I was talking to a friend a while ago and she asked me what the difference was between mineral, tonic and soda water. Now I admit that I had no idea and it got me to thinking, 'What is the difference?' I decided to Google the answer and found an interesting few websites that explained the difference. In order to start this explanation we need to define that there is a difference between carbonated water (water with bubbles) and still water. Often this is phrased as still or sparkling, with sparkling obviously being the carbonated water. There are many varieties around the world of sparkling water, in the United States it is often known as seltzer and in the United Kingdom it is usually just referred to as sparkling water. When you are offered still or sparkling water, generally it is plain water (still) or plain water with bubbles (sparkling). Now mineral water is different from plain water because in order to be named mineral water it must contain a level of naturally occurring minerals. So the source of the water would have to be a mineral spring somewhere. This water, as with plain water, can be either still or sparkling.
Soda water is similar to mineral water however the minerals are added to the water and are not naturally occurring, hence why this water cannot be named mineral water. Soda water is always carbonated and has never been made in a still variety. All of the waters mentioned above taste similar and I personally think, others might disagree, can easily be substituted with each other in either their still or sparkling varieties (obviously the still varieties will taste difference from the sparkling). Tonic water is the main exception, although it is a carbonated (sparkling) water, this type of water tastes very different to the other varieties. The reason for this, is because it contains a compound called quinine, this is what gives tonic water its bitter taste. Quinine is actually a medicinal compound which was the first effective treatment for malaria and originally tonic water was used for medicinal purposes. It is also this bitter taste which quinine adds to tonic water that ensures any other carbonated water added in place of tonic, to a gin and tonic, will give it a drastically different flavour. Hopefully you (and myself) will remember all these differences and next time somebody asks you why these waters have different names, you will be able to give them an answer!
Check out the kitchn website to read more about these water varieties.