Asked by Sean
With my new baby I spend a lot of time waiting for bottles of boiled water to cool before adding infant formula. Running then under a cold tap seems the most effective option, but would it ever be faster to slightly overfill the bottle and let some water evaporate, removing the most energetic molecules, before running under a tap?
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Short answer: No — for a normal baby bottle it won’t be faster in practice. Evaporation would have to remove a surprisingly large amount of heat and it’s slow from a narrow-neck bottle. Running the bottle under cold water (or using a cold water bath and swirling) will cool it much quicker.
Why:
- Heat to remove = mass × specific heat × ΔT. For example, a 250 mL bottle of boiling water cooled from 100 °C to 70 °C:
Q = 0.25 kg × 4.18 kJ/kg·K × 30 K ≈ 31 kJ.
- Evaporation removes heat by the latent heat Lv ≈ 2260 kJ/kg. To take away 31 kJ you must evaporate
Δm = Q / Lv ≈ 31 kJ / 2260 kJ/kg ≈ 0.0137 kg ≈ 13.7 mL.
That’s ~5–6% of the bottle volume — not “a little” water.
- Evaporation rate from a bottle mouth is limited by surface area, humidity and airflow. Typical passive evaporation from a narrow-neck bottle is slow (many minutes for tens of mL). Even with forced boiling you’d be wasting water and creating steam/scald risk.
- By contrast, forced convective cooling (holding the bottle under a cold tap, or immersing and swirling in cold water) provides a much higher heat flux. Reasonable heat-transfer estimates give cooling times on the order of tens of seconds to a few minutes — much faster than waiting for enough evaporation.
Other practical points:
- Letting hot water evaporate is wasteful and increases scald and contamination risks.
- Faster safe alternatives: run cold tap over the bottle while turning it or swirl the bottle in a bowl of cold water/ice-water, use a thermos of preheated water at the right temperature, or get a kettle with temperature control so you don’t have to cool so much.
So evaporation-before-tapping isn’t a practical speed-up for typical baby bottles.
Why:
- Heat to remove = mass × specific heat × ΔT. For example, a 250 mL bottle of boiling water cooled from 100 °C to 70 °C:
Q = 0.25 kg × 4.18 kJ/kg·K × 30 K ≈ 31 kJ.
- Evaporation removes heat by the latent heat Lv ≈ 2260 kJ/kg. To take away 31 kJ you must evaporate
Δm = Q / Lv ≈ 31 kJ / 2260 kJ/kg ≈ 0.0137 kg ≈ 13.7 mL.
That’s ~5–6% of the bottle volume — not “a little” water.
- Evaporation rate from a bottle mouth is limited by surface area, humidity and airflow. Typical passive evaporation from a narrow-neck bottle is slow (many minutes for tens of mL). Even with forced boiling you’d be wasting water and creating steam/scald risk.
- By contrast, forced convective cooling (holding the bottle under a cold tap, or immersing and swirling in cold water) provides a much higher heat flux. Reasonable heat-transfer estimates give cooling times on the order of tens of seconds to a few minutes — much faster than waiting for enough evaporation.
Other practical points:
- Letting hot water evaporate is wasteful and increases scald and contamination risks.
- Faster safe alternatives: run cold tap over the bottle while turning it or swirl the bottle in a bowl of cold water/ice-water, use a thermos of preheated water at the right temperature, or get a kettle with temperature control so you don’t have to cool so much.
So evaporation-before-tapping isn’t a practical speed-up for typical baby bottles.
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