4.1.5Electrochemistry
Investigating Cooling Curves for Stearic Acid — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions
5 min read 5 parts Lesson 9 of 20 Unit quiz
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 5Method
1
Place a boiling tube of molten stearic acid inside a beaker of warm water and clamp it so it stays upright.
2
Position a thermometer in the acid, keeping it away from the glass, and start the stopwatch.
3
Record the temperature every minute until it falls to about 40 °C.
4
At each reading, note whether the stearic acid looks like a solid or a liquid.
Part 2 of 5Variables and Equipment
Independent variable
- Independent variable Time, measured with a stopwatch that can record to 0.01 s.
Dependent variable
- Dependent variable Temperature, measured to 1 °C with the thermometer.
Control variables
- Control variables The volume and the initial mass of stearic acid must be kept the same for each run.
Warm water bath
- Warm water bath It cools the acid slowly and avoids cracking the glass, which can happen with cold water.
Part 3 of 5Interpreting the Cooling Curve
A flat section on a cooling curve shows the change of state.
Interpreting the Cooling Curve
Plateau
- Plateau A horizontal section shows that the temperature stays constant while the acid freezes, because latent heat is being released.
Melting point
- Melting point The temperature of the plateau gives the freezing point, which is the same as the melting point of the substance.
Sloping sections
- Sloping sections Steep drops show that the liquid or solid is cooling, with the temperature falling as energy is lost.
Evaluation
- Evaluation Repeat the readings, keep the thermometer at the same depth each time, and read scales at eye level to reduce errors.
| Time (min) | Temperature (°C) | State |
|---|---|---|
| 0 | 80 | Liquid |
| 2 | 76 | Liquid |
| 4 | 72 | Liquid |
| 6 | 69 | Freezing begins |
| 8 | 69 | Liquid and solid |
| 10 | 69 | Liquid and solid |
| 12 | 66 | Solid |
| 14 | 61 | Solid |
Part 4 of 5Worked Example
Question Using the illustrative readings in the table, state the melting point of stearic acid and explain how you decided.
- Step 1: Find the section where the temperature stays the same over several readings: 6 to 10 minutes.
- Step 2: Read the temperature on that flat section: 69 °C.
- Step 3: Recognise that the acid is changing state during the plateau, so this temperature is its freezing point and melting point.
Answer The melting point is 69 °C, taken from the plateau where the temperature stays constant during the change of state.
Part 5 of 5Exam Checklist & Tips
4.1.5 – Key Points for the Exam
- Cooling curve A graph of temperature against time as a substance cools.
- Plateau A horizontal section of a cooling curve, where the temperature stays constant during a change of state.
- Freezing point The temperature at which a liquid becomes a solid.
- Latent heat of fusion The energy released or absorbed when a substance changes between solid and liquid at constant temperature.
- Random error An error that varies between readings, reduced by repeating the experiment.
Common Mistakes
- Reading the melting point from the start of the graph rather than from the flat section.
- Letting the thermometer touch the glass, which gives a reading that is not the temperature of the acid.
- Saying the temperature falls during the plateau; it stays constant while the change of state happens.
- Recording only a few readings, which makes the plateau impossible to identify reliably.
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