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Chemistry: Chapter 2 Fall 2008

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SI Units For a measurement to make sense, it requires both a number and a unit. Many of the units you are familiar with, such as inches, feet, and degrees Fahrenheit, are not units that are used in science. Scientists use a set of measuring units called SI, or the International System of Units. The abbreviation stands for the French name Système International d'Unités. SI is a revised version of the metric system. There are seven primary base units you need to learn.

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Base Units

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Derived Units Additional SI units, called derived units, are made from combinations of base units.

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Metric Prefixes The metric unit for a given quantity is not always a convenient one to use. A metric prefix indicates how many times a unit should be multiplied or divided by 10. Learn these prefixes…

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Scientific Notation Scientific Notation is based on powers of the base number 10. The number 123,000,000,000 in scientific notation is written as : 1.23 X 10 11 The first number 1.23 is called the coefficient. It must be greater than or equal to 1 and less than 10. The second number is called the base. It must always be 10 in scientific notation. The base number 10 is always written in exponent form. In the number 1.23 x 1011 the number 11 is referred to as the exponent or power of ten.

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Dimensional Analysis Suppose you want to convert the height of Mount Everest, 8848 meters, into kilometers. Based on the prefix kilo-, you know that 1 kilometer is 1000 meters. This ratio gives you two possible conversion factors. Since you are converting from meters to kilometers, the number should get smaller. Multiplying by the conversion factor on the left yields a smaller number. Notice that the meter units cancel, leaving you with kilometers (the larger unit).

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Limits of Measurement Precision--Precision is a gauge of how exact a measurement is. The precision of a measurement depends on the number of digits in the answer. Significant figures are all the digits that are known in a measurement, plus the last digit that is estimated. The fewer the significant figures, the less precise the measurement is.

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Uncertainty When you make calculations with measurements, the uncertainty of the separate measurements must be correctly reflected in the final result. The precision of a calculated answer is limited by the least precise measurement used in the calculation. So if the least precise measurement in your calculation has two significant figures, then your calculated answer can have at most two significant figures.

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Accuracy Accuracy is the closeness of a measurement to the actual value of what is being measured. Although an instrument is precise, it does not have to be accurate.

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Organizing Data Scientists accumulate vast amounts of data by observing events and making measurements. Interpreting these data can be a difficult task if they are not organized. Scientists can organize their data by using data tables and graphs. These tools make it easier to spot patterns or trends in the data that can support or disprove a hypothesis

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Data Tables The simplest way to organize data is to present them in a table. The table relates two variables—an independent variable and a dependent variable. xy 12 34 56

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Line Graphs A line graph is useful for showing changes that occur in related variables. In a line graph, the independent variable is generally plotted on the horizontal axis, or x-axis. The dependent variable is plotted on the vertical axis, or y-axis, of the graph. A direct proportion is a relationship in which the ratio of two variables is constant. An inverse proportion, a relationship in which the product of two variables is a constant.

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Bar Graphs A bar graph is often used to compare a set of measurements, amounts, or changes. The bar graph makes it easy to see how the data for one thing compares with the data for another.

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Circle Graphs A circle graph is a divided circle that shows how a part or share of something relates to the whole.

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Communicating Data A crucial part of any scientific investigation is reporting the results. Scientists can communicate results by writing in scientific journals or speaking at conferences. Scientists also exchange information through conversations, e-mails, and Web sites. Young scientists often present their research at science fairs Different scientists may interpret the same data differently. This important notion is the basis for peer review, a process in which scientists examine other scientists' work. Peer review encourages comments, suggestions, questions, and criticism from other scientists. Peer review can also help determine if data were reported accurately and honestly.

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