Measurements – Base Units All measurements consist of two parts: a. the number b. the unit There are two systems of measurements used in the United States today. - English System – inches, feet, gallons, etc. - The Metric System or SI (International System of Units) – meters, liters, kilograms, etc. In scientific work, the metric system or the revised version of this system called the SI is used. The metric system is a decimal unit system, which means that conversion from one unit to another can be accomplished by multiplying or dividing by multiples of 10. There are seven base units; the other units are derived from these seven base units. During this course, we will use the first five. Base Units Quantity Quantity Symbol Unit Name Unit Abbreviation Length l meter m Mass m kilogram kg Time t second s Temperature T kelvin K Amount of Substance n mole mol Electric Current I ampere A Luminous Intensity Iv candela cd The meter is the SI unit of length. Kilometer, centimeter, or millimeter can also be used to measure lengths much larger or smaller than the meter. Length is a measure of distance. The kilogram is the SI unit of mass. The gram or milligram can also be used for smaller masses. Mass is not the same as Weight. Mass is the amount of matter in an object. Weight is a measure of the gravitational pull on matter. The second is the SI unit of time. In chemistry, chemical reactions may take seconds or fractions of a second to complete, so time can be measured in seconds, milliseconds, nanoseconds, or less. When measuring times much longer than about 100 seconds, we can express the time in minutes or hours. Temperature is the average kinetic energy of the particles that make up a substance. The Kelvin is the SI unit of temperature. The Kelvin scale is an absolute temperature scale, which means that lowest temperature is given the value zero. So, 0 Kelvin is the coldest temperature that could exist. The Celsius scale is used by much of the world, as well as by the scientific community. A third temperature scale is the Fahrenheit scale. Derived units – combination of SI base units. Derived units are produced by multiplying or dividing standard units. Derived units commonly used: Quantity Derivation Unit Abbreviation Unit Area length x width m2 squared meter Volume length x width x height m3 cubic meter Density mass/volume kg/m3 kilograms per cubic meter Molar mass mass/amount of substance kg/mol kilograms per mole Energy force x distance N ∙ m or J Joules Area – a measure of the extent of a surface. Area is measured in squared units. Square – side x side Rectangle – length x width Circle - п x radius2 Triangle – ½ base x height Volume- the amount of space occupied by an object. The SI unit of volume is the cubic meter (m3), however, this unit is inconvenient for expressing volume, so the cubic centimeter (cm3) is often used in the laboratory. The liter (L) is also used in chemistry. 1 liter = 1 cubic decimeter (dm3) = 1000 cubic centimeters (cm3) = 1000 milliliters (mL) So, 1 mL = 1 cm3 How to measure volume: 1. Mathematically (using a ruler) a. rectangular solids – length x width x height b. cube - side x side x side c. cylinder – п x radius2 x height d. sphere – 4/3 x п x radius3 * report your answer to the correct number of significant figures 2. Water displacement method a. measure how much water is displaced by the object. i.e. how much did the water rise after the object was introduced in the water. b. the amount of water displaced by the object is equal to the volume of the object * report your answer to the correct number of significant figures Density – is the ratio of mass to volume (mass/volume). The SI unit is kg/m3, however g/cm3 or g/mL are often used. For gases, density can be reported in g/L. How to find density: 1. measure the mass of the object to the correct number of significant figures 2. measure the volume of the object to the correct number of significant figures 3. divide the mass by the volume and round off the answer to the correct number of significant figures. .