CAPÍTULO 2. LAS CAJAS DE AHORRO ESPAÑOLAS COMO PRECURSORAS DE LA
3.2. El futuro de las Cajas de Ahorro tras las recientes reformas en el sector El Decreto-
Accurate student answers are shown. Actual answers may vary.
Investigate: Phase Change Safety
Study the procedure that you will follow. Discuss safety precautions that must be used and list them in the space below.
Goggles and aprons must be worn. Handle hot glassware carefully, using appropriate hot pads. Apply electrical safety rules: disconnect hot plate when not in use; prevent electrical equipment from coming in contact with water or wet surfaces. General lab safety and personal safety precautions are applicable.
1. Place 4–5 ice cubes (less than 100 g) and about 50 mL of water in a 250 mL beaker.
2. Connect the graphing calculator (TI-84) to CBL 2™ using the link. 3. Insert the temperature probe into Channel 1.
4. Select the DataMate program from APPS for data collection. 5. Complete setup of the calculator program for data collection as
instructed on the following pages.
6. Place the temperature probe in the ice-water mixture.
7. Place the beaker with the ice-water mixture, thermometer, and magnetic stirrer on a hot plate.
8. Ensure that the probe is not touching the sides or bottom of the beaker by suspending it in the ice-water mixture with a clamp and stand.
9. Turn on the hot plate and stirrer. Set the heat at the high setting and the stirrer at the medium setting.
10. Begin heating and start the data collection.
10 9 10 9 8 7 6 5 4 3 2 1 8 7 6 5 4 3 2 1
Changes
CHANGES
TE
11. When data collection is complete and the graph is displayed, analyze and discuss it with your lab group.
12. Data and graphs from the calculator may be transferred to a computer using an interface program such as TI-Connect™. The investigation report may be printed out then.
OR Data may be transferred manually to the data table, and graphs may be constructed.
Note
If data are collected manually, record the temperature every 30 seconds until the temperature reaches approximately 100 °C and remains constant over several readings. The water should be visibly boiling at this time.
Changes
CHANGES
TE
Data (for Manual Data Collection)
Time (30 sec intervals) Temp (°C) Time (30 sec intervals) Temp (°C) Time (30 sec intervals) Temp (°C) Time (30 sec intervals) Temp (°C) 0 2.0 25 52.5 50 101.0 75 1 2.0 26 55.0 51 101.0 76 2 2.0 27 60.0 52 77 3 2.0 28 64.0 53 78 4 2.0 29 67.5 54 79 5 2.5 30 70.0 55 80 6 3.0 31 73.5 56 81 7 3.0 32 76.5 57 82 8 4.0 33 80.0 58 83 9 5.0 34 83.0 59 84 10 7.0 35 85.0 60 85 11 8.5 36 87.0 61 86 12 9.5 37 88.0 62 87 13 12.0 38 89.0 63 88 14 14.0 39 92.0 64 89 15 17.0 40 90.0 65 90 16 20.0 41 94.0 66 91 17 23.5 42 96.0 67 92 18 26.5 43 98.0 68 93 19 28.5 44 99.5 69 94 20 32.0 45 100.5 70 95 21 34.0 46 101.5 71 96 22 37.5 47 101.0 72 97 23 44.5 48 101.0 73 98 24 49.5 49 101.0 74 99
Changes
CHANGES
TE
Temper ature in °CTime Intervals (Every 30 Seconds from Start)
Changes
CHANGES
TE
Analysis and Conclusions
1. In your investigation, identify the independent and the dependent variables.
The independent variable is the time interval, and the dependent variable is the temperature in Celsius.
2. How does the temperature of the ice-water mixture vary as it is continually heated?
The temperature stayed constant (close to 0 °C) for about 5 minutes and then rose steadily until it reached about 100 °C. It remained at 100 °C for about 6 minutes until we stopped data collection and heating.
3. What do the different sections of the graph represent?
The three sections in the graph represent melting ice, heating liquid, and boiling water.
