What s in a Scientist s Toolbox?

Transcription

1 What s in a Scientist s Toolbox? OBJECTIVES After completing this lab you will be able to: Make different kinds of scientific measurements with several common laboratory tools. Use assorted measurement scales, units and factors that influence variability in common laboratory measurements. Explain the precision and accuracy of a measurement. Describe how the number of significant figures reflect the precision of a measurement. PROBLEM / QUESTION How do we choose which scientific tools (instruments) to use when making measurements? PRIOR KNOWLEDGE Metric system International System of Units (SI) MATERIALS Station 1. Length Yellow metric ruler (0.5 cm divisions) Orange metric ruler (0.1 cm divisions) Wooden ruler (1 divisions) Tape measure (ft. & inches) Plastic rod Station 2. Time Wall clock/wrist watch (with sweeping second hand) Digital stopwatch (0:00:00) Timer, count up/count down (0:00) Drinking Happy Bird ~200 ml beaker for Happy Bird to drink from Station 3. Temperature 1 Dial thermometer (50 to 400 o F/5 o resolution) 2 Dial thermometer ( - 10 to +110 o C/2 o resolution) 1

2 Non-mercury thermometer ( - 10 to +110 o C/1 o resolution) Digital thermometer ( - 50 to +280 o C & -58 to +536 o F/0.1 o resolution) Station 4. Mass Spring scale (100 g x 1 g) 1 Electronic balance (0.01 g) 2 Electronic balance (0.001 g) 2 pan balance (50 g x 10 mg) Wooden block Station 5. Volume ml volumetric flask DI water Transfer pipets 50 ml graduated cylinder 100 ml beaker 125 ml Erlenmeyer flask Station 6. Accuracy and Precision Diagrams of various targets PRELAB A 1. Review the metric system 2. Review SI units in your text 3. Describe common measurements and various units used in everyday life. 4. Explain what you consider to be the purpose of measuring something. PRELAB B Part 1. Uncertainty in Measurement Figure 1. Centimeters (cm) Figure 1. A small section of a rod has been placed in between two different rulers. (Adapted from image at Centimeters (cm) Consult with your lab partner(s) and determine the length of the rod. 1. Use the left ruler. 2

3 2. Use the right ruler. 3. Report your answers on the board and discuss the results as a class. Part 2. Significant Figures (sig figs, for short) The number of significant figures (or significant digits) in a measurement, such as 1.49 cm, above, is equal to the number of digits that are known with some degree of confidence (1 and 4) plus the last digit (9), which is an estimate or approximation. As the sensitivity of the tool used to make a measurement improves, the number of significant figures increases, thus increasing the degree of certainty in the measurement. It s important to note that digital tools provide the estimated digit for us. All nonzero digits that are measured are significant; however, sometimes zeros are significant, while other times they are not. Examine Figure 2 below and measure the lengths of the objects. Think about the role of zeros in these measurements. Length of rod = Figure 2. Centimeter ruler pictured is not to scale. Length of rod = Figure 2. Centimeter rulers are not to scale. Think about how many zeros should be in each of the above measurements, particularly if you make your measurements properly with one uncertain digit. How can we show in the measurement that the second ruler is more sensitive (has more divisions)? Sig Fig Rules To determine the number of significant figures in a measurement, follow these simple rules: 1. Nonzero digits are significant. 3,971,874,351,298 has 13 sig figs 2. Zeros: There are three classes of zeros. a. Leading zeros are zeros that precede all nonzero digits. They are never significant 0.35 has 2 sig figs; has 3 sig figs b. Captive zeros are zeros between nonzero digits. They are always significant. 2,074,003 has 7 sig figs c. Trailing zeros are zeros at the right end of the number. They are significant only if the number contains a decimal point (or your teacher has a different rule). 5,000. has 4 sig figs; 38,000 has 2 sig figs; has 5 sig figs To which class of zeros in the rules above do the measurements in Figure 2 apply? Trailing zeros with a decimal point 3

4 Consult with your lab partner(s) and determine how many significant figures are in the following numbers. Number of Example Significant Comment (Cite Rule) Figures , Report your answers on the board and discuss the results as a class. PROCEDURE, DATA, OSERVATIONS & RESULTS Obtain a team number from your instructor to report class results on the board when you have completed all stations. My lab group (team) number is: Station 1. Length Carefully measure the length of the plastic rod with each of the 4 tools provided. Use proper units and the correct number of significant figures for your measurements and record your results in Table 1. Table 1. Length Measurements Yellow Metric Ruler Orange Metric Ruler Wooden Ruler Measuring Tape Station 2. Time Carefully measure how long it takes Happy Bird to take one drink of water with each of the 3 tools provided. Start/stop timing one drink of water when Happy Bird s beak touches the surface of the water. Use proper units and the correct number of significant figures for your measurements and record your results in Table 2. Table 2. Time Measurements Wall Clock/Wrist Watch Digital Stopwatch Timer, Count Up/Count Down 4

5 Station 3. Temperature Carefully measure the temperature of water dispensed from a DI water bottle with each of the 4 tools provided. Use proper units and the correct number of significant figures for your measurements and record your results in Table 3. Table 3. Temperature Measurements 1 Dial Thermometer (Ertco) 2 Dial Thermometer (Weston) Non-mercury Thermometer Digital Thermometer Station 4. Mass Note: always zero the scale or balance prior to taking measurements. Carefully measure the mass of the wooden block with each of the 4 tools provided. Use proper units and the correct number of significant figures for your measurements and record your results in Table 4. Table 4. Mass Measurements Spring Scale 1 Electronic Balance 2 Electronic Balance Two Pan Balance Station 5. Volume Note: extra care is necessary when measuring volume due to the curved surface of a liquid known as a meniscus. The proper place to make a volume measurement is at the bottom of the meniscus. To properly determine the location of the bottom of the meniscus, the level of the eye must be equal to the bottom of the meniscus. Fill a 25.00mL volumetric flask so that the meniscus touches the line, transfer the water from the flask to one of the tools provided and carefully measure the volume of DI water. Repeat this step for each of the 3 tools. Use proper units and the correct number of significant figures for your measurements and record your results in Table 5. Table 5. Volume Measurements Graduated Cylinder (50 ml) Beaker (100 ml) Erlenmeyer Flask (125 ml) 5

6 Station 6. Accuracy and Precision 1. Consult with your lab partner(s) and define accuracy as it relates to measurements. 2. Consult with your lab partner(s) and define precision as it related to measurements. 3. Review each target (1-4) and determine if the measurements represented by (X) illustrate accuracy or precision. Record your results in Table 6. Table 6. Accuracy and Precision Measurements 1 Target 2 Target 3 Target 4 Target ANALYSIS & DISCUSSION 1. Examine your measurement data for each station (1, 2 & 3) to determine which tool at each station exhibits the highest degree of certainty? Why? 2. Examine your measurement data for each station (3, 4, & 5) to determine which tool at each station exhibits the lowest degree of certainty? Why? 3. List possible sources of error for measurements taken at each station (1, 3, & 4). 6

7 4. Examine your data for each station (5 & 6) to determine which tool at the volume station is most accurate? Why? 5. Examine target 4 from station 6 to explain why the measurement represented by (X) is not precise? GOING FURTHER Examine your data to answer the following questions. 1. Which tool would you use to measure the height of your lab partner? Why? 2. If you wanted to measure someone s time running a 100meter sprint what tool would you use? Why? 3. If you wanted to measure someone s time running a marathon what tool would you use? Why? 4. If you wanted to monitor the temperature of meat when cooking a roast what tool would you use? Why? 7