As rain falls to Earth from the sky, it begins to dissolve carbon dioxide (CO₂) in the atmosphere. The result is the formation of carbonic acid (H₂CO₃). The carbonic acid, in turn, breaks down into hydrogen ions, which make the water acidic and form hydrogen carbonate ions (commonly called bicarbonate ions). Water dissolving CO₂ marks the beginning of water’s ability not only to dissolve rock but to make rock as well.
The pH scale measures the concentration of hydrogen ions in water. The pH of pure water is 7, but the pH of rainwater is between 5 and 6. The pH scale is logarithmic. Therefore, a decrease by only one digit, for example, indicates approximately 10 times more hydrogen ions in concentration.
To demonstrate how easy it is for water to take on CO₂, we devised an experiment. In this experiment, we demonstrate how the pH changes noticeably if we simply blow bubbles into the water.
After rain falls, much of it enters streams and rivers and collects in reservoirs. From there, it may be collected as drinking water. This water is likely to remain quite empty of minerals, as it has had only a small amount of contact with bedrock at the Earth’s surface. Similarly, this water is likely to retain a lower pH level than public water in hard water areas (see below) because it has had less opportunity to react with bedrock and will thus continue to be mildly acidic. In Melbourne, Australia, home of Barista Hustle’s offices, the pH of the public water supply is close to neutral. Most local governments offer information on public water quality. Here is a link to the water analysis for the Melbourne area.
Many parts of the world take their public water supply from aquifers. These underground regions between the Earth’s surface and solid bedrock contain cracks or loose material that water can easily penetrate.