Transcript Chemistry in Our Lives

Chemistry in Our Lives
Chemistry and Chemicals
What is chemistry?
Chemistry is the study of substances in terms of
Composition
What a material it made of
Structure
How the elementary particles are put together
Properties
The characteristics of the material
Reactions
How it behave with other substances
Chemical reactions happen when
• a car is started
• tarnish is removed from silver
• fertilizer is added to help plants
grow
• food is digested
• electricity is produced from
burning natural gas
• rust is formed on iron nails
Everything in our lives from materials
to life involve chemistry
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glass (SiO2)n
metal alloys
chemically treated water
plastics and polymers
baking soda, NaHCO3
foods
fertilizers and pesticides
living beings
Chemicals in Toothpaste
The Scientific Method
The scientific method is the
process used to explain
observations in nature.
The method involves:
• making observations
• forming a hypothesis
• doing experiments to test
the hypothesis
Everyday Scientific Thinking
• Observation: The sound from a CD in a CD
player skips.
• Hypothesis 1: The CD or player is faulty.
• Experiment 1: When the CD is replaced with another
one, the sound from the second CD is
OK.
• Hypothesis 2: The original CD has a defect.
• Experiment 2: When the original CD is played in another
player, the sound still skips.
• Theory:
The experimental results suggest that
the original CD has a defect.
Units of Measurement
In chemistry:
quantities are measured
experiments are performed
results are calculated
use numbers to report measurements,
results are compared to standards.
In a measurement of the thickness
of the skin fold at the waist,
calipers are used.
A measuring tool is used to
compare some dimension
of an object to a standard.
In every measurement, a number must be followed by a unit
to have any meaning.
Observe the following examples of measurements:
Number and Unit
35 m (meter)
0.25 L (liter)
225 lb (pound)
3.4 h (hour)
The Metric System (SI)
The metric system and SI (international
system) are
 related decimal systems based on 10
 used in most of the world
 used everywhere by scientists
Length
 is measured using a meter stick
 uses the unit meter
(m) in both the metric and SI systems
The unit of an inch
 is equal to exactly
 2.54 centimeters in
 the metric system
1 in. = 2.54 cm
Volume
 is the space occupied
 by a substance
 the unit of volume is the
liter (L) in the metric
system
1 L = 1.06 qt
The mass of an object
 is a measure of the quantity of material it contains
 the unit gram (g) or kilogram (1000 g) is used
What is the difference between mass and weight?
Weight is the result of the action of gravity on mass. Your weight on
the moon would be a lot less even though your mass would remain
the same
Despite this important difference, we will use these two terms interchangeably
The temperature
 indicates how hot or cold a
substance is
 the Celsius (C) scale is used
in the metric system
 the Kelvin (K) scale is also
used
 18 °C is 64 °F on this
thermometer
On the C scale, the melting point of ice is 0 C and boiling point of
water is 100 C
What is heat or cold? What does temperature really measure?
Time measurement
 the unit second (s) is
used in the metric
system.
 Time is based on an
atomic clock that uses a
frequency emitted by
cesium atoms
Scientific notation
 is used to write very large or very small numbers
 the width of a human hair (0.000 008 m) is written
8 x 10-6 m
 a large number such as 4 500 000 s is written
4.5 x 106 s
Scientific Notation
 A number in scientific notation contains a coefficient and a
power of 10.
coefficient
1.5
power of ten
x
102
coefficient
power of ten
7.35
x 10-4
 To write a number in scientific notation, the decimal point is
placed after the first digit.
 The spaces moved are shown as a power of ten.
52 000. = 5.2 x 104
0.00378 = 3.78 x 10-3
4 spaces left
3 spaces right
10-3/105 =
10-8
10-3*105 =
102
10-3 +105 =
105
100000 + 0.001 = 100000.001
Measurements
What is the length of this piece of wood?
What is the first digit? Any uncertainty in the digit?
What is the second digit? Any uncertainty in this digit?
What is the third digit? Any uncertainty in this digit?
