Investigation M3: Separating Mixtures into Component Parts

Transcription

1 Investigation M3: Separating Mixtures into Component Parts Goals: Use various methods to separate mixtures, make inferences from temperature/time graphs, and identify substances.

2 81 Activity M3.3: What happens when water boils? (Laboratory Activity) Equipment: Beakers, alcohol burners, tongs, matches, test tubes, two-hole stoppers to fit test tubes (with a thermometer inserted into one hole and a collecting tube inserted into the other hole), rubber tubing, thermometers, water, Cartesian coordinate graph paper, timer or equivalent, and safety goggles. Safety Precautions Wear safety goggles. Do not taste the liquids. (All liquids used are safe to touch.) Do not touch hot objects. Do not push thermometers or collecting tubes through rubber stoppers. Prepare the two-hole stoppers (for the following apparatus) before class so students do not break thermometers or glass tubing. Lubricate these items with glycerin. Don't push these items forcibly into the stoppers. Wear gloves to protect yourself from cuts. Time/temperature graphs are all part of the Heat and Conservation of Energy unit. You may want to combine this Lab Activity M3.3 with work from that unit. Thermometer 2-hole cork with glass tubing Distilling tube Boiling chips Condensing tube Alcohol burner M3.3(1) Instructor Materials 2001 American Association of Physics Teachers

3 82 1. WHAT'S YOUR IDEA? Does the temperature of water always rise when you heat it?? A naive idea is that temperature is always directly proportional to time for a substance being heated. 2. WHAT ARE THE GROUP'S IDEAS? How many of your classmates agree with you? How do their justifications differ from yours? What reasons do classmates give for disagreeing with you??? 3. MAKING OBSERVATIONS: Put 5 ml of cold water into a test tube. Insert a two-hole stopper into the test tube. The stopper should have a thermometer in one hole and a collecting tube in the other hole. Choose a stopper with the thermometer inserted far enough to keep the bulb close to the top of the test tube so that you measure the temperature of the water vapor just before it enters the collecting tube. (The thermometer bulb should not touch the test tube.) Put the loose end of the collecting tube into a second test tube. Suspend this test tube in a bath of cold water. Your apparatus should look like the preceding sketch. Use an alcohol burner to heat the water in the first test tube just enough to keep it boiling. Record the temperature of the water vapor every half minute. Continue to boil the water until the test tube is nearly dry. What volume of liquid was collected in the second test tube? What is the liquid collected in the second test tube? Take another 5 ml of cold water and use a stopper with the thermometer inserted far enough that the bulb is in the water. Repeat this activity, recording the temperature of the water every half minute. Continue to boil the water until the test tube containing it is nearly dry. Remove the rubber tubing from the test tube in the cold water bath before you stop applying heat to the boiling water. The heights of the flame and the test tube containing water need to be adjusted to keep the water boiling steadily instead of boiling over. If the rubber tubing is left in the test tube in the cold water bath, then low pressure from cooling of the other test tube may cause the water to move back into that test tube. Because cold water is moving into a hot test tube, that test tube might break. Students should conclude that distilled water is collected in the second test tube. M3.3(2) 2001 American Association of Physics Teachers Instructor Materials

4 83 4. MAKING SENSE: Make two graphs (one for water and one for water vapor) of temperature as a function of time. From your graphs, read the boiling point of water. Look again at the first question. Do you want to change your answer? Give additional support for your answer if you do not change your mind. Justify your new answer if you do change your mind. The students' graphs should tell them that the temperatures are constant for the boiling water and for the vapor from the boiling water. Some students will expect a continuation of the increasing time/temperature curve found for the initial heating of the water. Some students believe that the boiling water and its vapor will have different temperatures. M3.3(3) Instructor Materials 2001 American Association of Physics Teachers

5 84 Notes M3.3(4) 2001 American Association of Physics Teachers Instructor Materials