Causes of Water Acidity: Carbon Dioxide and Other Substances
Question: The most common cause of acidity of water is:
(A) Nitrogen
(B) Carbon dioxide
(C) Oxygen
(D) Alkalies
(A) Nitrogen
(B) Carbon dioxide
(C) Oxygen
(D) Alkalies
Answer: (B) Carbon dioxide
Explanation
The acidity of water is most commonly caused by the presence of dissolved carbon dioxide (CO₂). When CO₂ dissolves in water, it reacts to form carbonic acid (H₂CO₃), which lowers the pH, making the water acidic:
CO₂ + H₂O → H₂CO₃
Effects of Other Substances
While carbon dioxide is the primary cause of water acidity, other substances like nitrogen, oxygen, and alkalies also influence water chemistry. Below is a detailed comparison of their effects:
1. Nitrogen (N₂ and Nitrogen Compounds)
| Form | Effect on Water |
|---|---|
| Nitrogen gas (N₂) | Inert; no direct effect on water chemistry |
| Ammonia (NH₃) | Increases pH (alkaline); toxic to aquatic life at high levels |
| Nitrates (NO₃⁻) | Nutrient for algae; causes eutrophication and algal blooms |
| Nitrites (NO₂⁻) | Intermediate in nitrogen cycle; toxic to fish and harmful to humans |
| Total Kjeldahl Nitrogen | Indicator of organic pollution in wastewater |
2. Oxygen (O₂)
| Effect on Water | Details |
|---|---|
| Dissolved Oxygen (DO) | Essential for aquatic life (fish, insects) |
| Low DO levels (< 5 mg/L) | Leads to hypoxia or anoxic conditions; harmful to aquatic organisms |
| High DO levels | Generally good; but can cause gas bubble disease in extreme supersaturation |
| Oxidizing agent | Helps in natural purification and breakdown of organic matter |
3. Alkalies (e.g., NaOH, KOH, Ca(OH)₂)
| Effect on Water | Details |
|---|---|
| Raises pH | Makes water alkaline |
| Neutralizes acids | Used in water treatment to control corrosion |
| In high concentrations | Can be corrosive, toxic, and harmful to aquatic organisms |
| Examples used | Lime (Ca(OH)₂), soda ash (Na₂CO₃), sodium hydroxide (NaOH) |
Summary Table
| Substance | Effect |
|---|---|
| Carbon dioxide (CO₂) | Increases acidity by forming carbonic acid |
| Nitrogen | Can cause eutrophication, toxicity, or raise pH (NH₃) |
| Oxygen | Supports aquatic life, aids in biodegradation |
| Alkalies | Increase pH, neutralize acidity, can be harmful at high levels |
Effects of Substances in Water
| Substance | Form/Example | Effect on Water | Explanation |
|---|---|---|---|
| Nitrogen | N₂ gas | No chemical effect | Inert gas; does not react with water under normal conditions. |
| Nitrogen | Ammonia (NH₃) | Increases pH (alkaline); toxic at high levels | Forms due to organic waste breakdown; toxic to fish above 0.05 mg/L. |
| Nitrogen | Nitrates (NO₃⁻) | Causes eutrophication, algal bloom | Excess promotes plant/algae growth; leads to oxygen depletion. |
| Nitrogen | Nitrites (NO₂⁻) | Toxic to aquatic life and humans | Interferes with oxygen transport in blood (especially dangerous for infants). |
| Oxygen | Dissolved Oxygen (DO) | Essential for aquatic life | Required by fish and microorganisms for respiration. |
| Oxygen | Low DO (< 5 mg/L) | Harmful to fish; leads to hypoxia | Insufficient oxygen causes stress or death to aquatic species. |
| Oxygen | High DO (> 10 mg/L) | Usually good; may cause gas bubble disease if excessive | Over-saturation can harm fish due to gas bubble formation in blood. |
| Alkalies | Sodium hydroxide (NaOH) | Increases pH (alkaline nature) | Used to neutralize acidity in water treatment; raises pH above neutral. |
| Alkalies | Lime (Ca(OH)₂) | Reduces acidity; adjusts hardness | Commonly added to control corrosion and improve water stability. |
| Alkalies | Excess alkalies | Harmful to aquatic life; corrosive | Can irritate skin, eyes, and damage aquatic ecosystems at high concentration. |
Key Notes
- CO₂ is the primary cause of natural water acidity due to the formation of carbonic acid.
- Nitrogen compounds like NH₃ and NO₃⁻ can have indirect environmental impacts, such as eutrophication.
- Dissolved oxygen is critical for maintaining healthy aquatic ecosystems.
- Alkalies are used in water treatment to control pH but must be carefully managed to avoid harmful effects.
Note: Understanding the chemical influences on water quality is essential for environmental engineering, water treatment, and aquatic ecosystem management.
More solved questions of APSC WRD AE 2019, HERE
For APSC Water Resources Civil 2019 Question Paper, Click Here
For APSC Previous Year AE Question Paper, Click Here
For APSC Previous Year JE Question Paper, Click Here
Tags:
WRD PYQ