4. From your graph, identify the melting temperature of ice and the boiling temperature of water.
Melting temperature of ice = 0 °C Boiling temperature of water = 100 °C
5. Predict the shape of the graph if you cooled steam continually to change it to ice. Draw the predicted shape of the graph below. Identify the condensation temperature and the freezing temperature on the graph.
The graph will be reversed in the cooling process of steam to water, then to solid. Condensation (100 °C) Freezing (0 °C) cooling steam steam to water cooling water water to ice Temperature in °C Time Intervals
Changes
CHANGES
TE
6. Classify the following changes as endothermic or exothermic and give reasons for your classification.
Change Endothermic or Exothermic Reason
Melting Endothermic
Heat is absorbed when ice melts and changes to a liquid.
Freezing Exothermic
Heat is released when water freezes
to ice.
Boiling Endothermic when water boils and Heat is absorbed changes to vapor.
Condensation Exothermic
Heat is released when water vapor
condenses and changes to a liquid.
7. Why do you think the temperature stayed constant as the ice melted into water?
The energy absorbed was used to rearrange the particles of the solid into a more flexible arrangement of a liquid. Since the particles are not moving faster, there is no temperature change. After all the ice melted, the temperature increased, indicating that the water particles are moving faster.
8. Why do you think the temperature stayed constant as the water boiled?
In the boiling phase, the energy is used to change the liquid particles to vapor particles. The energy is not used to increase the movement of the water particles, so there is no increase in temperature.
9. What is the difference between evaporation and boiling?
Evaporation takes place on the surface of the liquid and happens at any temperature. For example, if you leave a bowl of water open to the air, it will evaporate. Boiling takes place when a substance is heated to a specific temperature. For example, water boils at 100 °C. Each liquid has a unique boiling temperature.
Changes
CHANGES
TE
10. Classify the following changes as physical or chemical and exothermic or endothermic. Give reasons for your classification.
Change Physical or Chemical Exothermic or Endothermic Reason
Melting Physical Endothermic
No new substance is formed and the change is reversible.
Heat is absorbed.
Freezing Physical Exothermic
No new substance is formed and the change is reversible.
Heat is released.
Boiling Physical Endothermic
No new substance is formed and the change is reversible.
Heat is absorbed.
Condensation Physical Exothermic
No new substance is formed and the change is reversible.
Heat is released.
Evaporation Physical Endothermic
No new substance is formed and the change is reversible.
Changes
CHANGES
TE
Notes: Phase Changes
All phase changes are physical changes because the chemical makeup of the substance does not change and the substance can easily be recovered.
The temperature remains constant during a phase change.
The energy absorbed during a phase change such as melting, evaporation, or boiling is used to overcome the forces holding the particles together.
In phase changes like freezing and condensation, energy is removed or released so that movement of the particles is slowed down and the particles align themselves to form a solid or liquid.
Temperature in °C
Heating Curve of a Solid to Vapor
boiling melting heating solid heating vapor heating liquid Time Intervals
Changes
CHANGES
TE
Application: Phase Changes
Two students conduct phase change experiments for a certain chemical using a graphing calculator and probes. One student works on the heating curve of the solid while the other student works on the cooling curve of the liquid. Both students decide to take temperature readings in Celsius every minute for 15 minutes. The two sets of graphs obtained by the students are shown below. Study the graphs carefully and answer the questions below.
1. What is the independent variable in the experiment? What is the dependent variable?
The independent variable is the time interval of 1 minute. The dependent variable is the temperature in Celsius.
2. Which set of graphs represents the heating of the solid? Which set of graphs represents the cooling of the liquid? How do you know?
Set A, B, C represents the cooling of a hot liquid because the temperature is decreasing.
Set P, Q, R represents the heating of a solid because the temperature is increasing.
3. Color or mark the melting point and the freezing point on the
respective graphs. What are the melting temperature and the freezing temperature of the substance?
Melting point and freezing point are the same temperature, 44 °C.
