So yesterday we talked about experiments, but…

Experiments Don’t Always Go According to Plan ► Accidental Discoveries  Teflon (page 48)  Mauve clothing dye (Page 49)

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.

Dimensional Analysis Using Conversion Factors

Système Internationale d’Unitès Units ► There are 7 SI Base Units QuantitySymbolUnitAbbr. Lengthlmeterm Massmkilogramkg Timetseconds ThermodynamictemperatureTkelvinK Amount of a substance nmolemol Electric current IampereA Luminous intensity IvIvIvIvcandelacd

Derived Units ► These units are derived by multiplying or dividing the base units. ► Examples  Speed – m/s  Area – m 2  Volume – m 3, but scientist tend to use the L or mL *1cm 3 = 1mL  Density = g/mL (D=m/V)

Example Density Problems (from page 858) 16) What is the density of an 84.7 g sample of an unknown substance if the sample occupies 49.6 cm 3 ? 17) What volume would be occupied by 7.75 g of a substance with a density of g/cm 3 ?

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 13; Table 3) Prefixes

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

Equalities Sometimes more than one

Let us look at some examples g kg g to kg x kg 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 =

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

Answers to Examples ► kg ► 4.5 m ► 3.5 dL ► MB ► 0.15 L (remember 1cm 3 = 1mL)