together
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Pure substance - only containing one type of molecule or atom
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Mixture - containing two or more pure substances
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Homogeneous mixture - mixture with the same composition throughout (looks like one thing)
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Heterogeneous mixture - mixture where one part looks different than another
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The picture above shows how all of these are organized
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We also classify matter as
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Solid - atoms closely packed together, having a set shape and volume
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Liquid - Atoms loosely together, having set volume but not shape
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Gas - Atoms far away from each other, having neither set volume or shape
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PROPERTIES
All compounds have physical and chemical properties
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Physical property - a property that doesn't change the composition (same thing before and after)
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These include boiling point, freezing point (the phase of matter s, l, g changes but the molecule is the same and these are considered physical changes), density, etc.
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Chemical property - a property that is displayed only when it interacts with something else.
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These include combustible (catches on fire - not the same afterwards so it is a chemical change).
UNITS
Standard units - most of the world has agreed on units to be used for certain measurements (international system of units)
Quantity - unit symbol
Length - meter m
Mass - kilogram kg
Time - second s
Temperature - Kelvin K
Amount of substance - mole mol
Electric current - ampere A
Luminous intensity - candela cd
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Volume - Liter L
Density - mass/volume g/cm3
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prefix symbol multiplier scientific
mega M 1,000,000 10^6
kilo k 1,000 10^3
deci d 0.1 10^-1
centi c 0.01 10^-2
milli m 0.001 10^-3
micro u 0.000001 10^-6
nano n 0.000000001 10^-9
PREFIXES
The great thing about the international system of measurements is that they are easily scaled by using prefixes. Some of these you may be familiar with such as kilo - which is 1000. For 1 kilometer, there are a 1000 meters. So the multiplier is how many of the original (base) unit.
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How many millimeters are there in 0.05 meters?
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Let's use dimensional analysis and the multiplier. Remember the multiplier applies to the original (base unit).
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0.05 m = ? mm
0.05 m x _ 1 mm = 50 m
0.001 m
Notice you divide by the multiplier as it applies to the meters and that is what you started with.
Practice by clicking the quiz below. This is a practice quiz. Try it then the answers are given in the second button
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SIGNIFICANT FIGURES
So now that you've had practice with dimensional analysis, let's make it more annoying. In chemistry we keep track of the significant digit. Unlike math where you keep all the digits in the calculator, we round to a significant figure.
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So let's say you measure a 1.15 length that you need to split into exactly 3 sections. So math you would:
1.15 / 3 = 0.38333333333
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In chemistry we see there are 3 digits in 1.15 so the answer will have 3 digits:
1.15/ 3 = 0.383
The 3 we are dividing by is considered an exact number and therefore doesn't limit the significant figures.
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So when multiplying or dividing the answer would have the least number of significant figures.
Example: 2.5 x 3.15 = 7.875 but as the 2.5 only has 2 digits:
2.5 x 3.15 = 7.9
Note: Rounding is done by looking at the digit we are rounding to (the 8 in 7.875 as we can only keep 2) then looking at the number to the right. If it is 5-9 then round up (8 becomes 9 in this case as there is a 7 to the right) and if it is 0-4 leave it alone.
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Addition and subtraction is by the least significant digit.
Example: 2.5 - 1.16 = 1.34 but as the first number didn't have the hundreds place, we don't know how accurate that place is so:
2.5 - 1.16 = 1.3
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Mastering significant figures will set you apart from many scientists. I have worked with Ph.D. level scientists who don't apply this well. It is one of the tested items on the AP test and was a huge topic (accuracy and precision) in my work at graduate school and in my job. Take the time to learn it and it will help.
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HOW TO GET SIGNIFICANT FIGURES
Chemistry is a laboratory science. The different equipment produces different levels of accuracy and therefore significant figures.
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Example 1: Mass
Let's say you have a sample to get the mass of but you have 3 different balances. Balances are great as they give you a digital display to tell you the value to write down. The first 2 look the same but the third has a glass box around it.
Balance 1: 2.5 grams
Balance 2: 2.47 grams
Balance 3: 2.4682 grams
Which is right? They all are, they just depend on the instrument you use. The last is the most accurate (analytical balance) and therefor has 5 significant figures. The first 2 are similar but one has an extra digit. I ran across this in my lab when I ordered the more accurate scales but the scales we already had only read 2.5 grams.
But now you can see how your instrument determines your significant figures.
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Example 2: Volume
Graduated cyliner
Graduated cylinder has markings as shown in the picture above. It shows the meniscus (the bubble looking thing in the water) just below the 3 mark (notice how the 3 isn't labeled but you figure it out from what you are given). Then you need to figure out the little marks. Since there are 5 sections between the 2 and 3, each mark is 0.2 mL. This will allow me to estimate that the volume is 2.94 mL You are allowed to estimate 1 digit pass the smallest mark so if it is 0.2 mL, you can estimate 0.02 mL.
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Volumetric Flask
A volumetric flask has only one line on it. It is marked at 100 mL (or whatever volume it is labelled) and you add sample to that volume. It is considered highly accurate. So you could write 100.000 mL but the limitation is that it can only measure that 1 volume.
