So yesterday we talked about experiments, but…

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

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

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

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.

Using Conversion Factors Dimensional Analysis Using Conversion Factors

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

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)

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 1.70766 g/cm3?

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)

Helpful Chart to remember Smaller Bigger deci (d) 10 deca (da) centi (c) 100 (hecto, h) milli (m) 1000 kilo (k) micro (m) 1000000 mega (M) nano (n) 1000000000 giga (G)

Equalities Sometimes more than one

Let us look at some examples 0.765 g to kg 1 kg 0.765 g x = 0.000765 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

Sometimes 2 steps are needed 23745 kg to milligrams 1000 mg 1000 g 23745 kg x x = 1 1 kg g 23745000000 mg

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