Unit 1: Introduction to
Chemistry
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Types of Measurements and Observations, Scientific Notation
Day 3 - Notes
• Measurement: a type of observation
• Qualitative measurements: descriptive–Ex: hot, cold, heavy, light, big,
blue, furry• Quantitative measurement:
observation made with a measuring instrument and includes both a number and a unit–Ex: ruler, balance, thermometer,
graduated cylinder, 13.5°C, 25kg, 17L
• Accuracy: How close a measurement is to the true or accepted value–Ex: Weighing a 50g mass
50.00g – accurate32.18g – not accurate49.99g – accurate
• Precision: How close multiple measurements are to each other– Ex: Take the weight of a 50g mass
Accurate, precise: Accurate, precise:50.00g 50.00g50.00g 49.99g50.00g 50.00g
Not accurate, precise:32.18g32.18g32.18g
An easy way to remember…
ACcurate = CorrectPRecision =
Reproducibility
Significance in Measurement
. Which of the following best describes the length of the beetle's body in the picture to the left?
a) Between 0 and 2 in b) Between 1 and 2 inc) Between 1.5 and 1.6 ind) Between 1.54 and 1.56
ine) Between 1.546 and
1.547 in
Significance in Measurement
The correct answer is . . .
(d), between 1.54 and 1.56 inches
Significance in Measurement
Measurements are often written as a single number rather than a range. The beetle's length in the previous frame was between 1.54 and
1.56 inches long. The single number that best represents the measurement is the
center of the range, 1.55 inches. When you write the measurement as a single number, it's
understood that the last figure (the second of the two 5’s in this case) had to be estimated. Consider measuring the length of the same object with two different rulers.
Significance in Measurement
. For each of the rulers, give the correct length measurement for the steel pellet as a single number rather than a range
Significance in Measurement
For the ruler on the left you should have had . . .1.4 in
For the ruler on the right, you should have had . . .1.47 in
Significance in Measurement
A zero will occur in the last place of a measurement if the measured value fell exactly on a scale division. For example, the
temperature on the thermometer should be recorded as 30.0°C.
Reporting the temperature as 30°C would imply that the measurement had been taken on a thermometer with scale marks 100°C apart!
Significance in Measurement Use the bottom of the
meniscus (the curved interface between air and liquid) as a point of reference in making measurements of volume in a graduated cylinder, pipet, or buret.
In reading any scale, your line of sight should be perpendicular to the scale to avoid 'parallax' reading errors.
Significance in Measurement
The graduated cylinder on the right has scale marks 0.1 mL apart, so it can be read to the nearest 0.01 mL.
Reading across the bottom of the meniscus, a reading of 5.72 mL is reasonable (5.73 mL or 5.71 mL are acceptable, too).
Significance in Measurement
Determine the volume readings for the two cylinders to the right, assuming each scale is in mL.
Significance in Measurement
For the cylinder on the left, you should have measured . . .
3.0 mL For the cylinder on the right, you should have
measured . . .0.35 mL