Required reading: Review of week 1
Introduction to Chemistry
Chemistry is the science that is concerned with the composition, structure and properties of matter. All of the objects we encounter on a daily basis, including pens, blankets, and refrigerators, and things we encounter in nature such as plants, animals, streams, and rocks are different forms of matter. Since chemistry as a science, deals with most of the objects we encounter on a daily basis, it’s difficult to exaggerate the influence chemistry has on modern science, technology and our ideas about our surroundings.
Scientific Method
The scientific method provides us with an overview of the way we make observations about natural phenomena and then formulate theories to summarize them. We first formulate a question about a specific observation.
The second thing we do is come up with a hypothesis. A hypothesis is a tentative explanation for some regularity of nature.
Third, we must conduct experiments to prove or disprove our hypothesis.
Finally, once a hypothesis is proven by experimentation we can officially call it a theory. What follows generally, is more experimentation to see if our theory holds up.
Law of Conservation of Mass
The law of conservation of mass states that the total mass remains constant during a chemical change or chemical reaction.
Matter
There are two principle ways in which we classify matter. We can classify matter by its state as either a solid, liquid or a gas. Otherwise we classify matter by its chemical structure as an element, compound or mixture.
Physical change vs. chemical change
A physical change is when matter changes form but its chemical composition remains the same. For instance, if you leave an ice cube on the kitchen counter it melts into its liquid form, but its chemical composition remains the same. A chemical change is one in which the composition of matter is transformed into new matter or several new kinds of matter. When iron combines with oxygen to form rust, this is a chemical change and this is the reason you shouldn’t leave your bike out in the rain.
Physical & Chemical Properties, Substance
a) Physical properties are characteristics of a material that can be observed without changing its chemical identity.
b) Chemical properties are material characteristics that involve chemical change.
A substance is a type of matter that cannot be separated into other kinds of matter by physical means. Consider that a mixture sodium chloride and water can be separated by physical means, but sodium chloride is substance that cannot be separated by physical means.
Mixtures
A mixture is any sample of matter that is not pure and that can be separated by a physical means to yield two or more substances. There are two types of mixtures, a homogenous mixture, is a mixture which has uniform composition and every part of the solution has the same properties. A heterogeneous mixture is composed of dissimilar parts.
Number of Significant Figures
Significant figures are digits in a measured number that include all certain digits plus a final one with some uncertainty. Significant figures are important because they give you information about how precise a measurement is. In order to determine the number of significant figures or precision of a measured number there are four rules that need to be memorized:
1) All nonzero digits are considered to be significant – 123, 0.123 and 0.00123 all have three significant figures.
2) Zeros between two nonzero digits are significant – 1203, 0.1203, 0.001203 all have four significant figures.
3) Leading zeros are not significant – 0.0023 has only two significant figures.
4) Trailing zeros in a number containing a decimal point are significant – 12.300 has five significant figures.
Scientific notation
Let’s say someone reports a measurement of 2,000 cm and you aren’t sure whether 1, 2, 3 or 4 significant figures are intended. A way to avoid the uncertainly is to report your number in scientific notation. Scientific notation is way to write a number in the form A x 10n, where A is a number with a nonzero digit to the left of decimal point and n is an integer or whole number.
If the measurement of 2,000 cm is precise to two significant figures it is written 2.0 x 103 cm. If it’s precise to three significant figures it is written 2.00 x 103 cm. Scientific notation is a convenient way to express very large or very small quantities that are difficult to express in decimal form.
Significant Figures in Calculations
How do we determine the number of significant figures to report in a measurement involving calculations? For this we follow two rules:
1) Multiplication/division: When multiplying or dividing measured quantities we report as many significant figures as there are in the measurement with the least number of significant figures.
2) Addition/subtraction: When adding or subtracting measured quantities report the same number of decimals in the answer as there with the least number of decimal places.
