Scientific Method. Why Study Chemistry? Why Study Chemistry? Chemistry has many applications to our everyday world. 1. Materials. Areas of Chemistry

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1 August 12, 2012 Introduction to Chemistry and Scientific Measurement What is Chemistry? Chemistry: is the study of the composition of matter and the changes that matter undergoes. Chapters 1 and 3 Why Study Chemistry? Why Study Chemistry? Chemistry has many applications to our everyday world. 1. Materials 2. Energy: Chemists play an essential role in finding ways to conserve energy, produce energy and store energy. 3. Medicine and Biotechnology -There are over 2000 prescription drugs. 4. Agriculture -Productivity: Measure the amount of edible food that is grown -Crop Production: Chemicals to attack insect pests. Areas of Chemistry Scientific Method There are five traditional areas of study 1. Organic Chemistry- the study of all chemicals containing carbon. 2. Inorganic Chemistry- the study of chemicals that, in general do not contain carbon. 3. Biochemistry- the study of processes that take place in organisms. 4. Analytical Chemistry- focuses on the composition of matter. 5. Physical Chemistry- the studies of mechanisms, rate and the energy transfer when matter undergoes a change. What do you already know about the scientific method?

2 The Scientific Method 1. Define the Problem: There can be no solution until a problem exists and is clearly recognized. This first step is usually the most difficult one. 2. Gather Information: The scientist derives information by observing nature. Information may also be gained from the liver, or through speaking with experts. The scientist must investigate and gather correct information before problems can be solved and conclusion made. 3. Propose a Hypothesis: Hypothesis is a compound of two Greek words. Hypo mean "less than" and thesis means "idea". So a hypothesis is "less than an idea." It is a guess that might possibly serve to explain the gathered information. Using the hypothesis, the scientist can plan new ways to check his or her ideas. The Scientific Method 4. Experiment: An experiment is a test to see if the hypothesis is correct. An experiment is an experience. Scientific experiments are controlled. A controlled experiment is regulated so only one factor is changed. The results of changing this factor are then compared with the unchanged event. 5. Develop a Theory: A theory is an idea. It is a tested conclusion. It is stronger than a hypothesis. Theories can be proven wrong and may be changed. People who are responsible for making decisions must be open minded. Sometimes a theory becomes so well tested and accepted that we call it a law. Very few theories ever reach this position. Variables Independent Variable: The variable that is changed in the experiment. Dependent Variable: The variable that is watched to see how it behaves because of the changes made to the independent variable. Many experiments include a control, which is a standard for a comparison. You should only change one variable at a time. Chapter 3: Scientific Measurement Measurements and the Metric System A Quantity is something that has magnitude, size or amount. A Quality is NOT a measurement. Qualitative data is descriptive in nature. Example: The sky is dark blue or you have big feet. Quantitative data is normally in a form of a measurement. Example: The temperature of the water is 20 C, or the mass of the container is 22.4 grams.

3 Models A model in science is more than a physical object; it is often an explanation of how a phenomena occurs and how data or events are related. They may be visual, verbal, mathematical or mental. Measurements: Fundamental to the experimental sciences. It is important to be able to make measurements and to decide whether a measurement is correct. Accuracy, Precision and Error 1. Accuracy: measure of how close a measurement comes to the actual or "true" value. 2. Precision: measure of how close a series of measurements are to one another. *To evaluate the accuracy of a measurement, the measured value must be compared to the correct value. *To evaluate the precision of a measurement, you must compare the values of two or more repeated measurements. Accuracy, Precision and Error We have two values when we talk about the error present. 1. Accepted value: The correct value based on reliable references 2. Experimental value: The value measured in the lab. The difference between the two is called the error. Error = experimental value - accepted value Percent Error: The absolute value of the error dived by the accepted value multiplied by 100 Percent Error = [error] X 100 accepted value Example Percent Error What is the percent error of a measured value of 114 lb if the person's actual weight is 107 lb? Percent Error = [error] X 100 accepted value Percent Error = [ ] X 100 =6.54% 107 Scientific Notation The process of writing numbers by the order of ten. We use this for very small and very large numbers. It also helps clarify the number of significant figures in a measurement. Example: 3,200,000,000 m = 3.2 x 10 9 m Or g = 4.12 x g

