So yesterday we talked about experiments but Experiments

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 welltested 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 Quantity Symbol Unit l Length meter m Mass kilogram t Time second T Thermodynamic kelvin temperature n Amount of a substance mole I Electric current ampere Iv Luminous intensity candela Abbr. m kg s K mol A cd

In Chemistry, we must remember to use a number (quantity) and unit (abbreviation) In other words, 5 is usually only half the answer.

Something else to consider ► 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) § Equalities ►Sometimes more than one

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

Let us look at some examples 0. 765 g to kg 0. 765 g x 1 kg 1000 g = 0. 000765 kg 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 23745 kg x 1000 g 1 kg x 1000 mg 1 g 23745000000 mg =

Try the Problems on the Metrics and Measurement Worksheet Groups 1, 3, 5, and 7 do the odd problems Groups 2, 4, 6 and 8 do the even problems

Answers to those problems 1. 2. 3. 4. 5. 0. 35 0. 950 27. 5 1 1 4. 5 7. 250 8. 0. 5 9. 7. 5 10. 15, 000 6.

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 m. L *1 cm 3 = 1 m. L § Density = g/m. L (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 1. 70766 g/cm 3?