SI system
The SI system is a standardized means of reporting measured quantities. The SI base units (Table 1) are like the building blocks of SI, where all other measurements are derived from. SI prefixes are added to the base unit to produce a multiple of the original unit. SI prefixes (Table 2) are used to indicate the power of ten. When an SI base unit is combined with the appropriate SI prefix, one can express any type of measurement. From the SI base units for standard measurements we can derive other units (Table 3). The number of derived units is unlimited and are obtained by means of mathematical operations.
Table 1. SI Base Units
UNIT NAME |
UNIT SYMBOL |
QUANTITY NAME |
meter |
m |
length |
kilogram |
kg |
mass |
second |
s |
time |
ampere |
A |
electric current |
kelvin |
K |
thermodynamic temperature |
mole |
mol |
amount of substance |
candela |
cd |
luminous intensity |
Table 2. SI Prefixes
MULTIPLE |
PREFIX |
SYMBOL |
106 |
mega |
M |
103 |
kilo |
k |
102 |
hecto |
h |
10 |
deka |
da |
10-1 |
deci |
d |
10-2 |
centi |
c |
10-3 |
mili |
m |
10-6 |
micro |
µ |
10-9 |
nano |
n |
10-12 |
pico |
p |
Table 3. Derived units
QUANTITY |
DEFINITION OF QUANTITY |
SI UNIT |
Area |
Length squared |
m2 |
Volume |
Length cubed |
m3 |
Density |
Mass per unit volume |
kg/m3 |
Speed |
Distance traveled per unit time |
m/s |
Acceleration |
Speed changed per unit time |
m/s2 |
Force |
Mass time acceleration of object |
kg*m/s2 (newton) |
Pressure |
Force per unit area |
kg/(m*s2) (pascal) |
Energy |
Force times distance traveled |
kg*m2/s2 (joule) |
Conversion factors
Conversion factors provide a roadmap for getting from one set of units to another for the same measured quantity. Let’s say we wanted to convert 15 cm3 to liters. 1 L = 1000 cm3 so converting from cm3 to liters isn’t difficult. You must first set up a conversion factor. Putting the units you want to cancel in the denominator and the desired units in the numerator.
cm3 cancels and our answer is 0.015 liters. The ratio 1L/103cm3 is called a conversion factor. Conversion factors allow you to convert between different units of the same measured quantity.
This method allows us to convert between U.S. and metric units. Say you wanted to convert 2.6 miles to kilometers. The conversion factor for miles to kilometers is 1.609 kilometer/mile. Therefore,
Rounded to two sig figs is 4.2 Km.
Nuclear Structure
The atomic number is the number of protons in the nucleus of the atom. Protons are nuclear particles with a charge equal to that of an electron; however, their mass is nearly 1800 times greater. Elements are groups of atoms that all of the same atomic number.
Neutrons are nuclear particles with a mass number nearly identical to that of a proton, but contain no charge.
The mass number is the total number of protons and neutrons together in the nucleus. This accounts for nearly all of the associated mass of an element. The mass associated with electrons are considered negligible since they are nearly 1800 times smaller than protons and neutrons.
Isotopes are elements who have the same atomic number, but different mass numbers. Since elements are defined by the number of protons this means the elements have a different number neutrons. Elements are a mixture of isotopes with each isotope having its own characteristic mass.
Conclusion
This week we discussed ways in which we classify matter. We can classify matter by its state as either a solid, liquid or a gas or otherwise we classify matter by its chemical constitution as an element, compound or mixture. The physical state of matter is dependent on temperature and pressure, whereas its chemical composition is constant. Chemists classify substances by physical properties many of which can be measured. This is why it’s important to have an understanding of the correct way to report measurements.
Measurement of different physical properties of matter is really an important practice in chemistry and all other sciences a like. The SI system of measurement provides a standardized way of reporting our measurements in science. This allows us to communicate our findings to one another.
Since Dalton’s day atomic theory has developed steadily and has been the cornerstone of chemistry ever since. The results of which have been enormous when you consider atoms can be rearranged through chemical reactions to give us all of the millions of compounds we encounter on a daily basis.
This fourth video lesson introduces atomic theory