On Becoming a Scientist
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Thinking Like a Scientist Activity 5

Science is built up with facts, as a house is with stones. But a collection of facts is no more a science than a heap of stones is a house.

--Jules-Henri Poincaré

Just the Facts



Objective

Students will investigate the importance of data collection and analysis.


About the Activity

There are three major parts to an experiment. The first part involves designing and setting up the investigation. The second part includes performing the investigation, making observations, and collecting data. The third part involves analysis, which is essential to understanding the implications of the observations, including whether or not the hypothesis is supported. The purpose of an experiment is to test a hypothesis, not to substantiate it. Thus, it is just as valid to prove that a hypothesis is wrong, as it is to prove the hypothesis is valid.

In this activity, students investigate how food additives affect growth of microorganisms.


Background Material

Bacteria and other microbes grow rapidly under optimal conditions. These organisms feed on many of the same substances as humans do, and can bring about undesirable chemical and physical changes in food. To prevent the growth of these organisms in food, people use processes such as canning, heating, irradiating, freezing, and pasteurizing. In addition, preservatives are often added to the food.


Materials

For each group:

  • 2 chicken bouillon cubes

  • Hot water

  • Measuring cup

  • Large beaker or glass jar

  • 7 clear baby-food jars or cups with lids

  • Salt

  • Sugar

  • Vinegar

  • Teaspoon and tablespoon measures

  • Masking tape

  • Pen

  • Paper


Instructions:

  1. Have students put a strip of masking tape on each of the seven jars. On each piece of tape they should write their names, class period, and jar number.
  2. Explain to students that they will be testing the effectiveness of sugar, salt, and vinegar on inhibiting bacterial growth. Have students dissolve the bouillon cubes in 2 cups of hot water. The resulting solution should then be divided evenly between the baby-food jars or cups.
  3. Have students add 1 teaspoon of salt to Jar 1, 1 teaspoon of vinegar to Jar 2, and 1 teaspoon of sugar to Jar 3. In Jars 4, 5, and 6, students should place 1 tablespoon of each material. Jar 7 is the control and should not have any additives. Remind students to label each jar to indicate the material and amount of material.
  4. Ask students to place the containers in a warm place overnight. Then have them carefully cover each container loosly with a lid, or other material.
  5. Have students develop a code based on the cloudiness of the liquid for recording observations. (For example, 0 = clear liquid, 1 = slightly cloudy (can see detail through the liquid), 10 = very cloudy (can't see anything through the liquid).) Have students check the containers each day for a week and record their observations in a chart.
  6. Have students draw conclusions about the effectiveness of each material as a food preservative and present their findings to the class.


Discussion Questions:

  1. How can you tell if bacteria are growing in the solution? Record which containers show bacterial growth and which do not. Of those which show growth, did all the growth begin at the same time? How can you tell?
  2. Does the amount of preservative change the results? Do you think the results would be different if you added more preservative to each container? Why or why not?
  3. Why was it important to check the containers over a period of a week, rather than just one or two days? Do you think the results would be different if you checked the containers over a longer period of time? Why or why not?
  4. What conclusions can you draw from this experiment? Explain your reasons.
  5. What questions do you have about this experiment? Describe an experiment you might do next to help answer your questions.



Activity 6: Let's Get Together


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