An acid is a compound of hydrogen with a non-metallic element or a group of elements that act as one, called a radical. The acid may be a gas soluble in water, as muriatic acid, or a liquid, such as sulphuric acid, or a solid, such as oxalic acid. All acids have a sharp, sour taste and most of them act on metals. The test used for determining whether or not a solution is an acid is to place a drop of the solution on a piece of blue litmus paper (paper dyed blue with the juice of a small plant). If the blue color changes to red, the solution is an acid. If the paper remains blue, the solution is not acid. Acids have different powers and uses. While some are healthful and are used for foods, others are poisonous. Acids are used very commonly in industry for dissolving metals (Fig. 67).