Aim: What is Cooling Curve or Heating Curve of Water?

Instructional Objectives:

1.By observing the cooling curve and heating curve of water, students will learn that during phase change, there is no temperature change.

2 Students should understand that melting point of a substance is the same temperature as its freezing point.

3. During phase change, kinetic energy does not change while potential energy changes.

CAUTION: Be careful when handling the hot water.

 Keep all papers away from the burner.

National Science Education Standards met: B, D, & E

NYS Curriculum connections: Phase change is the subject in Regents Chemistry of NYS Curriculum.

Background:

Students will cool water until it freezes and then heat a test tube with ice. They have real time collecting data, which will show as a curve on computer screen. From this information, students will be able to understand a phase change diagram (Cooling curve or heating curve )for substances

Teacherfs Introduction

Freezing temperature, the temperature at which a substance turns from liquid to solid, and melting temperature, the temperature at which a substance turns from a solid to a liquid, are characteristic physical properties. In this experiment, the cooling and warming behavior of a familiar substance, water, will be investigated. By examining graphs of the data, the freezing and melting temperatures of water will be determined and compared.

MATERIALS

Power Macintosh or Windows PC

400-mL beaker

Vernier computer interface

water

Logger Pro

10-mL graduated cylinder

Temperature Probe

ice

ring stand

salt

utility clamp

stirring rod

test tube

PROCEDURE

Part I: Freezing

1. Fill a 400-mL beaker 1/3 full with ice, then add 100 mL of water.

2. Put 5 mL of water into a test tube and use a utility clamp to fasten the test tube to a ring stand. The test tube should be clamped above the water bath. Place a Temperature Probe into the water inside the test tube.

3. Prepare the computer for data collection by opening the Experiment 2 folder from Chemistry with Computers. Then open the experiment file that matches the probe you are using. The vertical axis will have temperature scaled from –20 to 30‹C. The horizontal axis will have time scaled from 0 to 15 minutes.

4. When everything is ready, click  to begin data collection. Then lower the test tube into the ice-water bath.

5. Soon after lowering the test tube, add 5 spoons of salt to the beaker and stir with a stirring rod. Continue to stir the ice-water bath during Part I. Important: Stir enough to dissolve the salt.

6. Slightly, but continuously, move the probe during the first 10 minutes of Part I. Be careful to keep the probe in, and not above, the ice as it forms. When 10 minutes have gone by, stop moving the probe and allow it to freeze into the ice. Add more ice cubes to the beaker as the original ice cubes get smaller.

7. When 15 minutes have passed, data collection will stop. Keep the test tube submerged in the ice-water bath until Step 10.

 8. On the displayed graph, analyze the flat part of the curve to determine the freezing temperature of water:

Part II: Melting

9. Prepare the computer for data collection. From the Data menu, choose Store Latest Run. This stores the data so it can be used later.

 10. Click  to begin data collection. Then raise the test tube and fasten it in a position above the ice-water bath. Do not move the Temperature Probe during Part II.

 11. Dispose of the ice water. Obtain 250 mL of warm tap water in the beaker. When 12 minutes have passed, lower the test tube and its contents into this warm‑water bath.

 12. When 15 minutes have passed, data collection will stop.

 13. On the displayed graph, analyze the flat part of the curve to determine the melting temperature of water:

 14. To show a graph of temperature vs. time showing both data runs:

OBSERVATIONS

aSK STUDENTS TO OBSERVE:

1.  THE TEMPERATURE CHANGE DURING THE PROCESS OF  COOLING OR HEATING WATER.

2. wHEN IS THE TEMPERATURE CONSTANT DURING THE PROCESS?

3. wHAT IS THE MELTING POINT AND FREEZING POINT OF WATER

DATA TABLE

qUESTIONS FOR STUDENTS to discuss:

1. What happened to the water temperature during freezing? During melting?

2. According to your data and graph, what is the freezing temperature of water? The melting temperature? Express your answers to the nearest 0.1‹C.

3. How does the freezing temperature of water compare to its melting temperature?

 4. Tell if the kinetic energy of the water in the test tube increases, decreases, or remains the same in each of these time segments during the experiment.

a. when the temperature is changing at the beginning and end of Part I

b. when the temperature remains constant in Part I

c. when the temperature is changing at the beginning and end of Part II

d. when the temperature remains constant in Part II

5. In those parts of Question 4 in which there was no kinetic energy change, tell if potential energy increased or decreased.

Summary:

1. During phase change, temperature does not change, which can be observed as a flat part in the cooling or heating curve.
2. The temperature of melting is the same as the temperature of freezing.
3. During phase change, kinetic change does not change while potential energy changes.

Homework Assignment:

Questions:

1.  Describe the two curves. How are they similar? How are they different?

2. What is occurring in the region where the water temperature remained constant?

3. Compared with O0C ice and O0C water, do they have the same potential energy? If not, which one has more potential energy? Explain your answer.