المصدر: Units of Measurement (وحدات القياس ) في منتدى : الكيمياء العامة مقتبس Units of Measurement (وحدات القياس ) When we work problems or make measurements in the lab we will often need to use describe things quantitatively. For example it is not sufficient to say the sample is big, heavy or hot, we need to know how big, how heavy and how hot. In order to do this we need to have a standard system of units to which everything else can be compared. In most scientific fields measurements are reported in SI units (metric system) Base Units ( وحدات القياس الاساسية ) Base units must be arbitrarily defined. For the purposes of this course we will be concerned with the base units for length, mass and temperature. Length ® meter, m (1 m ~ 39.37 inches ~ 1.094 yards) Mass ® kilogram, kg (1 kg ~ 2.2 lbs) Temperature ® Kelvin, K Mass and Weight(الوزن والكتلة) It is worth noting that mass and weight are not equivalent. An object’s weight is a measure of the force of gravity on an object. It varies depending upon the distance of an object from the center of the earth (or planet upon which the object resides). The mass of an object is a measure of the quantity of matter and constant for a given sample regardless of location. Weight ® Dependent upon gravity Mass ® Constant for a given object Temperature Scales (مقاييس الحرارة) The Celsius temperature scale is also commonly used. This temperature scale was derived from the phase transition points of water. Melting point of water ® 0 ° C = 32 ° F Boiling point of water ® 100 ° C = 212 ° F Room Temperature ® 25 ° C = 77 ° F The conversion from Fahrenheit to Celsius is ° C = (° F – 32) ´ (5/9), however we will not be using the Fahrenheit scale at all in this class. In many cases it is more convenient to work with an absolute temperature scale. An absolute temperature scale is one where zero corresponds to the coldest possible temperature, absolute zero (-273.15 ° C). In the SI system the absolute temperature scale is Kelvins, K. The conversion between degrees Celsius and Kelvin is K = ° C + 273.15 Unless the temperature is known to the tenth or hundredth of a degree you can simply add 273 from the temperature in ° C to get temperature in K. Derived Units (وحدات القياس المشتقة ) Derived units can be derived from the base units already defined. Examples of derived units include volume, density and velocity Volume ® Liter, L (1 L ~ 1.05 quarts) The volume of an object describes how much space an object occupies, it can also be specified in units of cubic length (i.e. cm3, m3). There is a special relationship between cubic centimeters, grams and liters. 1 L = 1000 mL 1 mL = 1 cm3 = 1 g of H2O at STP This is how the gram was defined. If a liter was poured into a cubic container the cube would be 10 cm on each side, or 1 dm per side. ® 1 L = 1 dm3 Generally speaking mL’s are used as a unit of volume for liquids and cm3 are used for solids. Density is mass per unit volume r = m/V and is often reported in g/cm3 for solids, g/mL for liquids and g/L for gases. There are many other examples of derived units, such as velocity (m/s), acceleration (m/s2), force (N = kg-m/s2) Velocity is length per unit time v = l/t and is typically reported in units such as m/s, km/hr, miles/hour, etc.