Does Freezing a Water Bottle Make the Weight Increase?

Does freezing a water bottle make the weight increase?
- AHMANI (age 11)

Absolutely no. The weight is a measure of how many atoms you have. Ideally, you can seal a bottle very well such that not a single atom or molecule can be exchanged between the bottle and the surroundings. So whether there is a chemical reaction or a physical transition going on inside should not cause a change in the number of atoms, and therefore mass should not change.

Upon freezing, the volume of the material increases, but that is because the average separation between atoms increased, not their number. You can perhaps think of it like a spool of cotton fibre vs. a bag of raw cotton. Since it is made more compact, the same mass of the spool will occupy far less space than fluffy raw cotton. In case you were weighing it yourself and measuring a difference, I suggest cleaning the water condensed on the bottle very well, as it will cause an apparent increase. 

Tunc

(published on 12/17/2015)

Freezing reduces the energy -- of atoms which vibrate less. And since (as per Einstein) energy has mass and mass has weight, hence, freezing reduces the weight. But this reduction of weight is very little. So, if your question does not concern relativistic effects, then, as answered, freezing does not change the weight.
- Mehran (age 65)
Arlington Heights, IL

True, but an extremely small effect.

(published on 12/23/2015)