Chemistry 103A; Sections 5, 6, 7, 8; Lecture 2; 23 Aug 00
Chapter 1, continued
Classifications of matter (substances)
Mixtures
can be separated into component parts by physical means
most materials that we deal with are mixtures
heterogeneous mixtures
mechanical separation, filtration, etc
homogeneous mixtures
fractional distillation, etc
Pure substances
can not be separated into component parts by physical means
compounds
can be separated into simpler substances by chemical means
elements
cannot be separated into simpler substances by any means
91 elements occur naturally on earth
(Technetium, #43, is man made.)
21 more have been made (synthesized) by nuclear
reactions
A pure substance is either an element or a compound.
Properties of substances
We will be concerned with two classes of properties of substances, physical properties and chemical properties. We will spend lots of time looking at chemical properties later, but for now we will look at the physical properties of materials.
Physical properties are those properties that can be measured or otherwise observed.
For example, the color of a material is a physical property. Other physical properties include
melting point
boiling point
density
heat capacity (both of the latter will be defined later)
etc. (see Table 1.2, page 26, in your text).
Units of measurement
Before we can talk about measuring physical properties we need to discuss our units of measurement. We will use, mostly but not always, the SI system. SI is short for Système International d'Unitès.
The SI system is one version of a metric system. It is derived from what used to be called the mks system (for meter, kilogram, second) which was used mostly by physicists.
The SI system selects a set of "base units" from which all other units are derived. You can find a good description of the SI unit system at
http://physics.nist.gov/cuu/Units/index.html
Some of the base units of the SI system that we will use are:
quantity base unit symbol
mass kilogram kg
length meter m
time second s
temperature Kelvin K
amount of substance mole mol
electric current Ampere A
There is another set of units, called "derived units," which can be obtained from these. Examples of derived units are:
energy Joule J
electric potential volt V
power Watt W
volume cubic meter m3
pressure Paschal Pa
force Newton N
One can form larger and smaller units from the base and derived units by adding a prefix.
For example, the prefix "mega" (abbreviated M) indicates a unit one million times the initial unit. a megavolt, or MV, would be one million volts.
The prefix "milli" (abbrevited m) indicates a unit one thousand times smaller that the initial unit. Thus a millimeter, or mm, is one thousandth of a meter.
There is a list of prefixes in Table 1.4, page 37, in your text. For a complete list see
http://physics.nist.gov/cuu/Units/prefixes.html.We use some units which are not strictly in the SI system:
Volume units:
liter L
1 L = 1 thousandth of a m3
(Sometimes people call the liter a cubic decimeter, dm3.)
milliliter mL
1 mL = 1 cm3 = one thousandth of a liter
= one millionth of a m3
Pressure units
atmosphere atm
1 atm = 101325 Pa (exactly)
torr
1 atm = 760 torr (exactly)
bar
1 bar = 1.01325 atm = 100,000 Pa (exactly)
There are some conversions between SI and English units that we need to know:
The definition of the calorie (a unit of energy) is
1 cal = 4.184 J (exactly).
The "food Calorie" is really 1 kcal.
The definition of the inch is
1 inch = 2.54 cm (exactly).
You are required to know these definitions by memory.
Units of temperature
The SI unit of temperature is the Kelvin (abbreviated K, notice that there is no o sign on it). Kelvin "degrees" are the same size as Celsius (abbreviated C) degrees.
The Kelvin temperature scale is an "absolute" scale. There is a temperature, 0 K, which is the lowest attainable temperature.
0 oC = 273.15 K (exactly) is the "normal ice point," the melting point of ice at one atmosphere pressure. This equation defines the size of the Kelvin degree.
We also need to know how to convert between Fahrenheit temperatures and Celsius or Kelvin temperatures. The conversions from Fahrenheit to Celsius and back are:
The conversion from Celsius to Kelvin is:
K = oC + 273.15.
Somewhere along here we need to give a quick reminder about scientific notation and the use of significant figures in calculations.
In science we sometimes have to write very large or very small numbers. It is inconvenient (and dangerous) to try to write all the zero "place holders." So we use powers of 10 to tell us where the decimal point should be. For example:
602210000000000000000000 = 6.0221 ´ 1023
or
0.00000000005292 = 5.292 ´ 10 - 11
Significant figures (sig fig)
How many of the displayed digits have
meaning?
186282
has 6 sig fig
5.26 ´ 10- 9
has 3 sig fig
5.292 ´ 10- 9
has 4 sig fig
3.84
has 3 sig fig
3.840
has 4 sig fig
3.8400
has 5 sig fig
Zeros are treated differently depending on
where they are:
Place holders are not significant.
(These are zeros which only tell how
large or how small a number is.)
To avoid ambiguity use scientific notation.
Examples:
186000
0.000529
You can make the zeros in 186000
significant by writing 186000., or by writing
1.86000 ´ 105.
Imbedded zeros and zeros that are not
place holders are significant.
Examples:
2507
289007
4.18400
Here's an interesting one:
0.000360800
Rounding
Calculators give too many sig fig so we
must round off to the correct significance.
Look at the figure next to the right of your
least significant figure.
If it is 0 to 4, leave the least significant
figure alone (round down).
If it is 5 to 9, add 1 to the least significant
figure (round up).
Example:
1.92548 has 6 sig fig
5 sig fig 1.9255
4 sig fig 1.925 ¬ watch it!
3 sig fig 1.93
2 sig fig 1.9
1 sig fig 2
Keeping track of significant figures in calculations:
1. multiplication and division
Use smallest number of significant figures
Example
2. addition and subtraction
Use the smallest number of decimal places.
Example:
Some numbers have "infinite significance."
Examples
integers (used for counting)
"a square has 4 sides"
definitions
2.54 cm/inch
5280 ft/mile
12 inch/foot
etc.
These numbers are almost always used in
multiplication or division. The number of
significant figures in the answer is never
limited by the number of significant figures
in these numbers.
2.54 cm/inch can be written
2.540 cm/inch
2.5400 cm/inch
2.54000 cm/inch, etc
with as many significant figures as required.
Units - most numbers have units
186282 mile/sec
5.29 x 10-9 cm
We can do algebra on units the same as on numbers.
You should carry the with you in calculations.
Incorrect units at the end of a calculation is an indication of error.
Physical Properties of Materials
Density
Density of a measure of "concentration of mass," how much mass is crammed into a certain volume.
Definition:
This equation can be rearranged into other algebraically equivalent forms:
Examples:
The density of water at 25oC is 0.997 g/cm3, what is the mass of 1.00 L of water?
Diamond has a density of 3.513 g/cm3, what is the volume of a 0.300 g (1.5 carat) diamond?
Copyright 2000, W. R. Salzman