Acids in the narrower sense are all compounds that can function as proton donors, i.e. are able to transfer protons (H+) to a reaction partner. In aqueous solution, the reactant is essentially water. As a result of this process, in which oxonium ions (H3O+) are formed, the pH value of the solution decreases. Acids react with bases to form water and salts. A base is thus able to neutralize an acid  it is the counterpart of acid. In a broader sense, however, various acid-base concepts serve to describe a much broader range of chemical reactions that go far beyond the above-mentioned processes.