4 Write in scientific notation 1. 2,030, Practice Problems x x x x 10-2 Significant Digits Reverse the scientific notation x x x x ,600, Significant Digits: all the digits known to be precise in a measurement plus the last estimated digit. A digit that must be guessed is called uncertain. A measurement always has some degree of uncertainty. Significant Digit Rules Example: Significant Digits How many significant digits are in each measurement? m 2. 40,506 mm x10 m meter sticks m 6. 98,000 m Measuring Length Measuring Temperature

5 Measuring Volume Correctly record the measurement using significant digits Correctly record the measurement using significant digits Significant Figures in Calculations A calculated answer cannot be more precise than the least precise measurement from which it was calculated. We will discuss two main calculations 1. Addition and Subtraction 2. Multiplication and Division Answer: (ml) 1. Addition and Subtraction The answer must be rounded so that it contains the same number of digits to the right of the decimal point as there are in the measurement with the smallest number of digits to the right of the decimal point. 2. Multiplication and Division The product or quotient should be rounded off to the same number of significant figures as in the measurement with the fewest significant figures. 2. Addition and Subtraction m m m =? = meters 3. Multiplication and Division 7.55 m x 0.34 m = 2.6 m 2

6 Use Scientific Notation to express the number 480 to: a. One SF: b. Two SF: c. Three SF: d. Four SF: Rounding 1. Look at the number behind the one you're rounding. 2. If it is 0 to 4, don't change it 3. If it is 5 to 9 make it one bigger Example: Round to: a. four sig figs b. three sig figs c. two sig figs d. one sig fig Practice a b c d. (0.102 x x 273) 1.01 e. (2.00 x 10 6 ) (3.00 x 10-7 ) SI Units The International System of Units (SI from the French) is the modern form of the metric system. It is the world's most widely used system of units both in everyday commerce and in science. 1. SI unit is a standard of measurement that is used throughout the world. 2. There are 7 base units and most others are derived from those seven. 3. To be derived means to be any combination of the main seven.

7 Dimensional Analysis Dimensional Analysis: is a way to analyze and solve problems using the units, or dimensions, of the measurements. It is important when working these problems to list the knowns and unknowns. "Factor-Label" Method: units are arranged in such a way that all units cancel except for the unit that is of interest. Conversions A conversion factor: is a ratio of equivalent measurements. Example: 1 dollar = 4 quarters = 10 dimes = 20 nickels = 100 pennies Because the conversion factors are equal to each other, they can be multiplied by other factors without changing the size of the quantity of measured. DA is used when converting from one unit of measure to a different unit so a conversion factor must be used. A measurement always has a number and a unit! Steps for conversions: 1. Identify initial and final units. 2. Line up conversion factors so units cancel. 3.Multiply all top number and divide by all bottom numbers. 4. Check units and answer. ALWAYS include the units!! Conversion Factors Example: The relationship between grams and kilograms is 1000 g = 1 kg 1000 g and 1 kg 1 kg 1000g 10 9 nm = 1 m 10 9 nm 1 m and 1 m 10 9 nm