4
4.5
4.56
Definition of a significant figure:
Significant digits include all digits with no uncertainty plus one estimation
• . l8. . . . l . . . . l9. . . . l . . . . l10. . cm
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What is the length of the red line?
1) 9.38 cm
2) 9.39 cm
3) 9.40 cm
9.38, or 9.39, 9.40 is less likely
Measurement
Number of
Significant
Figures
38.15 cm
4
5.6 ft
2
120.55 m
5
0.0055 in
1200 m
2
2
A. Exact numbers are obtained by
2. counting
3. definition
B. Measured numbers are obtained by
1. using some measuring tool
Classify each of the following as exact (E) or
measured (M) numbers. Explain your answer.
A. __ Gold melts at 1064 °C.
B. __ 1 yard = 3 feet
C. __ The diameter of a red blood cell is 6 x 10-4 cm.
D. __ There are 6 hats on the shelf.
E. __ The atom sodium has 11 protons and 12 neutrons.
Significant Figures
In calculations:
 Answers must have the same
number of significant figures
as the measured numbers.
 Calculator answers must often
be rounded off.
 Rounding rules are used to
obtain the correct number of
significant figures.
Rounding Off
 When the first digit dropped is 4 or less, the retained
numbers remain the same.
To round 45.832 to 3 significant figures
drop the digits 32 = 45.8
 When the first digit dropped is 5 or greater,
the last retained digit is increased by 1.
To round 2.4884 to 2 significant figures
drop the digits 884 = 2.5 (increase by 0.1)
Multiplication and Division
When multiplying or dividing use
 the same number of significant figures (SF) as the
measurement with the fewest significant figures
Example:
110.5 x 0.048 = 5.304 = 5.3
4SFs
2SFs
calculator
2SFs
Addition and Subtraction
When adding or subtracting, use
 the same number of decimal places as the
measurement with the fewest decimal places
25.2 one decimal place
+ 1.34 two decimal places
26.54 calculated answer
26.5
final answer (with one decimal place)
For each calculation, round the answer to give the
correct number of decimal places.
A. 235.05 + 19.6 + 2 =
1) 257
2) 256.7
3) 256.65
B. 58.925 – 18.2 =
1) 40.725
2) 40.73
3) 40.7
An equality
 states the same measurement in two different units
 can be written using the relationships between two
metric units
Example: 1 meter is the same as 100 cm and 1000 mm.
1 m = 100 cm
1 m = 1000 mm
1m/100cm = 1;
1m/1000mm = 1
1 = 100cm/1m;
1 = 1000mm/1m
volume has the dimensions of
length cubed
 Several equalities can be
written for mass
1 kg = 1000 g
1 g = 1000 mg
1 mg = 0.001 g
Some Common Equalities
• An injured person loses 0.30 pints of blood.
How many milliliters of blood would that be?
0.30pt*1qt/2pt = 0.15qt;
0.30pt*2pt/1qt = 0.60pt2/qt
0.15qt*946mL/qt = 141.9 mL;
140 mL
• If a person weighs 200 pounds, how many
kiograms does the person weight?
• 200 lb*1 kg/2.2 lb = 90.9 kg
• 200 lb*2.2 lb/1 kg = 440 lb2/kg
If the thickness of the skin fold at the
waist indicates an 11% body fat, how
much fat is in a person with a mass
of 86 kg?
11 % fat means 11kg/100kg body weight
86 kg
x
11 kg fat = 9.5 kg of fat
100 kg
Density
Density
compares the mass of an object to its volume
is the mass of a substance divided by its volume
Density expression:
D = mass
= g or g
volume
mL
cm3
= g/cm3
Osmium is a very dense metal. What is its density in g/cm3 if 50.0 g of osmium has a volume of
2.22 cm3?
1) 2.25 g/cm3
2) 22.5 g/cm3
3) 111 g/cm3
• The density of the zinc object
can be calculated from its
mass and volume.
d = 68.6g/(45.0-35.5)mL; 68.6g/9.5 mL
d = 7.2 g/mL