21

Table 2.4 The Relationship of the Literand Milliliter

Unit Symbol Equivalence

liter L 1 L � 1000 mLmilliliter mL � 10�3 L � 1 mL1

1000 L

100mL

90

80

70

60

50

40

30

20

10

Figure 2.3A 100-mL graduated cylinder.

Another important measurable quantity is mass, which can be definedas the quantity of matter present in an object. The fundamental SI unit ofmass is the kilogram. Because the metric system, which existed before theSI system, used the gram as the fundamental unit, the prefixes for the var-ious mass units are based on the gram, as shown in Table 2.5.

In the laboratory we determine the mass of an object by using a bal-ance. A balance compares the mass of the object to a set of standard masses

C H E M I S T R Y I N F O C U S

Measurement: Past, Present, and Future

Measurement lies at the heart of doing science. Weobtain the data for formulating laws and testingtheories by doing measurements. Measurementsalso have very practical importance; they tell us ifour drinking water is safe, whether we are anemic,and the exact amount of gasoline we put in ourcars at the filling station.

Although the fundamental measuring deviceswe consider in this chapter are still widely used,new measuring techniques are being developedevery day to meet the challenges of our increas-ingly sophisticated world. For example, engines inmodern automobiles have oxygen sensors thatanalyze the oxygen content in the exhaust gases.This information is sent to the computer that controls the engine functions sothat instantaneous adjustmentscan be made in spark timing andair– fuel mixtures to provide effi-cient power with minimum air pollution.

As another example, considerairline safety: How do we rapidly,conveniently, and accuratelydetermine whether a given pieceof baggage contains an explosivedevice? A thorough hand-search ofeach piece of luggage is out of thequestion. Scientists are now devel-oping a screening procedure thatbombards the luggage with high-energy particles that cause anysubstance present to emit radia-

tion characteristic of that substance. This radiationis monitored to identify luggage with unusuallylarge quantities of nitrogen, because most chemi-cal explosives are based on compounds containingnitrogen.

Scientists are also examining the natural worldto find supersensitive detectors because manyorganisms are sensitive to tiny amounts of chemi-cals in their environments—recall, for example, thesensitive noses of bloodhounds. One of these nat-ural measuring devices uses the sensory hairs fromHawaiian red swimming crabs, which are conn-ected to electrical analyzers and used to detecthormones down to levels of 10�8 g/L. Likewise, tissues from pineapple cores can be used to

detect tiny amounts of hydrogenperoxide.

These types of advances inmeasuring devices have led to anunexpected problem: detecting allkinds of substances in our foodand drinking water scares us.Although these substances werealways there, we didn’t worry somuch when we couldn’t detectthem. Now that we know they arepresent what should we do aboutthem? How can we assess whetherthese trace substances are harm-ful or benign? Risk assessmenthas become much more compli-cated as our sophistication in tak-ing measurements has increased.

A pollution control officer mea-suring the oxygen content ofriver water.

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