1. So yesterday we talked about experiments, but…

2. Experiments Don’t Always Go According to Plan
Accidental Discoveries
Teflon
Mauve clothing dye

3. Theories and Laws
When a hypothesis withstands repeated testing, it may become a theory – a well-tested explanation of observations
Theories are explanations, not facts
They can be disproved, but can never be completely proven.
Law is a statement or mathematical expression that reliably describes a behavior in the natural world

4. The main difference
While a theory is an attempt to explain the cause of certain events in the natural world, a scientific law describes the events

5. Models
A model is a representation of an object, a system, a process, or an idea.
A model is used to help describe or communicate information about ideas or theories that are difficult to observe or explain otherwise.

6. Using Conversion Factors
Dimensional Analysis
Using Conversion Factors

7. Système Internationale d’Unitès Units
There are 7 SI Base Units
Quantity
Symbol
Unit
Abbr.
Length l
meter m
Mass
kilogram kg
Time t
second s
Thermodynamic
temperature T
kelvin K
Amount of a substance n
mole
mol
Electric current I
ampere A
Luminous intensity Iv
candela cd

8. Derived Units
These units are derived by multiplying or dividing the base units.
Examples
Speed – m/s
Area – m2
Volume – m3, but scientist tend to use the L or mL *1cm3 = 1mL
Density = g/mL (D=m/V)

9. Example Density Problems
What is the density of an 84.7 g sample of an unknown substance if the sample occupies 49.6 cm3?
What volume would be occupied by 7.75 g of a substance with a density of g/cm3?

10. Somethings…
These base units may be too large or small depending on the situation
Therefore we need to use dimensional analysis – conversion factors
For this we need:
Prefixes (page 33, table 2)

11. Helpful Chart to remember
Smaller Bigger deci (d) 10 deca (da) centi (c) 100 (hecto, h) milli (m) 1000 kilo (k) micro (m) mega (M) nano (n) giga (G)

12. Equalities Sometimes more than one

13. Let us look at some examples
0.765 g to kg 1 kg
0.765 g x = kg
1000 g
1) Rewrite what is given
2) Set up the equalities so that the unit you start with is canceled out and the new one is left.
3) Fill in the numerical values
4) Multiply by the top numbers; divide by the bottom numbers

14. Sometimes 2 steps are needed
23745 kg to milligrams
1000 mg
1000 g
23745 kg x x = 1 1 kg g mg

15. Answers to Examples 0.950 kg 4.5 m 3.5 dL 60000 MB
0.15 L (remember 1cm3 = 1mL)