Pure rain water is naturally :

Question

Accepted Answer

Correct answer: A — Concept: The pH scale measures the hydrogen-ion activity of a solution: at 25 °C a value of 7 is neutral, a value below 7 is acidic and a value above 7 is basic. Whenever an acid-forming gas dissolves in water it releases hydrogen ions and pulls the pH down, so the acidity of any natural water is decided by the gases and solutes it has been in contact with. Application: A falling rain droplet is in contact with air, and air always carries carbon dioxide (about 420 parts per million today), so the droplet takes up that gas until the two are in equilibrium. The dissolved gas reacts with water to give carbonic acid: CO2 + H2O ⇌ H2CO3 Carbonic acid is a weak acid, so it ionises only partially and releases a small quantity of hydrogen ions: H2CO3 ⇌ H+ + HCO3− That small excess of hydrogen ions fixes the equilibrium pH of clean, unpolluted rain at about 5.6 — below the neutral value of 7, but only a little below it. Cross-check with the other pH bands: pH value Condition that produces it About 7.0 Distilled water kept in a sealed vessel away from air, where the hydrogen-ion and hydroxide-ion concentrations are equal Above 7.0 Dissolved alkaline matter such as carbonate dust or ammonia raising the hydroxide-ion concentration — a local dust effect, not the atmospheric baseline About 5.6 Equilibrium with atmospheric carbon dioxide alone, the baseline for clean rain Below about 5.0 Sulphur dioxide and nitrogen oxides from fuel burning dissolving and forming sulphuric and nitric acids — acid rain, a pollution effect So rain water in its pure, unpolluted state, in equilibrium with atmospheric carbon dioxide alone, is naturally mildly acidic at a pH of about 5.6.

Answer

A. Mild Acidic

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B. Mild Basic

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C. Neutral

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D. Strong Acidic

pH value	Condition that produces it
About 7.0	Distilled water kept in a sealed vessel away from air, where the hydrogen-ion and hydroxide-ion concentrations are equal
Above 7.0	Dissolved alkaline matter such as carbonate dust or ammonia raising the hydroxide-ion concentration — a local dust effect, not the atmospheric baseline
About 5.6	Equilibrium with atmospheric carbon dioxide alone, the baseline for clean rain
Below about 5.0	Sulphur dioxide and nitrogen oxides from fuel burning dissolving and forming sulphuric and nitric acids — acid rain, a pollution effect