Lab. #1

Qualitative Observations of a Chemical Reaction

 

Background

Scientists rely heavily on experimentation. A good scientist must observe and interpret what is happening. Observing means using the senses: seeing, smelling, touching, hearing, and sometimes tasting. NEVER TASTE CHEMICALS UNLESS INSTRUCTED TO BY YOUR TEACHER.

When scientists make observations, they try to be objective. Being objective means putting aside any preconceived notions. Scientists are interested in what really occurs, not in what they wish would occur.

After observations are made, scientists must make interpretations. Interpretations are based on previous knowledge and experience. Because people have different experiences, one scientist may interpret observations in one way while another may interpret the same observations to mean something else. When we interpret, we attempt to make sense out of observations. Scientists never assume that their interpretations are correct until they test them fully and repeatedly. After complete testing, scientists then come to their conclusions.

 

PURPOSE

In this investigation you will make some qualitative observations of a chemical reaction. That is, no measurements will be made. During a chemical reaction, one or more substances change into one or more other substances. The burning of wood, wax, oil, gasoline, and coal are examples of a chemical reaction known as combustion. The reaction you will study in this investigation is a combustion reaction.

 

MATERIALS

Beaker 250 ml                                      candle (2 cm diameter) Paper towels   

Erlenmeyer flask 125 ml                        matches                                    aluminum foil

Glass square                                         toth picks                                 cobalt chloride paper

Metric ruler                                          limewater solution

Microslide                                            string

                                               

PROCEDURE

Record observations for each step.

  1. Note appearance, odor, and feel of the unlit candle.
  2. Light the candle and allow it to burn for several minutes. Note any changes. Briefly describe the burning candle.
  3. Blow out the flame and immediately place a lighted match in the “smoke” about 2 centimeters above the wick.  Note the result.
  4. Drip a small amount of liquid from the bowl of the candle onto a microslide. Try to light it and note the result.
  5. Place a toothpick into the soft candle next to the unlit wick to form a wooden wick.  Light the toothpick and note the result.
  6. Place a length of string about 4 cm long on the glass square. Light it and observe its behavior.
  7. Make a slit in a small piece of aluminum foil. Light the candle. Place the foil between the base of the flame and the liquid in the candle bowl. Note the behavior of the flame.
  8. Invert a 250-mL beaker over the lit candle. See Figure 1-3. Note any substance that collects on the inside of the beaker. Test the liquid with cobalt chloride paper.
  9. Invert a 125-mL Erlenmeyer flask over the lit candle for several minutes. Remove the flask, turn it right side up, and add about 10 mL of the clear limewater solution. Stopper and shake the flask. Note any change in the limewater solution.
  10. Place the copper coil around the lit wick. Remove it. Describe what you observe repeat a couple of times and see what happens.

 

CONCLUSIONS AND QUESTIONS

Answer ach question completely, addressing each part of the question

  1. What phases (solid, liquid, gas) are present in the unlit candle? In the burning candle? Which phase appears to take part in the chemical reaction?
  2. What part does the wick play in the burning of the candle? What properties should the wick have? Explain the result when aluminum is placed between the liquid and the wick. Is the wick part of the chemical reaction?
  3. What two substances are indicated by the cobalt chloride and limewater tests? Is it possible that other substances are produced when the candle burns? Explain.
  4. A source of energy is needed to start the burning of the candle. What energy source is used? Did the reaction give off or absorb heat?
  5. How would you explain the copper coil observation?
  6. Give an example illustrating the difference between observation and interpretation.