8 Dimensional Analysis Example: How many seconds are in a workday that lasts exactly eight hours? (28,800 s ) Sample Problem 3.6 Examples (27 students) Known Time worked = 8 hours 1 hour= 60 minutes 1 minute= 60 seconds Unknown seconds worked =? s Practice Problem # 30 Practice Problem # 31 (67 students) (86.4 F) Converting Between Units We will be using the prefixes to convert between metric units such as grams to kilograms. There will be three methods in which you can set up your conversion. I will prefer one way to start and then you may work the other ways if they make more sense! 1. Keeping with the chart: 1km = 10 3 m 1cm = 10-2 m 2. Keeping the base unit having one! 1m=10-3 km 1m = 10 2 cm Example: 5.2 cm to meters Metrics and Measurement Handout! 3. Seeing the jumps of the power of ten! Example: 5.2 cm to m There is a difference of 2 powers of ten. Example Problems: One Step 1. What is the mass in grams of 64.9 mg? 10-2 and 10 0 Rule: Write the smallest unit on the top with the power of ten = 10 2 cm 1 m Then, Look and see how your initial unit will cancel with the conversion factor! 5.2 cm 10 2 cm 1 m What must you do with the units in order to be left with meters?

9 Example Problems: One Step 2. What is the length in centimeters of m? Example Problems: One Step 3. Your European hairdresser wants to cut your hair 8.0 cm shorter. How many inches will he be cutting off? Converting Between Units (Multistep) When converting between units, it is often necessary to use more than one conversion factor. Sample Problem 3.8 What is cm in micrometers? Knowns: length= cm = 7.3 x 10-2 cm 10 2 cm = 1 m 1 m = 10 6 µm Unknown: length =?µm Example Problems: Multistep 4. How many cm are in 3.1 km? 7.3x10-2 cm x 1 m x 10 6 µm = 7.3x10 2 µm 10 2 cm 1 m Example Problems: Multistep 5. How many milliliters are in 1.00 quart of milk? Example Problems: Multistep 6. How many meters are in 3.3 inches?

10 Example Problems: Compound 7. How many mph are in 4.55 m/s? Example Problems: Compound 8. How many g/in are in 0.21 kg/ft? Convert the following: km to meters mg to grams g to centigrams cm to millimeters Converting Between Units (Extra) Answers: m x 10-3 g cg x 10-1 mm Converting Between Units (Multistep) Answers: Let's Practice 1. Practice Problem 34 page x 10-8 cm 2. Practice problem x 10 8 dm 3. Practice Problem 36 page x 10 4 kg/m 3 4. Practice Problem x RBC/L Temperature Conversions We will never use Farenheit as a measure of temperature. * The Celsius scale sets the freezing point of water at 0 C and the boiling point of water at 100 C. * The Kelvin scale sets the freezing point of water to kelvins (K) and the boiling point is K. A change of one degree on the Celsius scale is equivalent to one kelvin on the Kelvin scale. The zero point on the Kelvin scale, O K or absolute zero, is equal to C Temperature Conversion Practice 1. Normal body temperature is 98.6 F. Convert this temperature to Celsius and Kelvin 2. Water boils at 100 C. What is the boiling point of water in degrees Fahrenheit and Kelvin? 3. Liquid nitrogen has a boiling point of 77 K. What is this temperature on the Fahrenheit scale? K= C C = K Tc=(5/9)(Tf-32) Tf=(9/5)(Tc+32)

11 Density Which is heavier, a pound of lead or a pound of feathers? A pound of lead has the same mass as a pound of feathers. People are really thinking about the density. Density: the ratio of the mass of an object to its volume. Density = mass volume Density Example: What did we see with the column that we made? * It is most often expressed in g/cm 3 or g/ml * Density is an intensive property that depends only on the composition of a substance, not on the size of the sample. * The density of some common substances are found on page 90 of your textbook. Density Problems gram ice cube raises the level or water in a 10 ml graduated cylinder 1.2 ml. What is the density of the ice cube? (Answer:0.92 g/ml) 2. Suppose you dropped a gold cube into a 10 ml graduated cylinder containing 8.50 ml of water. The level of the water rises to ml. You know that gold has a density of 19.3 g/cm 3. What is the mass of the gold cube? (Answer: 42.5 g)

12 Density changes with temperature: Density decreases as temperature increases. Why?