Sulphur burns in air to form an acidic oxide, which is sulfur dioxide (SO2).
When sulfur burns in air, it reacts with oxygen to form sulfur dioxide gas. Sulfur dioxide is an acidic oxide because it reacts with water in the air to form sulfurous acid, which has a pH lower than 7. This means that it can react with bases to form salts and water. Sulfur dioxide is also a major air pollutant that can cause respiratory problems and acid rain when it dissolves in rainwater.
In summary, when sulfur burns in air, it forms sulfur dioxide, which is an acidic oxide that can react with water to form sulfurous acid and can contribute to air pollution and acid rain.