Magnesium ribbon in ethanoic (acetic) acid. Undiluted ethanoic acid (left), also known as glacial acetic acid, is not reactive with magnesium. However, when it is diluted in a ratio of 2:5 (right) there is a fairly strong reaction. It is more usual for dilute acid to be less reactive than concentrated. It's the presence of hydrogen ions in the diluted acid that causes the reaction to take place. In the undiluted ethanoic acid there are no hydrogen ions as the molecules have covalent bonds not ionic bonds.