01.04 Heating and Cooling Curves

Heating and cooling curves graphically represent the temperature changes of a substance as it undergoes heating or cooling, illustrating the energy changes during state transitions.

Melting Point Measurement

Procedure:

1. Sample Preparation: Place a powdered solid in a melting-point tube.
2. Heating: Insert the tube into an oil or water bath and gradually heat it.
3. Observation:
   • Initial Heating: Temperature rises steadily as the solid heats up.
   • Melting Phase: Temperature plateaus as the solid melts; energy is used to break intermolecular bonds, not to increase temperature.
   • Post-Melting: Temperature rises again once the entire solid has melted and the liquid begins to heat.

Example:

• Substance A (Paraffin Wax): Melts at ~55°C.
• Observation: As paraffin wax is heated, the temperature rises to 55°C, remains constant during melting, then rises again as the liquid paraffin is heated further.

Boiling Point Measurement

Procedure:

1. Setup: Similar to melting point measurement using a boiling-point tube.
2. Heating: Heat the liquid in a controlled environment.
3. Observation:
   • Initial Heating: Temperature rises as the liquid heats up.
   • Boiling Phase: Temperature plateaus as the liquid boils; energy is used for the phase transition from liquid to gas.
   • Post-Boiling: Temperature rises again once boiling is complete and gas is heated.

Example:

• Substance B (Water): Boils at 100°C.
• Observation: Water temperature increases to 100°C, stays constant during boiling, then rises if heating continues.

Cooling Curve

Procedure:

1. Heating: Heat a liquid until it melts.
2. Cooling: Allow the liquid to cool while recording the temperature at regular intervals.
3. Observation:
   • Initial Cooling: Temperature decreases as the liquid loses heat.
   • Freezing Phase: Temperature plateaus as the liquid solidifies; heat is released during the phase transition.
   • Post-Freezing: Temperature continues to drop as the solid cools further.

Example:

• Water Cooling: Starts at 80°C, cools down, plateaus at 0°C during freezing, then continues to decrease to -20°C.

Key Concepts

• Heat Energy and Phase Transitions:
  • Addition of Heat: Causes particles to move faster, leading to melting or boiling.
  • Removal of Heat: Causes particles to move slower, leading to freezing or condensation.
• Energy Absorption and Release:
  • Energy Absorbed: During melting and boiling (endothermic processes).
  • Energy Released: During freezing and condensation (exothermic processes).
• Plateaus in Heating/Cooling Curves:
  • Indicate phase changes where temperature remains constant despite ongoing energy transfer.

Experimental Skills 1.1: Plotting a Cooling Curve

Materials Needed

• Two beakers (250 cm³)
• Bunsen burner
• Tripod and gauze
• Heat-resistant mat
• Stopwatch or timer
• Two boiling tubes labeled A and B
• Two stirring thermometers (range: -10°C to 100°C)
• Samples:
  • Sample A: Paraffin wax (m.p. ~55°C)
  • Sample B: Octadecanoic acid (Stearic acid, m.p. 70°C) or Phenyl salicylate (Salol, m.p. 43°C)

Safety Precautions

• Protective Gear: Always wear safety goggles and heat-resistant gloves.
• Bunsen Burner: Use the appropriate flame type as instructed by your teacher.
• Chemical Handling: Be aware of the concentrations and volumes; handle all chemicals with care.
• Environment: Conduct the experiment in a well-ventilated area on a heat-resistant surface.

Procedure

1. Prepare Water-Bath:
   • Fill a 250 cm³ beaker three-quarters full with water.
   • Place it on a tripod with gauze and heat using the Bunsen burner.
   • Heat until the water reaches 90°C, monitored by the thermometer.
2. Melt Samples:
   • Place boiling tubes containing Samples A and B into the hot water-bath.
   • Allow the solids to melt completely.
3. Insert Thermometers:
   • Once melted, insert a thermometer into each boiling tube ensuring the bulb is submerged in the liquid.
4. Begin Cooling:
   • Remove the tubes from the water-bath.
   • Place them in an empty beaker for support.
5. Record Temperatures:
   • Immediately record the initial temperature of each sample.
   • Start the stopwatch and record the temperature every minute until it reaches 40°C.
6. Plot the Graph:
   • For each sample, plot temperature (y-axis) against time (x-axis).
   • Identify and mark the plateau where solidification occurs.

