Can you theoretically have an endless supply of water if you can find a way to collect the water that condensates on the outside of a water bottle?
In: Chemistry
* There is gas-phase water in the air. Air can only hold so much water, any more than that and it can’t stay a gas and condenses into liquid droplets. That’s what condensation is. That’s also what dew is, and frost.
* The amount of water that air can hold depends on temperature. The warmer the air, the more water it can hold. Condensation happens when warm humid air gets cooled down by the cold walls of the water bottle. As it cools, its water-holding capacity gets less. Once the cooling air can’t hold the water it previously could, it has to get rid of it – the water condenses into liquid droplets that deposit on the nearest surfaces (aka the water bottle).
* Once the water bottle isn’t colder than the air around it, the condensation stops. So if the bottle warms up, no condensation. An empty bottle, or good insulation, no condensation. If the outdoor air temp gets as cold as the bottle contents, no condensation. That’s why a very cold drink on a very hot day gives you the most condensation.
So, as a water source, the limits are:
* How much you’re able to keep the condensing surface cold (which takes energy, or adding ice – which also takes energy)
* How much humidity is in the air to begin with. You can only condense out as much water as the air is holding to begin with.
Your idea is so good that they’re actually using it, by the way! [Fog collection as a water source](https://en.wikipedia.org/wiki/Fog_collection) is useful in specific climates (eg high deserts in Chile) that get very little rain but often have fog (which is very humid air that’s 100% full of water vapour). There are also plants and even animals such as the [Namib Desert Beetle](https://en.wikipedia.org/wiki/Stenocara_gracilipes) with body parts specifically designed to promote condensation and collect it as their primary water source:
>To drink water, the S. gracilipes stands on a small ridge of sand. Facing into the breeze, the beetle catches fog droplets on its hardened wings. Its head faces upwind, and its stiff, bumpy outer wings are spread against the damp breeze. Minute water droplets from the fog gather and stick to hydrophilic (water-loving) bumps on its wings. Accumulation continues until the combined droplet weight overcomes the water’s electrostatic attraction to the bumps; at that point it will roll down the beetle’s back to its mouthparts.
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