Freezing tonic water causes it to become cloudy and opaque due to the separation of the quinine molecules. These molecules are normally held in suspension by the water molecules, but when the water freezes, they are forced out of the solution and begin to clump together. This causes the tonic water to take on a milky appearance.

When the tonic water is thawed, the quinine molecules will eventually disperse and the water will return to its original clear state. However, the process may take some time, depending on the temperature at which the tonic water was frozen.

The change in appearance of tonic water when frozen is not harmful, and the water is still safe to drink. However, some people may find the cloudy appearance unappealing, so it is best to avoid freezing tonic water if you want to maintain its original clarity.