Data Collection and Graph Plotting

• Axes:
  • X-axis: Time (minutes)
  • Y-axis: Temperature (°C)
• Plotting Points:
  • Carefully plot each temperature reading at the corresponding time interval.
  • Use a ruler for neat axes and consistent scaling.
• Drawing the Cooling Curve:
  • Connect the points smoothly, noting the plateau during freezing.
  • Label key points indicating phase transitions.

Self-Assessment Checklist

Total Marks (Out of 14): ____

Reflection:

• Identify strengths and areas for improvement based on your score.
• Discuss with your teacher for personalized feedback.

Example Questions and Answers

Question 1: Pure Substances

Which of the two substances (A and B) is a pure substance? Explain your answer.

Answer:

• Substance A (Paraffin Wax) is a pure substance because it has a specific melting point (~55°C) and consists of only one type of molecule.
• Substance B could be a pure substance if it is either octadecanoic acid or phenyl salicylate, each having distinct melting points.

Question 2: Improving the Method

Explain any ways in which your method could be improved to give more reliable results.

Answer:

• Use of a Thermostat-Controlled Water-Bath: Ensures a consistent temperature during heating.
• Calibrated Thermometers: Use thermometers that are calibrated for accurate readings.
• Stirring Samples: Stir samples gently to ensure uniform temperature distribution.
• Multiple Trials: Conduct the experiment multiple times to average out any anomalies.
• Insulation: Use insulated containers to minimize heat loss during cooling.

Question 3: Sketching a Cooling Curve

Sketch a cooling curve for water from 80°C to -20°C, noting what is taking place in the different regions of the graph.

Answer:

• 80°C to 0°C: Temperature decreases as water cools (liquid state).
• 0°C Plateau: Water begins to freeze; temperature remains constant as heat is released during the phase change from liquid to solid.
• 0°C to -20°C: Temperature continues to decrease as ice cools further (solid state).

Diagram Description:

1. Downward Slope: From 80°C to 0°C.
2. Horizontal Plateau: At 0°C.
3. Downward Slope: From 0°C to -20°C.

Question 4: Energy Changes in State Transitions

Energy is needed to overcome the forces of attraction holding the particles in position in a solid. Energy is absorbed during melting. Figure 1.12 shows how energy is involved in the different changes of state. Complete Figure 1.12 by providing labels for the positions A, B, and C.

Answer:

• Position A: Melting (Solid to Liquid) – Energy Absorbed
• Position B: Boiling (Liquid to Gas) – Energy Absorbed
• Position C: Condensation (Gas to Liquid) or Freezing (Liquid to Solid) – Energy Released

Question 5: Heating Curve States

As an alternative to following the cooling of a substance, it is possible to draw a heating curve. Figure 1.13 shows the heating curve for substance X. What physical state, or states, will X be in at points A, B, C, and D on the curve?

Answer:

• Point A: Solid (before melting starts)
• Point B: Solid and Liquid (during melting)
• Point C: Liquid (before boiling starts)
• Point D: Gas (after boiling is complete)

Additional Tips for Success

• Understand the Concepts: Focus on the relationship between energy and state changes.
• Practice Graphs: Regularly plot heating and cooling curves to become comfortable with graph interpretation.
• Memorize Key Points: Such as specific melting and boiling points of common substances.
• Safety First: Always prioritize safety procedures during experiments.
• Review Past Papers: Familiarize yourself with the format and types of questions asked in exams.
• Seek Feedback: Use the self-assessment checklist and seek teacher feedback to improve your experimental and graphing skills.