What is the pH of rainwater and why and what does it mean?

Answer 1

The pH scale is a simple numerical scale from 1 to 14. If you take a piece of Litmus paper and dip it in a solution (we'll go with rain water if you like) and it comes out neutral, the pH value would be 7, right in the middle. It would be neither acidic, nor alkaline, and that's how rainwater is supposed to be...pure. If pH is below 7, the water is acidic and if above 7 it is alkaline. Thus, values of 6 and 8 would be slightly acidic and slightly alkaline, respectively. A pH value of 1 or 14 would be an extremely dangerous substance (either very, very acidic or very very alkaline).

The pH of modern rainwater is between 5.5 and 6.0 because of carbon dioxide that's dissolved in the atmosphere. This CO2 reacts with the H2O (water vapor) in the atmosphere for the following reaction:

CO2 + H2O ---> H2CO3 (carbonic acid)

then that dissociates:

H2CO3 ---> H+ + HCO3- (bicarbonate ion)

the bicarbonate ion can then dissociate:

HCO3- ---> H+ + CO3- (carbonate).

All of these reactions are reversible as well and is what makes bicarbonate a really good buffer. Bicarbonate is the same buffer used in human blood.

More Information:

Rain water on average in 2009 had a pH between 4.3 and 5.6

And a value below 7 on the pH scale indicates acidity. 7 is neutral .Above 7 is alkaline.

Low pH rain is called acid rain and it a major cause in a lot of erosion of ancient structures.

In plants, pH of the soil for the abundance of plants should be near neutral or slightly acidic. Some plants though such as azalea and blueberry bushes require a very acid soil around 4.5 to 5 for best growth. Soil pH affects how certain nutrients can be absorbed by plants.

An alkaline substance counteracts a acid substance.

Lime can be used to raise soil pH. Sulfur can be used to lower soil pH.

Your body should also be somewhat pH balanced. Urine tests can tell if your body is on the acid or alkaline side.