2.6 SI Units

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2.6 SI Units

• The International System of Units, SI, is a
  revised version of the metric system
• Correct units along with numerical values are
  critical when communicating measurements.
• The are seven base SI units (Table 2.1) of which
  other SI units are derived.
• Sometimes non-SI units are preferred for
  convenience or practical reasons

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2.6 SI Units Table 2.2
Quantity SI Base or Derived Unit Non-SI Unit
Length meter (m)
Volume cubic meter (m3) liter
Mass kilogram (kg)
Density grams per cubic centimeter (g/cm3) grams per mililiter (g/mL)
Temperature kelvin (K) degree Celcius (C)
Time second (s)
Pressure Pascal (Pa) atmosphere (atm) milimeter of mercury (mm Hg)
Energy joule (J) calorie (cal)
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Common SI Prefixes

• Units larger than the base unit

Tera T e12 base units termeter (Tm)
Giga G e9 base units gigameter (Gm)
Mega M e6 base units megameter (Mm)
Kilo k e3 base units kilometer (km)
Hecto h e2 base units hectometer (hm)
Deka da e1 base units decameter (dam)
Base Unit e0 base units meter (m)
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Common SI Prefixes

• Units smaller than the base unit

Base Unit e0 base units meter (m)
Deci d e-1 base units decimeter (dm)
Centi c e-2 base units centimeter (cm)
Milli m e-3 base units millimeter (mm)
Micro µ e-6 base units micrometer (µm)
Nano n e-9 base units Nanometer (nm)
Pico p e-12 base units picometer (pm)
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Common SI Prefixes

• A mnemonic device can be used to memorize these
  common prefixes in the correct order
• The Great Monarch King Henry Died By Drinking
  Chocolate Mocha Milk Not Pilsner

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2.7 Units of Length

• The basic unit of length is the meter
• Prefixes can be used with the base unit to more
  easily represent small or large measurements
• Example A hyphen (12 point font) measures about
  0.001 m or 1 mm.
• Example A marathon race is approximately
• 42,000 m or 42 km.

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2.7 Concept Practice

• 15. Use the tables in the text to order these
  lengths from smallest to largest.
• a. centimeter
• b. micrometer
• c. kilometer
• d. millimeter
• e. meter
• f. decimeter

- 3
- 1 (smallest)
- 6 (largest)
- 2
- 5
- 4
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2.8 Units of Volume

• The space occupied by any sample of matter is
  called its volume
• The volume of rectangular solids can be
  calculated by multiplying the length by width by
  height
• Units are cubed because you are measuring in 3
  dimensions
• Volume of liquids can be measured with a
  graduated cylinder, a pipet, a buret, or a
  volumetric flask

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2.8 Units of Volume

• A convenient unit of measurement for volume in
  everyday use is the liter (L)
• Milliliters (mL) are commonly used for smaller
  volume measurements and liters (L) for larger
  measurements
• 1 mL 1 cm3
• 10 cm x 10 cm x 10 cm 1000 cm3 1 L

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2.8 Units of Volume
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2.8 Concept Practice

• 17. From what unit is a measure of volume
  derived?
• A Volume is a length measurement cubed.

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2.8 Practice

• 18. What is the volume of a paperback book 21 cm
  tall, 12 cm wide, and 3.5 cm thick?
• A 882 cm3 ? 880 cm3 8.8 x 102 cm3
• 19. What is the volume of a glass cylinder with
  an inside diameter of 6.0 cm and a height of 28
  cm?
• Vpr2h
• A 790 cm3 7.9 x 102 cm3

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2.9 Units of Mass

• A person on the moon would weigh 1/6 of his/her
  weight on Earth.
• This is because the force of gravity on the moon
  is approximately 1/6 of its force of Earth.
• Weight is a force it is a measure of the pull
  on a given mass by gravity it can change by
  location.
• Mass is the quantity of matter an object contains
• Mass remains constant regardless of location.
• Mass v. Weight

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2.9 Units of Mass

• The kilogram is the basic SI unit of mass
• It is defined as the mass of 1 L of water at 4C.
• A gram, which is a more commonly used unit of
  mass, is 1/1000 of a kilogram
• 1 gram the mass of 1 cm3 of water at 4C.

