Measurement of Matter SI Metric Units

Chemistry is a quantitative science, which means that in many cases we can measure a property of a substance and compare it with a standard having a known value of the property. We express the measurement as the product of a number and a unit. The unit indicates the standard against which the measured quantity is being compared. When we say that the length of the playing field in football is 100 yd, we mean that the field is 100 times as long as a standard of length called the yard (yd). In this section, we will introduce some basic units of measurement that are important to chemists. 

The scientific system of measurement is called the Systkme Internationale d Unites (International System of Units) and is abbreviated SI. It is a modern version of the metric system, a system based on the unit of length called a meter (m). The meter was originally defined as 1/10,000,000 of the distance from the equator to the North Pole and translated into the length of a metal bar kept in Paris. Unfortunately, the length of the bar is subject to change with temperature, and it cannot be exactly reproduced. The SI system substitutes for the 

Standard meter bar an unchanging, reproducible quantity 1 meter is the distance traveled by light in a vacuum in 1/299,792,458 of a second. Length is one of the seven fundamental quantities in the SI system (see Table 1.1). All other physical quantities have units that can be derived from these seven. SI is a decimal system. Quantities differing from the base unit by powers of ten are noted by the use of prefixes. For example, the prefix kilo means one thousand (10 ) times the base unit it is abbreviated as k. Thus 1 kilometer = 1000 meters, or 1 km = 1000 m. The SI prefixes are listed in Table 1.2. 

Most measurements in chemistry are made in SI units. Sometimes we must convert between SI units, as when converting kilometers to meters. At other times we must convert measurements expressed in non-Sl units into SI units, or from SI units into non-Sl units. In all these cases, we can use a conversion-factor or a series of conversion factors in a scheme called a conversion pathway. Later in this chapter, we will apply conversion pathways in a method of problem solving known as dimensional analysis. The method itself is described in some detail in Appendix A. 

Mass describes the quantity of matter in an object. In SI the standard of mass is 1 kilogram (kg), which is a fairly large unit for most applications in chemistry. More commonly we use the unit gram (g). 

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The Scientific Method 1-2 Properties of Matter 1-3 Classification of Matter 1-4 Measurement of Matter SI (Metric) Units... 

Accurate measurement is crucial to scientific experimentation. The units used are those of the Systeme Internationale (SI units). There are seven fundamental SI units, together with other derived units Mass, the amount of matter an object contains, is measured in kilograms (kg) length is measured in meters (m) temperature is measured in kelvins (K) and volume is measured in cubic meters (m3). The more familiar metric liter (L) and milliliter (mL) are also still used for measuring volume, and the Celsius degree (°C) is still used for measuring temperature. Density is an intensive physical property that relates mass to volume. 

Another important measurable quantity is mass, which can be defined as the quantity of matter present in an object. The fundamental SI unit of mass is the kilogram. Because the metric system, which existed before the SI system, used the gram as the fundamental unit, the prefixes for the various mass units are based on the gram, as shown in Table 5.5. 

The development of the metric system, which served as the basis of the International System of Units (Le Systeme International d Unites known as SI), occurred during the French Revolution in the mid-eighteenth century. This coincided with the beginning of the age of modern science, especially chemistry and physics, as the value of physical measurements in the conduct of those pursuits became apparent. As scientific activities became more precise and founded on sound theory, the common nature of science demanded an equally consistent system of units and measurements. The units in the SI have been defined by international accord to provide consistency in all fields of endeavor. The basic units are defined for only seven fundamental properties of matter. All other consistent units are derived as functions of these seven fundamental units. 

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