Measurement in Science Unit 1 Lesson 2 Lecture Notes

Essential Questions 1. What are the sources of uncertainty in measurement? 2. How do you record a measurement with the correct # of significant figures using both digital and non-digital equipment?

(1) Intro to Measurement Because scientific theories are based on observation and experimentation, careful measurements are very important in science, especially in Chemistry

(1) Intro to Measurement No measurement is perfect. All contain some degree of “built-in” uncertainty. Limitations in precision of lab equipment Estimation requires some subjective judgment This leads to random error

(1) Intro to Measurement You CAN’T eliminate all uncertainty in measurement You CAN record measurements in a way that tells others how much uncertainty there is It’s all about the number of digits we record

(2) Recording Measurements How many digits should you record? Record all “certain digits” those for which there is a graduation (marking) on your instrument Record one “estimated digit” the digit that is between two graduations can’t be measured directly so it must be estimated

(2) Recording Measurements All measured digits + one estimated digit are collectively known as “significant figures” or “sig figs”

(2) Recording Measurements Rulers, meter sticks, triple beam balances, graduated cylinders, and thermometers are a few of the instruments you might use in lab Beakers and flasks are not used when precise measurements are needed, graduated cylinders or pipettes are better for precise measurement

(2) Recording Measurements Step 1: determine the value of each increment on the measuring device This ruler is labeled at every 1 cm There are 10 lines between each 1 cm mark Each line represents 0.1 cm (or 1 mm)

(2) Recording Measurements Step 2 record all certain digits + one estimated digit You can record with certainty 2.7 cm The bar is about halfway between 2.7 and 2.8 cm Record the length as 2.85 cm

(2) Recording Measurements Give the length of the object shown using each ruler (unit = cm) Which ruler gives the most accurate measurement? Ruler A: 6.2 cm Ruler B: 62.2 cm A B A. 6.2 cm B. 62.2 cm

(3) How to Measure Liquid Volume Measure from the bottom of the curved surface of the liquid (the meniscus) at eye level Viewing at an angle will lead to inaccurate readings In the illustration the volume is between 9.7 mL and 9.8 mL and you can estimate here to 0.1 mL since marks are given every 1 mL. Doing so gives a volume of 9.76 mL.

(3) How to Measure Liquid Volume Step 1 : determine the value of each increment on the device The cylinder is labeled at every 5 mL There are 10 lines btwn each 5 mL mark Each line represents 0.5 mL Step 2: record all certain digits + 1 estimated digit You can record with certainty 11.5 mL Meniscus is right at the 11.5 mL mark Record the volume as 11.50 mL

(3) How to Measure Liquid Volume: practice reading each of the following 32.6 mL 33.6 mL 8.46 mL 32.6 mL 33.6 mL 8.46 mL A B C

(3) How to Measure Liquid Volume The more graduations an instrument has, the more precise the measurements you can make with it Always measure using the smallest possible device e.g., if you are measuring 9 mL of fluid, use the 10 mL cylinder instead of the 100-mL cylinder

(4) The Volume of Solids For regular shaped solids, measure the dimensions and apply these formulas:

(4) The Volume of Solids Calculate the volume of these objects. Cylinder: Volume of a cylinder = 3.14 x Radius x Radius x Length Volume = 3.14 x Radius x Radius x Length Radius = 4.5 x .5 = 2.25 inches Length = 11.5 - .75 = 10.75 (.75 represents enclosure material thickness) V = 3.14 x 2.25 x 2.25 x 10.75V = 170.98 cubic inches

(4) The Volume of Solids For irregular shaped solids, use the water displacement method The amount that the water level in the cylinder rises when the object is dropped in = the volume of the object What is this cartoon’s volumn?

(4) The Volume of Solids What is the volume of this dinosaur?

(5) Measuring with Digital Equipment Some science lab equipment is digital balance, pH meter, gas pressure probe, accelerometer The rules for recording digital measurements are slightly different (but easier!)

(5) Measuring with Digital Equipment The # on the digital display has one estimated digit included Just record what you see on the display The last digit may fluctuate. Choose the number that seems to “stick”

(5) Measuring with Digital Equipment There is a special procedure for measuring mass on a digital balance that ensures you don’t include the mass of the container in your measurement Never place items to be massed directly onto scale-always use a container

(5) Measuring with Digital Equipment Turn scale on Set mode to grams Press “mode” until a “g” appears Place empty container on platform Never place item directly on balance. Press “tare” This will zero the balance so that the mass of the container will not be included in the readout. Add substance to be weighed Record displayed measurement Last digit displayed is the estimated one Last digit may flicker. Record the digit that the device seems to “prefer.” Repeat twice and take average of 3 measurements.

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