Are there any chemical reactions which result in the density of a water decreasing? I'm not a chemist or a physicist, I'm not even college educated, so forgive me if I use the wrong terminology or do not understand the question I am asking.
I understand that dissolving salt into water is a chemical reaction that results in the substance having a higher density (Salt water having a higher density than freshwater). I was wondering if there is anything you can dissolve into water which would decrease its density? Or is that just outright impossible?
My question arises from my curiosity about oscillating chemical reactions, I know that dissolving salt into water is a chemical change, but it also changes its physical property of density. I was wondering if there is something you could mix in to the salt water to bring its density back down. I understand that's not what an oscillating chemical reaction is at all, I'm just interested in making the density of a bucket of water change back and forth.
 A: For example, you can add alcohol to water to decrease water's density.
A: Yes you can.
If you put some substance that is lower in solubility H20 molecules will tend to bond with the substance you put in. And the existing salt (NaCl) will stack on the bottom of a container. Finally the density of salt with H20(water) swaps quickly depending on how high soluble material you put. If you are not familiar with the concept of solubility please refer to the link (wikipedia).
A: To lower water density you need some addition that makes the water molecules less densely packed (and itself is not heavy enough to add much weight to the volume). The problem is that the molecules tend to attract each other with hydrogen bonds, producing an average distance of 0.31 nm. This is not too different from the interatomic distances inside the molecule: it is a rather dense structure, despite the molecules jostling with each other and sometimes forming very brief bigger spaces. So adding something to keep the molecules apart would need to be nearly everywhere and somehow block the hydrogen bonds.
Hydrogen has a solubility of 0.00016 and helium even less: it does not look plausible that one could "jam open" water by adding them, and while ions generate cage complexes around themselves they do not change the density downward.
Still, by supercooling water under low pressure and allowing it to freeze into a glass one can get ice forms that are apparently lower density than normal crystalline ice. This can also be used to very briefly produce a low density liquid form. But it is not really a chemical reaction.
