Physical Properties - Freezing & Melting Points

No technology Setup

Teacher Notes:

Chm.2.1.1 Explain the energetic nature of phase changes.
Chm.2.1.2 Explain heating and cooling curves, heat of fusion, heat of vaporization, heat, melting point, and boiling point).

Chm.2.1.3 Interpret the data presented in phase diagrams. 

In this lab, students will analyze data from graphs made using independent and dependent variables. They will find the relation between the melting point and boiling point of water.

This lab will not include heat of vaporization but can be shown later in this explanation or in another lab.

Helping Students Learn How to Make a Data Table

The data analysis in this lab is a double line graph.  There are no mathmatical calculations in this lab unless you change the lab.

To teach students to make a data table, they must read the lab.  In the first lab, I teach them how to pick out what must be part of the data table.  This probably will not be the first lab.  Some may still need help in making the data table.  If they ask, I ask them what they think are the dependent (controlled)  and independent manipulated) variables.  This would be a good lab in which to discuss variables.  In this lab the variables will be time and temperature.

If a mini lecture on graphing has not been given to the student, the following overhead can be helpful for a quick review. Use the info on an overhead projector or project the information on a smart board.  (Never assume that all students know this information from previous classes. If all do, move on.) Discuss what is included in a well-constructed graph, such as labeled x-axis and y-axis (including units), an appropriate title, neatness and readability.  Students always need to know what is expectedl

This lab will help most students understand that there is a correlation between freezing and melting point.  This lab can be done using technology or no technology.  Both methods will be explained.  Students need help in understanding that there are many experimental errors in this lab but the teacher does not need to state these nor tell the correlation between freezing and melting points.  This needs to be discovered by the student.  The teacher can answer any questions but not tell the expected data to collect.  The students always record data that they truly observe, not what they think they should observe.  The teacher should observe all setups and "might" make recommendations if necessary.

The thermometer should not be used as a stirring instrument.  Remind the students as often as necessary.  Provide plastic spoons for stirring.

Conclusion

In many labs, I added conclusion questions that they might see on a test.  I feel that it makes a connection with what was discussed in class. Also, it can be an aid when studying and reviewing for a test.  Yes, I asked the students to write the questions in the lab book so that they would know what the question was when reviewing for a test.

Experimental Errors
See lab "The Law of Conservation of Energy" to read explanation of experimental errors.

Helping Students To Think       (Important for every Lab)

During a lab, I rarely give additional instructions (beyond the written lab instructions) or give  answers to an experiment, but will answer any questions that will help them.  Yes, it causes frustration, but they learn that I am teaching them how to think.  Sometimes I will answer a question with a question, to again, help them with a thinking process.  I think it is important for students to use a marbled lab book. The Purpose (or Question), Hypothesis, Materials, Procedure and Data Table must be written in the lab book before a student may begin the lab.  (My signature proves that it was done). Every teacher can form their own opinion about this technique but I found that students take more ownership of their lab work.  It also is a good way to organized their lab work.

Technology
In this lab, a temperature probe will replace the thermometer.  This lab can be completed using a CBL or LabPro with a temperature probe and TI calculator.  The computer can be used also if available.  This lab has been completed successfully with all three Vernier products.  Pasco has great science sensors, also. Then, there are probes for classrooms having iPads.  These companies include instructions on the use of the temperature probe, including a temperature lab  such as the CBL temperature lab for Vernier .  Students of all ages love using technology in the science classroom, but in order for it to work smoothly as possible, the teacher needs to have completed the lab with the technology prior to instructing students. It also takes time to set up the technology but becomes easier each time -  well, most of the time. I have found that there will be some students very savvy in the use of the equipment and can help other frustrated students.  Student lab assistants can become helpful with this task also. But again, the teacher needs to know the problems and can help students work through problems encountered during the lab.

Freezing and Melting Points Lab

Question:  What is the freezing point and melting point of water?

Hypothesis:  ?  (Some students will need help.)

Materials: alcohol thermometer, ring stand, utility clamp, test tube, 400-mL beaker, water, 10-mL graduated cylinder, ice, rock salt, plastic spoon, stop watch

Procedure:  

1. Fill the 400-ml beaker 1/2 full of ice and add enough water to fill the beaker to 3/4 full of ice and water.
2. Put 5 mol of water into a test tube and use a test tube clamp to fasten the test tube to a ring stand. (Be careful not to break the test tube by clamping too tight too.)  Clamp the test tube above the water bath.  Place the thermometer into the water inside the test tube. Record the temperature.
3. When the equipment is ready, lower the test tube into the ice-water bath.  Begin the stop watch and record the temperature of the thermometer every 30 seconds for 15 minutes.
4. As soon as the test tube is lowered into the beaker, add 5 spoons of salt to the beaker and stir with a plastic spoon.  Do not use the thermometer to stir!  
5. Add more ice to the ice-water bath as the ice in the beaker begins to melt. Remember to keep recording the temperature every 30 seconds.
6. When 15 minutes have passed, stop collecting data but keep the test tube submered in the ice-water bath.  Using the data collected, determine the freezing temperature of the water in the test tube.
7. Begin a new data list.  Raise the test tube and clamp it above the ice-water bath.  Do not move the thermometer.
8. Remove the ice water from under the test tube.  Start stop watch and begin collecting temperature data every 30 seconds.  Pour the ice-water in a disposal bucket provided by your teacher.  Rinse the beaker and put 250 mL warm water in the beaker.  When 12 minutes have passed, lower the test tube and thermometer into the warm-water bath.
9. When 15 minutes have passed, stop collecting temperature and time data. Using the data collected during the melting, determine the melting temperature of the water in the test tube.
10. Graph the data with time as the independent variable and temperature as the dependent variable.  Graph the temperatures in the first 15 minutes and the second 15 minutes on the same x and y axes(total of 30 minutes).  The difference in the two line graphs can be supported with a line and broken line, or different colors.  Label where phase changes occur and the heat of fusion. Be sure to make a key for your graph.

Data Table:  (Includes the dependent and independent variables)

Data Analysis:  (This would include the graph on graph paper)

Conclusion:  

Questions:  Write the question and answer in complete questions.

1. What does phase change mean ans where do they occur in this lab.
2. What is the dependent variable and independent variable in this experiment?
3. Describe what happens to the water temperature as it freezes.  Describe  what happens to the water temperature as it melts.  
4. Why are there flat lines on the graph?
5. Using the data from the graph, what was the freezing temperature of the water? What was the melting temperature of the water?
6.  Using the independent and dependent variables, describe the relation between the melting point and boiling point of water.

Remember that there has to be a paragraph about the experimental errors and what changes would be made if the lab was repeated.