Which Wrap Keeps Water Warmest?
Four identical plastic bottles were filled with 250 mL of water at 60 °C. Bottle A was left bare. Bottle B was wrapped in one cotton sock. Bottle C was wrapped in one layer of aluminum foil. Bottle D was wrapped in a wool sock with foil over the outside. All four were sealed and placed on the same table in a 20 °C room, 30 cm apart. Students recorded the water temperature every 10 minutes. No bottle was opened during the test.
| Minutes | A bare | B cotton | C foil | D wool + foil | | 0 | 60 | 60 | 60 | 60 | | 10 | 52 | 55 | 54 | 57 | | 20 | 46 | 51 | 49 | 54 | | 30 | 41 | 47 | 45 | 51 | | 40 | 37 | 44 | 42 | 48 | | 50 | 34 | 41 | 39 | 45 |
1. Which bottle cooled most slowly? Use the 50-minute numbers to compare all four, and explain what the winning cover is doing.
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Bottle D cooled most slowly. After 50 minutes it was 45 °C, a drop of only 15 °C. Bottle A fell 26 °C (to 34 °C), B fell 19 °C (to 41 °C), and C fell 21 °C (to 39 °C).
The winning cover works in two ways. Wool is mostly tiny pockets of trapped air, and still air carries heat poorly, so the wool slows heat moving from the water to the room. The foil adds a shiny outer surface that reflects some radiated heat back toward the bottle. Combining them beats either one alone.
Foil by itself (C) helped only a little, because metal conducts heat quickly, so a thin sheet of it is a poor insulator. Its small effect comes from reflecting radiation, not from trapping heat.
2. Predict bottle D's temperature at 60 minutes. Show the pattern you used, and give one reason the real result might be slightly different.
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Look at D's drops: 60 → 57 → 54 → 51 → 48 → 45. Every 10 minutes it loses exactly 3 °C, so the next reading should be about 42 °C.
A 42 °C prediction is reasonable but may be slightly wrong in one direction: the room is 20 °C, and the closer the water gets to room temperature, the more slowly it loses heat. The 3 °C per 10 minutes pattern cannot continue forever, because once the water reaches 20 °C it stops cooling. So the real 60-minute reading is likely to be a little above 42 °C rather than below it.
Also, this is a single trial, so small measurement errors in each reading would shift the prediction by a degree or two.
3. The four bottles are now placed in front of a fan that blows room-temperature air across them. Predict which bottle cools fastest and which cools slowest, and explain what has changed.
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The bare bottle A should cool fastest, and D should still cool slowest, though everything will cool faster than before.
Normally each bottle sits inside a thin layer of air that the bottle has warmed. That warm air stays put and slows further heat loss. A fan constantly strips this warm layer away and replaces it with cooler room air, so heat leaves the surface faster. A bottle with almost no insulation is affected most, because there is nothing but that air layer protecting it.
D still has trapped air inside the wool fibres. The fan cannot blow that trapped air out, so D keeps most of its advantage, but the foil surface itself will now lose heat to the moving air more quickly than before.