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Chemistry: Chapter 2

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.  

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Chemistry: Chapter 2

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  1. Chemistry: Chapter 2 Fall 2008

  2. 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.

  3. Base Units

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

  5. 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…

  6. 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 1011 • 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.

  7. 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).

  8. 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.

  9. 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.

  10. 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.

  11. Presenting Scientific Data

  12. 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

  13. 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.

  14. 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.

  15. 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.

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

  17. 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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