Consider the following statements: 1. ‘When pH of rain water lies in the range of 6 to 7, it is called acid rain. 2. Tooth enamel starts to corrode when pH of mouth is greater than 7. 3. Under acidic conditions farmers would treat the soil with quick lime. How many of the statements given above are correct?

Acid Rain

• Rainfall that has a lower pH than normal, due to the presence of acidic components in the atmosphere, such as sulphur dioxide (SO₂) and nitrogen oxides (NOₓ), which combine with water vapor to form sulfuric acid (H₂SO₄) and nitric acid (HNO₃)
• Acid Rain pH: Acid rain usually has a pH less than 5.6. In many polluted areas, the pH of acid rain can drop to between 4.0 and 5.0, and in severe cases, it can be even lower. Hence, statement 1 is incorrect.

Tooth enamel

• Tooth enamel starts to demineralize, or lose its mineral content, when the pH of the mouth drops below a certain level. For the formation or remineralization of tooth enamel, maintaining a pH above this critical level is essential.

pH Threshold for Tooth Enamel

• Demineralization: Tooth enamel begins to demineralize when the pH of the mouth drops below 5.5. At this pH, acids from food and drink, or produced by bacteria in the mouth, start to dissolve the calcium and phosphate in the enamel, weakening it. Hence, statement 2 is incorrect.
• Remineralization: For enamel to remineralize and repair itself, the pH needs to be above 5.5

Quicklime

• Quick Lime: Also known as calcium oxide (CaO), it is a compound obtained by heating limestone (calcium carbonate) in a kiln to produce calcium oxide and carbon dioxide.
• Under acidic conditions, farmers would treat the soil with quicklime. Hence, statement 3 is correct.

Purpose of Application

• Soil Acidity: Acidic soils (with a low pH) can hinder plant growth and reduce crop yields. Adding quick lime helps to neutralize soil acidity, making the soil more suitable for planting.
• pH Adjustment: Quick lime reacts with water and soil acids to form calcium hydroxide (slaked lime) and raise the soil pH to more neutral levels.

Benefits

• Improved Soil Structure: Helps in enhancing the soil structure and
• increasing its nutrient availability.
• Enhanced Fertility: By raising the soil pH, quick lime makes essential nutrients more available to plants, improving overall soil fertility and crop yields.