Scientific Observations Problem What observations indicate that a chemical reaction has occurred? What is the difference between simple physical mixing of ingredients and an actual chemical reaction?
Introduction You may already feel that you are a good observer but there is more to good observation than you might think. Observing scientifically requires concentration, attention to detail, the effective use of measuring tools, ingenuity and patience. It also takes practice. Before you come to lab, try making scientific observations at home. Find a candle and light it. See how many observations you can write down for the burning candle. Michael Faraday, a 19th century chemist, found many such things to observe in a burning candle. In fact he wrote a book on the subject and gave talks about it. Compare the observations you made about your candle to those your teacher gives you in class. Those observations that are descriptive are called qualitative. Observations that involve numerical quantities are quantitative – these are called measurements. Chemists often measure temperature, time, mass, or volume. Now you can practice your skills of scientific observation by watching what happens when several substances are mixed together.
Prelaboratory Assignment 3. Read the Introduction and Procedure before you begin. 4. Make observations of a burning candle. (Never leave a burning candle unattended) 5. Answer the Prelaboratory Questions. 1. Which of the following statements is not a scientific observation? For those that are not observations, briefly explain why they are not. a. The wire is 18 cm long. b. The solution is blue-green in color. c. The reaction is producing a red-brown solid. It must be rust. d. The gas is being produced at a rate of 6.5 mL/second. 2. What distinguishes an observation from a conclusion?
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Small beaker (100 mL) Blue-green chemical Aluminum foil (15 cm x 15 cm) Glass stirring rod
Safety 1. Wear safety goggles and a lab apron at all times in the laboratory. 2. No food or drink is allowed in the laboratory at any time. 3. Observe odors by gently waving the fumes toward your face. Do not sniff directly from a container. 4. Unknown chemicals can be caustic. Avoid contact with your skin. 5. If you are using a mercury thermometer, be very careful. Mercury is extremely toxic. If you break a mercury thermometer, report it to your teacher immediately.
Procedure Note: In this experiment you may use the thermometer at any time. No specific directions for its use will be given. 1. Fill the small beaker half-full with tap water. Add one rounded spoonful of the blue-green chemical to the water gently without stirring. 2. Write your observations of what is happening at each step of the procedure as a list. 3. Note the appearance of the solution in the beaker. Stir the chemical into the water gently. Record any observations. 4. Form the piece of aluminum foil into a loosely rolled tube about the size of a hot dog. Stand the aluminum tube in the solution in your beaker and wait several minutes. Record your observations of what happens. 5. When you are convinced that the reaction is finished, remove the remaining aluminum foil, if any, from the beaker and place it in the trash or solid waste disposal container provided by your teacher.
Cleaning Up 1. Carefully pour the solution that remains in the beaker into the proper disposal container. Do not pour any solid into this container. 2. Place any remaining solid from the bottom of the beaker in the solid waste container. 3. Thoroughly wash your hands before leaving the laboratory World of Chemistry
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Analysis and Conclusions Complete the Analysis and Conclusions section for this experiment either on your Report Sheet or in your lab report as directed by your teacher. 1. 2. 3. 4. 5.
Do your observations from Procedure, step 2 indicate a chemical reaction? Explain. Do your observations from Procedure, step 4 indicate a chemical reaction? Explain. Do your observations from Procedure, step 6 indicate a chemical reaction? Explain. What are some clues that a chemical reaction has taken place? Identify the three qualitative observations you made that you think are most noticeable. Were any of your observations quantitative? If so, which? 6. Classify each of your observations as qualitative or quantitative. 7. Is dissolving a solid in water a chemical reaction? Why or why not? 8. Is a metal disappearing into a solution a chemical reaction? Why or why not?
Something Extra 1. Design an experiment that would separate the blue-green chemical from the water. 2. What name is given to chemical reactions which produce energy as heat? What are those reactions that consume energy as heat called?
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