Acid rain, primarily caused by the release of sulfur and nitrogen oxides into the atmosphere, has a detrimental effect on historical monuments and buildings, especially those made of limestone, marble, and other calcareous stones. When acid rain interacts with calcium carbonate (the primary component of these materials), it forms calcium sulphate, carbon dioxide, and water. This chemical reaction not only weakens the stone's structure but also causes it to become more porous and susceptible to erosion. Many iconic structures worldwide, including the Taj Mahal in India and the Parthenon in Greece, have shown signs of damage due to the corrosive effects of acid rain.

Yes, amphiprotic substances have various practical applications outside the lab. For instance, baking soda, an amphiprotic compound, is commonly used in cooking as a leavening agent. When combined with an acid, it produces carbon dioxide gas, causing the dough or batter to rise. Additionally, amphiprotic substances are also employed in certain industrial processes, like waste treatment, to neutralise effluents. In medicine, amphiprotic compounds can serve as buffers to maintain the pH levels of solutions, ensuring they remain within a specific range, which is essential for many biological functions and pharmaceutical formulations.

Metal oxides, when dissolved in water, tend to produce hydroxides, thereby behaving as bases. For instance, calcium oxide (CaO) reacts with water to produce calcium hydroxide (Ca(OH)₂). Non-metal oxides, on the other hand, typically form acids when they come into contact with water. An example of this is sulfur dioxide (SO₂) which reacts with water to form sulfurous acid (H₂SO₃). The inherent difference lies in the electron configurations and atomic structures of metals and non-metals. Metal oxides have the ability to donate electrons to water, resulting in basic solutions, while non-metal oxides accept electrons from water, leading to acidic solutions.

While pure water is neutral with a pH of 7 at 25°C, natural rainwater typically has a pH between 5.5 and 6.5, making it slightly acidic. This slight acidity arises from the reaction of water vapour with carbon dioxide (CO₂) in the atmosphere to form carbonic acid (H₂CO₃), a weak acid. The reaction is: CO₂ + H₂O -> H₂CO₃. Thus, even in the absence of significant pollutants, rainwater will inherently be a bit acidic due to the natural presence of carbon dioxide in the atmosphere.

Water is an amphiprotic substance, which means it can act as both an acid and a base. In the auto-ionisation process, one water molecule donates a proton to another, acting as an acid, while the second water molecule accepts this proton, behaving as a base. The process is represented by the equation: 2H₂O (l) -> H₃O+ (aq) + OH- (aq). This auto-ionisation is responsible for the presence of small amounts of H₃O+ and OH- ions in pure water. Though the concentration of these ions is minute, it's crucial for establishing the pH scale and explaining why pure water has a neutral pH of 7 at 25°C.