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The most acidic rain in the Solar System is found on the planet Venus, where the working fluid in the cycle of evaporation, condensation and precipitation is a sulphuric acid solution (rather than water, as on Earth). Studies of the Venusian cloud layer have shown the concentration of sulphuric acid in this solution (sulphuric acid is too unstable to exist in its pure form, so it bonds with water) ranges from 70–99%. The strength of the concentration increases as the altitude of the clouds decreases, meaning that rain on Venus has a pH of around -1.2.
The pH scale is used to specify how acidic or alkaline (basic) a water-based solution is. It is typically given as a number between 0 and 14, where 0 is a strong acid, 14 is a strong base and 7 is neutral. For extremely powerful acids or bases, values greater than 14 or less than zero are possible, though owing to the way pH is calculated (it represents the concentration of hydrogen ions in a given solution), they can't be directly measured.
On Earth, rain is typically slightly acidic, with a pH of between 5 and 5.5, though both atmospheric pollution and volcanic activity can decease the pH by introducing small quantities of acid to the water. Acid rain caused by human activity usually has a pH of around 4 (about as acidic as tomato juice or beer) while acid rain caused by volcanic eruptions can have a pH as low as 2 (lemon juice or vinegar). On Saturn's moon Titan – where the working fluid in its cycle of evaporation, condensation and precipitation is methane – rain probably has a pH of around 7.
The acid rain on Venus is caused by the reaction of sulphur dioxide and water in the planet's atmosphere. Despite being many times more corrosive than the most acidic rain on Earth, Venusian rainstorms are not a significant contributor to surface erosion. Surface temperatures on Venus average around 473°C (884°F) and sulphuric acid evaporates at 300°C (572°F); this means that the acid rain likely boils away at an altitude of around 30,000 m (98,425 ft).