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2.9 Concept Practice

• 20. As you climbed a mountain and the force of
  gravity decreased, would your weight increase,
  decrease, or remain constant? How would your mass
  change? Explain.
• A Your weight would decrease mass would remain
  constant.
• 21. How many grams are in each of these
  quantities?
• a. 1 cg b. 1 µg c. 1 kg d. 1mg
• A 0.01g 0.000001g 1000g 0.001 g

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2.10 Density

• Density is the ratio of the mass of an object to
  its volume.
• Equation ? D mass/volume
• Common units g/cm3 or g/mL
• Example 10.0 cm3 of lead has a mass 114 g
• Density (of lead) 114 g / 10.0 cm3 11.4
  g/cm3
• See Table 2.7, page 46

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2.10 Density

• Density determines if an object will float in a
  fluid substance.
• Examples Ice in water hot air rises
• Density can be used to identify substances
• See Table 2.8, page 46

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2.10 Concept Practice

• 22. The density of silver is 10.5 g/cm3 at 20C.
  What happens to the density of a 68-g bar of
  silver that is cut in half?
• A Its density does not change.

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2.10 Concept Practice

• 23. A student finds a shiny piece of metal that
  she thinks is aluminum. In the lab, she
  determines that the metal has a volume of 245
  cm3 and a mass of 612 g. Is the metal aluminum?
• A Density 2.50 g/cm3 the metal is not
  aluminum.
• 24. A plastic ball with a volume of 19.7 cm3 has
  a mass of 15.8 g. Would this ball sink or float
  in a container of gasoline?
• A Density 0.802 g/cm3 the ball will sink.

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2.10 Specific Gravity (Relative Density)

• Specific gravity is a comparison of the density
  of a substance to the density of a reference
  substance, usually at the same temperature.
• Water at 4C, which has a density of 1 g/cm3, is
  commonly used as a reference substance.
• Specific gravity density of substance (g/cm3)
• density of water (g/cm3)
• Because units cancel, a measurement of specific
  gravity has no units
• A hydrometer can be used to measure the specific
  gravity of a liquid.

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2.11 Concept Practice

• 25. Why doesnt a measurement of specific gravity
  have a unit?
• A Because it is a ratio of two density
  measurements, the density units cancel out.
• 26. Use the values in Table 2.8 to calculate the
  specific gravity of the following substances.
• a. Aluminum b. Mercury c. ice
• A 2.70 13.6 0.917

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2.12 Measuring Temperature

• Temperature determines the direction of heat
  transfer between two objects in contact with each
  other.
• Heat moves from the object at the higher
  temperature to the object at a lower temperature.
• Temperature is a measure of the degree of hotness
  or coldness of an object.
• Almost all substances expand with an increase in
  temperature and contract with a decrease in
  temperature
• An important exception is water

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2.12 Measuring Temperature

• There are various temperature scales
• On the Celsius temperature scale the freezing
  point of water is taken as 0C and the boiling
  point of water at 100C

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2.12 Measuring Temperature

• The Kelvin scale (or absolute scale) is another
  temperature scale that is used
• On the Kelvin scale the freezing point of water
  is
• 273 K and the boiling point is 373 K (degrees
  are not used).
• 1C 1 Kelvin
• The zero point (0 K) on the Kelvin scale is
  called absolute zero and is equal to -273C
• Absolute zero is where all molecular motion stops

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2.12 Measuring Temperature

• Converting Temperatures
• K C 273
• C K - 273

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2.12 Concept Practice

• 27. Surgical Instruments may be sterilized by
  heating at 170C for 1.5 hours. Convert 170C to
  kelvins.
• A K 170C 273 443 K
• 28. The boiling point of the element argon is 87
  K. What is the boiling point of argon in C?
• A C 87 K 273 -186C

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2.13 Evaluating Measurements

• Accuracy in measurement depends on the quality of
  the measuring instrument and the skill of the
  person using the instrument.
• Errors in measurement could have various causes
• In order to evaluate the accuracy of a
  measurement, you must be able to compare it to
  the true or accepted value.

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2.13 Evaluating Measurements

• accepted value the true or correct value based
  or reliable references
• experimental value the measured value
  determined in the experiment
• The difference between the accepted value and the
  experimental value is the error.
• error accepted value experimental value

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2.13 Evaluating Measurements

• The percent error is the error divided by the
  accepted value, expressed as a percentage of the
  accepted value.
• Percent Error x 100
• An error can be positive or negative, but an
  absolute value of error is used so that the
  percentage is positive

error AV
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2.13 Concept Practice

• 32. A student estimated the volume of a liquid in
  a beaker as 200 mL. When she poured the liquid
  into a graduated cylinder she measured the value
  as 200 mL. What is the percent error of the
  estimated volume from the beaker, taking the
  graduated cylinder measurement as the accepted
  value?
• A Percent Error x 100 4

200 mL - 208 mL 200 mL