pH of dust free and unpolluted rainwater is 5.6 due to dissolution of
CO2
Understanding the pH of Unpolluted Rainwater
Pure water, in theory, has a neutral pH of 7. This is because it auto-ionizes slightly to form equal amounts of hydrogen ions (\(H^+\)) and hydroxide ions (\(OH^-\)). However, rainwater is not perfectly pure water. As rain falls through the Earth's atmosphere, it comes into contact with various gases. Some of these gases dissolve in the water droplets, influencing the water's chemical composition and, consequently, its pH.
Why Normal Rainwater is Slightly Acidic
The question states that dust-free and unpolluted rainwater has a pH of 5.6. This indicates that even in the absence of pollution, rainwater is slightly acidic (pH < 7). This natural acidity is primarily caused by the dissolution of a common atmospheric gas.
| Gas in Atmosphere | Interaction with Water | Resulting Compound | Effect on pH |
|---|---|---|---|
| Carbon Dioxide (\(CO_2\)) | Dissolves and reacts | Carbonic Acid (\(H_2CO_3\)) | Decreases pH (slight acidity) |
| Sulfur Dioxide (\(SO_2\)) (Pollutant) | Dissolves and reacts | Sulfurous Acid (\(H_2SO_3\)) / Sulfuric Acid (\(H_2SO_4\)) | Significantly decreases pH (acid rain) |
| Nitrogen Oxides (\(NO_x\)) (Pollutant) | Dissolves and reacts | Nitrous Acid (\(HNO_2\)) / Nitric Acid (\(HNO_3\)) | Significantly decreases pH (acid rain) |
The Role of Carbon Dioxide in Rainwater pH
The Earth's atmosphere naturally contains carbon dioxide (\(CO_2\)). When rainwater falls, \(CO_2\) from the air dissolves in the water droplets. This dissolved \(CO_2\) then reacts reversibly with water to form carbonic acid (\(H_2CO_3\)), a weak acid.
The chemical reaction is:
\(CO_{2(g)} + H_2O_{(l)} \rightleftharpoons H_2CO_{3(aq)}\)
Carbonic acid is a weak acid, meaning it does not fully dissociate in water. However, it does partially dissociate, releasing hydrogen ions (\(H^+\)) into the water:
\(H_2CO_{3(aq)} \rightleftharpoons H^+_{(aq)} + HCO_{3(aq)}^-\)
The presence of these hydrogen ions increases the acidity of the water, lowering its pH from neutral (7) to around 5.6. This pH of 5.6 is considered the normal, natural pH of unpolluted rainwater due to this dissolution of atmospheric \(CO_2\).
Analyzing the Options
Let's consider the given options:
- SO\(_2\): Sulfur dioxide is a significant air pollutant, primarily from burning fossil fuels. When it dissolves in rainwater, it forms sulfurous acid (\(H_2SO_3\)) and can be further oxidized to sulfuric acid (\(H_2SO_4\)). These are strong acids that cause acid rain, which typically has a pH much lower than 5.6, often below 5 or even 4. Since the question specifies unpolluted rainwater, \(SO_2\) is not the cause of the natural pH 5.6.
- CO\(_2\): Carbon dioxide is a natural component of the atmosphere. Its dissolution and reaction with water to form carbonic acid is responsible for the natural slight acidity (pH 5.6) of unpolluted rainwater, as explained above.
- NO\(_2\): Nitrogen dioxide is another major air pollutant, often from vehicle emissions and industrial processes. It dissolves in rainwater to form nitric acid (\(HNO_3\)) and nitrous acid (\(HNO_2\)). These strong acids also contribute to acid rain, causing pH values significantly lower than 5.6. Thus, \(NO_2\) is not the cause of the natural pH 5.6 in unpolluted rainwater.
- HO\(_2\): This chemical formula usually refers to the hydroperoxyl radical. While present in the atmosphere in trace amounts and involved in complex atmospheric chemistry, it does not dissolve in rainwater and react to form an acid that significantly affects the bulk pH to 5.6 in the way \(CO_2\) does. Its impact on the overall acidity of rainwater is negligible compared to \(CO_2\).
Therefore, the natural acidity of unpolluted rainwater, resulting in a pH of 5.6, is due to the dissolution of atmospheric carbon dioxide.
Revision Table: Factors Affecting Rainwater pH
| Factor | Chemical Involved | Effect on pH | Notes |
|---|---|---|---|
| Natural Process | Atmospheric \(CO_2\) | Slight decrease (pH ~5.6) | Forms weak carbonic acid |
| Pollution | \(SO_2\), \(NO_x\) | Significant decrease (pH < 5.6) | Forms strong sulfuric/nitric acids (Acid Rain) |
| Dust/Particulates | Basic compounds (e.g., Calcium Carbonate) | Increase (pH > 5.6) | Can neutralize acidity in polluted areas |
Additional Information: Acid Rain vs. Natural Rainwater pH
It is important to distinguish between the natural acidity of rainwater and acid rain.
- Natural Acidity: This is caused by the natural presence of \(CO_2\) in the atmosphere dissolving in rainwater, resulting in a pH of around 5.6. This is a normal environmental condition.
- Acid Rain: This term refers to precipitation (rain, snow, fog) that is significantly more acidic than normal, typically with a pH below 5.6. Acid rain is caused by man-made emissions of pollutants like \(SO_2\) and \(NO_x\), which form strong acids in the atmosphere. Acid rain can have harmful effects on the environment, including damage to forests, aquatic ecosystems, and buildings.
The question specifically asks about the pH of unpolluted rainwater, which is naturally acidic due to \(CO_2\).
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Reason (R) : CFCs are highly unreactive.
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(ii) \(HCO_3^-\)
iii) \(CO_3^{2-}\)
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the pH of natural waters is controlled mainly by
