Change of State: Cooling Curve for Molten Wax
Objectives
- To observe the change of state from liquid to solid in wax
- To plot a cooling curve for molten wax
- To understand the relationship between temperature and time during phase changes
- To identify the freezing point of wax
Materials Required
- Wax candle (paraffin wax)
- Small beaker (100 mL)
- Thermometer (-10°C to 110°C)
- Stopwatch or timer
- Bunsen burner
- Tripod stand
- Wire gauze
- Heat-resistant gloves
- Retort stand with clamp
- Graph paper or graphing software
- Safety goggles
Theoretical Background
When a substance changes from one state to another, the process is known as a phase change. Energy is either absorbed or released during phase changes. For example, when a liquid solidifies (freezes), heat energy is released to the surroundings.
During a phase change, the temperature remains constant while energy is being absorbed or released. This energy is known as latent heat. Latent heat is the energy absorbed or released during a phase change without a change in temperature.
The equation for latent heat is:
\[ Q = mL \]
Where:
- \( Q \) = heat energy transferred (J)
- \( m \) = mass of substance (kg)
- \( L \) = specific latent heat of fusion or vaporization (J/kg)
For the freezing process (liquid to solid), we use the specific latent heat of fusion \( L_f \).
A cooling curve is a graph that shows how the temperature of a substance changes as it cools over time. The shape of a cooling curve reveals important information about phase changes.
Key features of a cooling curve:
- Slopes indicate cooling in a single phase (liquid or solid)
- Plateaus (horizontal sections) indicate phase changes
- The temperature at which a plateau occurs during cooling represents the freezing point
The rate of cooling can be described by Newton's Law of Cooling:
\[ \frac{dT}{dt} = -k(T - T_s) \]
Where:
- \( \frac{dT}{dt} \) = rate of temperature change
- \( k \) = cooling constant
- \( T \) = temperature of the object
- \( T_s \) = temperature of the surroundings
Safety Precautions
- Wear safety goggles and heat-resistant gloves when handling hot materials
- Be careful when using the Bunsen burner - tie back long hair and loose clothing
- Handle the thermometer with care to avoid breakage
- Hot wax can cause burns - use caution when handling
- Ensure the workspace is well-ventilated
- Keep all flammable materials away from the Bunsen burner
Procedure
- Set up the apparatus as shown in the diagram. Place the tripod stand and wire gauze over the Bunsen burner.
- Place small pieces of candle wax (approximately 50g) in the beaker.
- Heat the beaker gently until all the wax has melted completely. Do not overheat.
- Turn off the Bunsen burner once all the wax has melted.
- Clamp the thermometer to the retort stand so that the bulb is immersed in the molten wax but not touching the bottom of the beaker.
- Record the initial temperature of the molten wax.
- Start the stopwatch and record the temperature every 30 seconds until the wax has completely solidified and the temperature has fallen at least 15°C below the freezing point.
- Enter your readings in the data table provided.
- Plot a graph of temperature (y-axis) against time (x-axis).
- Analyze the cooling curve to identify the freezing point of the wax.
Experimental Setup
Figure 1: Experimental setup for observing cooling curve of wax
Data Collection
| Time (seconds) | Temperature (°C) | Observations |
|---|---|---|
| 0 | ||
| 30 | ||
| 60 | ||
| 90 | ||
| 120 | ||
| 150 | ||
| 180 | ||
| 210 | ||
| 240 | ||
| 270 | ||
| 300 | ||
| Continue recording data every 30 seconds until the wax has completely solidified and cooled | ||
Analysis
Graphing
Plot a graph of temperature (y-axis) against time (x-axis) using your recorded data. This will be your cooling curve.
Figure 2: Example of a cooling curve for wax
Questions for Analysis
- What is the freezing point of the wax used in this experiment?
- During which time interval did the wax change from liquid to solid state?
- Describe the shape of your cooling curve. Identify the different parts of the curve and explain what is happening at each stage.
- Why does the temperature remain constant during the freezing process?
- Calculate the rate of cooling (in °C/min):
- Before the freezing began
- After the freezing was complete
To calculate the rate of cooling, use the formula:
\[ \text{Rate of cooling} = \frac{\Delta T}{\Delta t} = \frac{T_2 - T_1}{t_2 - t_1} \]
Where:
- \( \Delta T \) = change in temperature (°C)
- \( \Delta t \) = change in time (minutes)
- \( T_1 \) = initial temperature
- \( T_2 \) = final temperature
- \( t_1 \) = initial time
- \( t_2 \) = final time
Select two points from the cooling curve for each calculation (before freezing began and after freezing was complete).
Discussion Questions
- How does the cooling curve for wax compare to what you would expect for a pure substance like water?
- Why might the freezing point of wax be different from the values in reference books?
- What sources of error might be present in this experiment?
- How could the experimental procedure be improved to reduce these errors?
- In real-world applications, why is understanding the cooling curve of a substance important? Give examples.
Extension Activities
- Investigate how adding impurities to the wax affects its freezing point.
- Compare the cooling curves of different types of wax (beeswax, paraffin, etc.).
- Determine the specific latent heat of fusion for the wax.
- Design an experiment to investigate the effect of surface area on the cooling rate of molten wax.
To determine the specific latent heat of fusion, you would need:
- A known mass of wax (\(m\))
- A calorimeter to measure energy transferred as heat (\(Q\))
Then use the equation:
\[ L_f = \frac{Q}{m} \]
Where:
- \( L_f \) = specific latent heat of fusion (J/kg)
- \( Q \) = heat energy transferred (J)
- \( m \) = mass of substance (kg)
Conclusion
Write a conclusion summarizing what you have learned from this experiment. Include:
- The freezing point of the wax
- An explanation of the shape of the cooling curve
- The relationship between temperature, time, and phase changes
- Any interesting observations or unexpected results
- Sources of error and how they might have affected your results