In this experiment, you will investigate which of three insulating materials is most effective at reducing the rate of heat loss from a beaker of hot water....
Question 1 Report
In this experiment, you will investigate which of three insulating materials is most effective at reducing the rate of heat loss from a beaker of hot water.
Fig. 3.1 shows the apparatus for one of the beakers. Four identical glass beakers are used, each containing 150 ml of hot water. The first beaker is wrapped in a 1.0 cm thick layer of cotton wool held in place by an elastic band. The second beaker is wrapped in a 1.0 cm thick layer of bubble wrap. The third beaker is wrapped in a 1.0 cm thick layer of aluminium foil. The fourth beaker is left unwrapped and serves as a control. Each beaker is covered with a cardboard lid to reduce heat loss from the top surface. A thermometer passes through a hole in each lid so that the bulb sits in the centre of the water.
At the start of the experiment the water in all four beakers is at 82 °C. A stopwatch is started and the temperature is recorded every 3 minutes for 18 minutes. The room temperature is 20 °C and remains approximately constant throughout.
Table 3.1 shows the results. The unwrapped beaker cools most rapidly, falling to 52 °C after 18 minutes. The cotton wool beaker retains the most heat, reaching 68 °C after 18 minutes. The bubble wrap beaker reaches 64 °C after 18 minutes. The aluminium foil beaker reaches 58 °C after 18 minutes. The rate of cooling is greatest in the first few minutes for all beakers and decreases as the water temperature approaches room temperature.
The thickness of each insulating layer is measured with a ruler before the experiment begins. All beakers are placed on the same wooden bench, spaced apart so that one beaker does not warm the air around another. The water for all four beakers is drawn from the same heated source and poured simultaneously.
| Time / min | Cotton wool / °C | Bubble wrap / °C | Aluminium foil / °C | No insulation / °C |
|---|---|---|---|---|
| 0 | 82 | 82 | 82 | 82 |
| 3 | 79 | 78 | 76 | 73 |
| 6 | 74 | 71 | 66 | |
| 9 | 74 | 71 | 67 | |
| 12 | 72 | 68 | 64 | 57 |
| 15 | 70 | 66 | 60 | 54 |
| 18 | 68 | 64 | 58 | 52 |
Table 3.1
(a) Record the starting temperature of the water shown in Table 3.1.
Starting temperature = ........................ °C [1]
(b) Complete the two missing values in Table 3.1. [2]
(c) State which insulating material is the most effective at reducing heat loss. [1]
(d) Explain, using data from Table 3.1, how you reached your conclusion in (c). [2]
(e) Describe one precaution taken to ensure a fair comparison between the four beakers. [1]
(f) State why cardboard lids are placed on top of the beakers. [1]
(g) Plan an experiment to investigate whether doubling the thickness of the cotton wool further reduces the rate of cooling. Include what you would change, what you would measure and what you would keep the same. [2]
(h) State one source of inaccuracy in this experiment. [1]
Answer Details
(a) The starting temperature of the water shown in Table 3.1 is 82 °C. [1]
(b) The two missing values in Table 3.1:
- Cotton wool at 6 minutes: the value must lie between 79 °C (at 3 min) and 74 °C (at 9 min). The pattern shows a roughly even decrease, so the value is approximately 76 °C. [1]
- No insulation at 9 minutes: the value must lie between 66 °C (at 6 min) and 57 °C (at 12 min). Following the pattern, the value is approximately 61 °C. [1]
The completed table:
| Time / min | Cotton wool / °C | Bubble wrap / °C | Aluminium foil / °C | No insulation / °C |
|---|---|---|---|---|
| 0 | 82 | 82 | 82 | 82 |
| 3 | 79 | 78 | 76 | 73 |
| 6 | 76 | 74 | 71 | 66 |
| 9 | 74 | 71 | 67 | 61 |
| 12 | 72 | 68 | 64 | 57 |
| 15 | 70 | 66 | 60 | 54 |
| 18 | 68 | 64 | 58 | 52 |
(c) The most effective insulating material is cotton wool. [1]
(d) The cotton wool beaker has the highest temperature after 18 minutes (68 °C), meaning it lost the least heat. [1] Its total temperature drop is only \( 82 - 68 = 14\,{}^{\circ}\text{C} \), which is the smallest of all four beakers (compared to 18 °C for bubble wrap, 24 °C for aluminium foil, and 30 °C for no insulation). [1]
Cotton wool traps a large volume of still air within its fibres. Still air is an excellent thermal insulator because it has low thermal conductivity and, when trapped in small pockets, cannot circulate by convection.
(e) One precaution: use the same volume and starting temperature of water in each beaker. [1] This ensures a fair comparison where the only difference is the insulating material.
(f) Cardboard lids are placed on top to reduce heat loss by evaporation and convection from the top surface of the water. [1] Without lids, hot water vapour would escape and carry thermal energy with it, and warm air above the water would rise and be replaced by cooler air.
(g) To investigate whether doubling the cotton wool thickness further reduces cooling:
- Change: the thickness of the cotton wool (compare 1.0 cm with 2.0 cm). [1]
- Measure: the temperature of the water at regular time intervals (e.g. every 3 minutes for 18 minutes), or the final temperature after a fixed time.
- Keep the same: volume of water, starting temperature, type and size of beaker, cardboard lid, room temperature. [1]
If the 2.0 cm cotton wool beaker retains more heat (higher final temperature or smaller temperature drop), then doubling the thickness does improve insulation.
(h) One source of inaccuracy: it is difficult to wrap each beaker with exactly the same thickness of insulating material. [1] Uneven thickness would affect the rate of heat loss inconsistently. Another valid source: heat escapes through the lid or around the thermometer hole, which is not controlled by the insulation.
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