What is Dissolved in Rainwater?

Rainwater primarily contains dissolved gases, with dissolved solids generally being absent, except in specific environments like coastal areas.

While often perceived as pure, rainwater dissolves various substances from the atmosphere as it falls. Based on the provided reference, rainwater is typically devoid of all dissolved solids but contains key dissolved components.

Primary Dissolved Substances in Rainwater

The main substances dissolved in rainwater are dissolved gases picked up from the atmosphere. These include:

• Oxides of Carbon: Primarily carbon dioxide (CO₂).
• Oxides of Nitrogen: Such as nitrogen dioxide (NO₂).
• Oxides of Sulphur: Such as sulphur dioxide (SO₂).

These dissolved gases react with water to form weak acids, which is why rainwater is naturally slightly acidic.

Impact on pH

The presence of dissolved gases like oxides of carbon, nitrogen, and sulphur significantly influences the pH of rainwater. As the reference states, these gases result in pH values of about 5.5 or lower. This slight acidity is a natural characteristic of rainwater and is why it is often referred to as "acid rain" when the pH drops significantly below 5.5 due to higher concentrations of these pollutants.

Coastal Influence

In addition to atmospheric gases, the composition of rainwater can be affected by its environment. For instance, in coastal areas, rainwater may also contain dissolved sodium chloride (common salt). The reference specifies that in these regions, rainwater may contain up to 15 mg/l of sodium chloride originating from sea-spray.

Summary of Dissolved Substances

Here's a quick overview based on the reference:

This composition highlights that while rainwater is relatively pure compared to ground or surface water, it is not chemically inert and actively interacts with atmospheric components.