2. Collecting Evidence Introduction to Measurement Chemistry and Physics of Forensics

3. Types of Observations and Measurements We make QUALITATIVE observations of reactions – Some examples include: Changes in color Physical state We also make QUANTITATIVE MEASUREMENTS, which involve numbers

4. Stating a Measurement In every measurement there is a….  Number followed by a  Unit from a measuring device The number should also be as precise as the measurement!

5. Standards of Measurement When we measure, we use a measuring tool to compare some dimension of an object to a standard For example, at one time the standard for length was the king’s foot. What are some problems with this standard?

6. STANDARD UNITS OF MEASUREMENT Standard units of measurement are called SI units which is based on the metric system Length Mass Volume Time Temperature Meter, m Kilogram, kg Seconds, s Celsius degrees, ˚C Kelvin, K Liter, L

7. NOTE – Mass is NOT Weight Mass is the amount of matter in grams – Measured with a BALANCE Weight is the force exerted by mass, only when present with gravity – Measured in pounds with a SCALE Can you hear me now?

8. Some Tools for Measurement Which tool(s) would you use to measure: A. Temperature B. Volume C. Time D. Weight

9. Learning Check Match L) length M) mass V) volume ____ A. A bag of tomatoes is 4.6 kg. ____ B. A person is 2.0 m tall. ____ C. A medication contains 0.50 g Aspirin. ____ D. A bottle contains 1.5 L of water. M L M V

10. Metric Prefixes Sometimes, the base SI unit is not always convenient so instead, we use different prefixes! Kilo- means 1000 of that unit – 1 kilometer (km) = 1000 meters (m) Centi- means 1/100 of that unit – 1 meter (m) = 100 centimeters (cm) – 1 dollar = 100 cents Milli- means 1/1000 of that unit – 1 Liter (L) = 1000 milliliters (mL)

11. Metric Prefixes

12. Metric Prefixes in Action

13. Learning Check 1. 1. 1000 m = 1 ___a) mm b) km c) dm 2. 0.001 g = 1 ___ a) mg b) kg c) dg 3. 0.001 L = 1 ___a) mL b) cL c) dL 4. 0.01 m = 1 ___ a) mm b) cm c) dm

14. Learning Check Select the unit you would use to measure 1. Your height a) millimeters b) meters c) kilometers 2. Your mass a) milligramsb) grams c) kilograms 3. The distance between two cities a) millimetersb) meters c) kilometers 4. The width of an artery a) millimetersb) meters c) kilometers

15. Parts of a Measurement In addition to a number and a unit, all measurements also have some uncertainty associated with them All measurements contain some uncertainty because… – Errors occur – The numbers reported in a measurement are limited by the measuring tool Uncertainty is measured with: Accuracy How close to the true value Precision How close to each other

16. Three targets with three arrows each to shoot. Uncertainty and Measurements Both accurate and precise Precise but not accurate Neither accurate nor precise How do they compare?

17. Taking Uncertainty into Account when Measuring When making a measurement, you must include the known digits PLUS one estimated digit We can only estimate one place beyond what we are sure of What does that mean?!

18. Let’s Try It!. l 2.... I.... I 3....I.... I 4.. cm First digit (known)= 2 2.?? cm Second digit (known)= 0.7 2.7? cm Third digit (estimated) between 0.05- 0.07 Length reported=2.75 cm or2.74 cm or2.76 cm Is one answer better than the other? NO!

19. Known + Estimated Digits In 2.76 cm… Known digits 2 and 7 are 100% certain The third digit 6 is estimated (uncertain) In the reported length, all three digits (2.76 cm) are significant including the estimated one

20. Learning Check. l 8.... I.... I 9....I.... I 10.. cm What is the length of the line? 1) 9.6 cm 2) 9.62 cm 3) 9.63 cm How does your answer compare with your neighbor’s answer? Why or why not?

21. Another Example. l 3.... I.... I 4.... I.... I 5.. cm What is the length of the line? First digit 5.?? cm Second digit 5.0? cm Last (estimated) digit is 5.00 cm

22. Always estimate ONE place past the smallest mark!

23. USING MEASUREMENTS FOR CALCULATIONS…YAY!

24. DENSITY - An important and useful physical property Mercury 13.6 g/cm 3 21.5 g/cm 3 Aluminum 2.7 g/cm 3 Platinum

25. DENSITYDENSITY Density is an INTENSIVE property of matter – Means it does NOT depend on quantity of matter – Another example is temperature This is different than an EXTENSIVE property of matter – Depends on quantity of matter – Examples include mass and volume Styrofoam Brick

26. How to Calculate Density from Measurements 1.Measure the length, width, height of an object and calculate volume in cubic centimeters V = l x w x h OR 2.Measure the volume of a liquid using a graduated cylinder cylinder 3. Measure the mass of the object using a balance 4. Calculate the density by dividing mass and volume

27. Learning Check Osmium is a very dense metal. What is its density in g/cm 3 if 50.0 g of the metal occupies a volume of 2.22cm 3 ? 1) 2.25 g/cm 3 2)22.5 g/cm 3 3)111 g/cm 3

28. Solution Placing the mass and volume of the osmium metal into the density setup, we obtain: D = mass = 50.00 g = volume2.22 cm 3 = 22.522522 g/cm 3 = 22.5 g/cm 3

29. Measuring the Volume of a Weird Object by Displacement A solid displaces a matching volume of water when the solid is placed in water 33 mL 25 mL

30. Learning Check What is the density (g/cm 3 ) of 48 g of a metal if the metal raises the level of water in a graduated cylinder from 25 mL to 33 mL? 1) 0.2 g/ cm 3 2) 6 g/m 3 3) 252 g/cm 3 33 mL 25 mL