A P B Q C R
Changes
CHANGES
TE
Elaborate
Teacher Notes
Investigate: ChangesStudents perform several chemical investigations. Each investigation allows the opportunity to apply laboratory process skills as well as understanding of physical and chemical changes. Students will observe and analyze their observations carefully to determine the attributes of the change. Independent student inquiry is emphasized. The Elaborate section may also be used as a performance assessment.
Learning Outcome
• Observe several changes while adhering to proper laboratory procedures.
• Determine if a change has occurred based on observations.
• Classify the change as exothermic or endothermic and give logical reasons for classification.
• Classify the change as physical or chemical and give logical reasons for classification.
Advance Preparation
Collect lab materials and equipment needed for this investigation. The suggested chemicals below may be substituted with other chemicals at the teacher’s discretion. Obtain the chemicals listed below and place them in individual containers with labels as shown. The containers may be dropper bottles or disposable pipettes for the liquids and sample jars for the solids. About 10 g of solid and 25 mL of liquid should be sufficient. Each student group will need a set of chemicals.
Chemicals Labels
Sodium bicarbonate Solid A Vinegar Liquid B Potassium iodide solution 0.1 M Liquid C Lead nitrate solution 0.1 M Liquid D Calcium chloride Solid E Soy wax (melted before the
investigation and kept in hot water bath)
Liquid F (place in test tube) Mineral oil Liquid G
Water Liquid H
Materials List For each station
• A set of chemicals as described below • Test tubes
• Test tube rack • Stirring rod Station 1 • Solid A • Liquid B • Plastic scoop Station 2 • Liquid C • Liquid D Station 3 • Solid E • Water in a plastic cup • Plastic scoop • Pipette Station 4 • Hot plate
• Large beaker with water for hot bath • Liquid F
• Small beaker • Water in a plastic
cup
• Test tube holder
Station 5
• Liquid G • Liquid H
Changes
CHANGES
TE
Place the soy wax in test tubes, one for each student group. These test tubes are labeled “Liquid F.” Heat the soy wax test tubes in a hot water bath to melt the wax. The liquid may be obtained by the student groups when they are ready to conduct that part of the investigation.
Note: Soy wax is nontoxic and is washable with soap and hot water.
Activity
1. Allow students to read and discuss the information on the laboratory sheet.
2. Review laboratory safety with attention to chemical, heat, and electrical safety.
3. Ensure that all student groups have the necessary equipment and materials to conduct the investigation.
4. Allow students time to conduct the investigation, discuss, and complete the laboratory record sheet.
5. Monitor students and intervene only when necessary.
6. After students have completed the investigation and cleaned up, conduct a class discussion to review student understanding of the major concepts in the lesson. Accept all answers before guiding student reasoning, using probing questions as necessary.
Discussion Questions
1. Which investigations represent a chemical change? Which were physical changes?
1 and 2 were chemical changes. 3 and 4 were physical changes. There is no change in 5.
2. How do you know that 1 and 2 were chemical changes?
Bubbles of gas were produced in the first investigation, and yellow precipitate was produced in the second. (Usage of proper learned vocabulary is stressed.)
Changes
CHANGES
TE
3. What were the reasons for classification in 3, 4, and 5?
In 3, there was no observable change other than the temperature change, which alone is not sufficient to classify it as a chemical change.
In 4, the hot liquid changed to a solid and floated on the water. It may be freezing of the liquid rather than a chemical change.
In 5, the two liquids did not mix; they remained separate layers, so there was no change.
4. Which investigations were exothermic? How do you know?
3 and 4 were exothermic because in both cases the water temperature increased.
5. Which investigations were endothermic? How do you know?
1 was endothermic because the temperature of the contents in the test tube dropped. Heat was absorbed by the chemical change/ reaction.
6. In which investigation was mass conserved?
Mass is always conserved in a change. In all the investigations, the total mass before the change equals the total mass after the change.
7. In which investigation was energy conserved?
Energy is always conserved in a change. In an exothermic process, heat given off by the change is absorbed by the surroundings. In an endothermic process, heat needed for the change is absorbed from the surroundings.