Required Practical: Investigating Thermal Insulation

Step-by-Step Method:

1. First, place a small beaker inside a larger beaker.
2. Fill the gap between the two beakers with your first insulating material, ensuring it is not overly compressed.
3. Boil water in a kettle and use a measuring cylinder to pour exactly $80\text{ cm}^3$ of hot water into the small beaker.
4. Place a cardboard lid over the large beaker. The lid must have a small hole in it.
5. Insert a thermometer (with a standard resolution of $1\text{ }^\circ\text{C}$) through the hole so the bulb is submerged in the water but not touching the bottom.
6. Record the initial temperature, start a stopwatch, and record the temperature every 3 minutes for 15 minutes.
7. Repeat the entire process using different materials in the gap, plus one "control" run with only air in the gap (no insulation).

Investigation 2: Comparing Thickness of Material

The second investigation looks at how the number of layers of a single material changes its insulating properties.

Variables for Investigation 2:

Step-by-Step Method:

1. First, wrap a beaker in a set number of layers (e.g., 2 layers) of newspaper, securing them tightly with rubber bands.
2. Pour $80\text{ cm}^3$ of hot water into the beaker.
3. Place a cardboard lid on the beaker, insert a thermometer, and wait for the temperature to fall to your chosen starting temperature (e.g., $80\text{ }^\circ\text{C}$).
4. Start the stopwatch and record the temperature every 3 minutes for 15 minutes.
5. Repeat the process using 4, 6, and 8 layers, ensuring you start at the exact same temperature each time.

Analysing Data: Cooling Curves

A cooling curve is a temperature-time graph showing how an object loses thermal energy to its surroundings. Time is plotted on the x-axis and temperature on the y-axis.

The curve is non-linear and is steepest at the beginning because the temperature gradient (the difference in temperature between the hot water and the room) is at its greatest. As the water cools, the curve levels off, eventually reaching an asymptote at room temperature.

The best insulator will produce the curve with the shallowest gradient, as it retains thermal energy the longest.

Calculating Temperature Change and Cooling Rate

You may be asked to calculate the overall temperature drop or the rate of cooling.

Equations:

Worked Example:

A beaker of water starts at $80\text{ }^\circ\text{C}$ and drops to $55\text{ }^\circ\text{C}$ after 15 minutes. Calculate the rate of cooling in $^\circ\text{C/min}$.

Step 1: Calculate the temperature change ($\Delta T$).

• $\Delta T = 80 - 55 = 25\text{ }^\circ\text{C}$

Step 2: Substitute into the rate equation.

• $\text{Rate} = \frac{25}{15}$

Step 3: Calculate the final answer.

• $\text{Rate} = 1.67\text{ }^\circ\text{C/min}$

(Higher Tier Note: To find the rate of cooling at a specific, exact moment in time, you must draw a tangent to the curve at that specific time and calculate its gradient).

Evaluating the Experiment

To get the most accurate results, you must manage errors and safety carefully.

• The Lid: A lid is essential because it prevents heat loss via convection and evaporation from the surface, ensuring you are strictly testing the insulation around the sides.
• Systematic Error: If you reuse a beaker that is already warm from a previous run, it will artificially slow down the cooling. Always use a room-temperature beaker for a new run to avoid this systematic error.
• Improving Accuracy: Replacing a standard thermometer with a digital temperature probe (or data logger) increases the resolution to $0.1\text{ }^\circ\text{C}$ and removes the random error of misreading the scale (parallax error).
• Safety: Hot water can scald. Always conduct this experiment standing up so you can move away quickly if a beaker spills, and use a heatproof mat.