Required Practical: Investigating Thermal Insulation

What the investigation finds out

A beaker of hot water transfers thermal energy to its cooler surroundings, so the water's temperature falls. Wrapping the beaker in an insulating material reduces this unwanted transfer. An effective insulator therefore produces a smaller temperature decrease in a fixed time, or a lower rate of cooling.

This required practical has two related parts:

1. compare different insulating materials;
2. investigate a factor affecting insulation, such as the thickness or number of layers of one material.

The two questions must be tested separately. Changing both material and thickness at once would make it impossible to tell which change caused a difference in cooling.

Apparatus and arrangement

The basic apparatus includes hot water from a kettle, 100 cm³ and 250 cm³ beakers, thermometers, a stopwatch, cardboard, scissors, rubber bands and insulating materials. Suitable materials include newspaper, corrugated cardboard, bubble wrap, polystyrene balls and cotton wool.

For a comparison of materials, place insulation inside a larger beaker and put a smaller beaker inside it. Add hot water to the smaller beaker. A cardboard lid with a hole for the thermometer helps to reduce energy transfer from the exposed water surface and keeps this source of energy loss similar in every test.

For the thickness investigation, wrap otherwise identical beakers in different numbers of layers of the same material. Include a beaker with no insulation as a control.

Comparing different materials

Use the same arrangement for each material. Place an equal mass of each insulating material around the inner beaker, pour in the same volume of hot water and make the starting temperatures as similar as possible. Fit the lid and thermometer, start the stopwatch and record the temperature at regular intervals—for example, every 3 minutes for 15 or 20 minutes. Repeat using each material and include a no-insulation test.

Here, the type of insulating material is the independent variable. Temperature is measured as the water cools; the temperature decrease or cooling rate is then used as the dependent outcome. Important control variables include:

• the volume of water;
• the starting temperature;
• the sizes and materials of the beakers;
• the mass of insulation and the way it is packed;
• the lid and thermometer arrangement;
• the measurement times and total cooling time;
• surrounding conditions, such as room temperature and draughts.

Equal masses make the comparison between materials fair. Without this control, one material might appear better simply because more of it was used.

Investigating thickness

Choose one insulating material and alter only its thickness, perhaps by using 0, 2, 4 and 6 layers of newspaper. Keep the water volume, starting temperature, container, lid, timing and surrounding conditions constant.

The thickness or number of layers is now the independent variable. Measure temperature at the same times and compare either the temperature decrease over a fixed interval or the shape of each cooling curve. The expected pattern is that greater thickness produces slower cooling because it reduces the rate of thermal energy transfer to the surroundings.

Recording and processing the results

A results table should give time in minutes and temperature in °C for every material or thickness. Calculate the temperature decrease using

$\text{temperature decrease}=\text{starting temperature}-\text{final temperature}$

For a common time interval, a simple average rate of cooling can be compared using

$\text{average rate of cooling}=\frac{\text{temperature decrease}}{\text{time}}$

Plot temperature against time, with time on the x-axis and temperature on the y-axis. The line usually curves rather than remaining straight because the rate of energy transfer changes as the temperature difference between the water and its surroundings becomes smaller. Over the same interval, the most effective insulation gives the smallest fall in temperature and generally the shallowest cooling curve.

The AQA technician data illustrate the comparison. Over 15 minutes, the uninsulated water falls from 85 °C to 57 °C, a decrease of 28 °C. Water insulated with newspaper falls from 86 °C to 63 °C, a smaller decrease of 23 °C. In the thickness data, water with no newspaper falls by 28 °C, whereas water surrounded by six layers falls from 86 °C to 66 °C, a decrease of only 20 °C. These results support the conclusions that newspaper reduces cooling and that increasing its thickness improves its insulating effect under these conditions.

Different starting temperatures mean that final temperatures alone should not be compared blindly. Comparing the change over the same time is fairer, although making the starting temperatures equal would improve the experiment.

Making the evidence trustworthy

Repeat each condition and calculate a mean temperature at each time, or a mean temperature decrease. Repeats reveal anomalous readings and reduce the influence of random variation. Use thermometers with the same resolution, read them at eye level and keep their bulbs at the same depth without touching the beaker.

Start each run promptly after adding the hot water. Even better, test the conditions simultaneously using several matched beakers, because room conditions may change between trials. Prevent draughts and keep insulation dry and arranged consistently. These precautions improve validity by making the independent variable the only important difference between tests.

The uninsulated control is essential: it shows how rapidly the water cools in the same apparatus without the insulating treatment. It allows the reduction in cooling caused by each material to be judged.

Safety

Hot water and steam from a kettle can burn or scald. Place the beakers on a stable, heat-resistant surface, pour carefully and follow the teacher's instructions for handling the kettle. Cut cardboard carefully with scissors and wipe up spilled water promptly.