 # Chemistry: Chapter 2 Fall 2008. SI Units  For a measurement to make sense, it requires both a number and a unit.  Many of the units you are familiar.

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Chemistry: Chapter 2 Fall 2008

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.

Base Units

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

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…

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.

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).

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.

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.

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.

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

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

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.

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.

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

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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