What is the ion in sulphuric acid which causes it to be acidic?
The answer to one of your questions is all we are permitted each day. Sulphuric acid is a very strong acid. This acid is a clear, colorless, odorless, viscous liquid that is very corrosive. It is created by dissolving sulphur trioxide in water (this trioxide is made from sulphuric dioxide). One molecule of Sulphuric Acid, (H2H04) is created by combining two atoms of hydrogen, one atom of sulphur, and four atoms of oxygen. Sulphuric Acid does not have a flash point, however, the melting point is 10.2 C degrees; boiling point is 338C degrees, and its density is 1.94. Besides being a very strong acid,”it forms ions of hydrogen or hydronium (H+ or H3O+), hydrogen sulfate (HSO4-), and sulfate (SO42-).“ It is also an oxidizing and dehydrating agent and chars many organic materials." The molecules can be tested with litmus paper to determine its acidity. When water is added the the mixture creates heat. If you as sugar water the reaction leaves a blob of carbon. An interesting fact is this is the exact compound that makes our eyes tear when we cry while slicing an onion. The onion is sliced, the gas inside the onion is released into the air, hits the water in our eyes and forms acid; thus the tears.
Many things, including distilled water have the H+ ion; however, the H+ ion in aqueous solution must be greater than 1X10^-7 in order to be classified as "acidic". Most high school students will simply work with acids in the context of water solutions. So that simple answer is: [H+]; period.