Thermal insulation required practical

Aim: To investigate how the type or thickness of an insulating material affects how quickly a container of hot water cools down.

Variables

• Independent: Type of insulating material (or the number of layers of the same material)
• Dependent: Temperature of the water after a fixed time (or the rate of cooling)
• Control: Volume of hot water; Starting temperature of the water; Size and type of beaker; Total time of cooling and the time between readings; Room temperature and whether a lid is used

Equipment

• Small beaker inside a larger beaker
• Kettle for hot water
• Measuring cylinder
• Thermometer
• Stopwatch
• Cardboard lids with a hole for the thermometer
• Insulating materials, e.g. bubble wrap, cotton wool, newspaper, polystyrene, felt
• Elastic bands or tape

Method

1. Place the small beaker inside the large beaker and fill the gap with the first insulating material (or wrap a set number of layers around a single beaker).
2. Use a measuring cylinder to pour the same volume of hot water, for example 80 cm³, into the small beaker.
3. Cover the beaker with a cardboard lid and push the thermometer through the hole into the water.
4. Record the starting temperature and start the stopwatch at once.
5. Record the temperature every 2 minutes for 20 minutes.
6. Empty the beaker and repeat with each of the other materials (or a different number of layers), starting from the same water temperature each time.
7. Repeat with no insulation as a control for comparison.
8. Work out the temperature drop over the 20 minutes for each material.

Safety

• Take care with water from a kettle: pour slowly to avoid scalding and keep the beaker on a stable, flat surface.
• Mop up spills straight away so nobody slips.
• Handle glass thermometers and beakers carefully and report any breakages.

Results and calculations

Record temperature against time for each material in a table. Plot a cooling curve (temperature on the y-axis, time on the x-axis) for each material on the same axes; the material with the smallest temperature drop, and the shallowest curve, is the best insulator. Temperature drop = starting temperature − final temperature.

Common mistakes and improvements

• Starting each test at a different water temperature makes comparisons unfair, because hotter water loses energy faster: let the water cool to the same starting temperature every time before starting the clock.
• Leaving off the lid lets energy escape by evaporation and convection from the surface, which hides the effect of the insulation: use the same lid for every test.
• Packing the materials to different thicknesses: use the same thickness or number of layers so only the type of material changes.
• Taking one set of readings only: repeat each material and calculate a mean to spot anomalies.

Exam tips

• Explain the result in terms of energy transfer: a good insulator has low thermal conductivity, so energy is transferred to the surroundings more slowly.
• Know how to read a cooling curve: a steeper curve means a faster rate of cooling.
• Many materials work well because they trap air, which is a poor conductor; mention this if asked why.
• When asked to name control variables, pick ones that actually affect the result, such as volume of water and starting temperature, not vague ones like 'same equipment'.
• Link to real life: loft insulation and thicker walls reduce the rate of energy transfer from a house.