Liquid water is essential to life on our planet, and chemistry involving the characteristic ions of water, [latex]\ce{H+}[/latex] and [latex]\ce{OH^{−}}[/latex] is widely encountered in nature and society. As introduced in another chapter of this text, acid-base chemistry involves the transfer of hydrogen ions from donors (acids) to acceptors (bases). These [latex]\ce{H+}[/latex] transfer reactions are reversible, and the equilibria established by acid-base systems are essential aspects of phenomena ranging from sinkhole formation (Figure 1) to oxygen transport in the human body. This module will further explore acid-base chemistry with an emphasis on the equilibrium aspects of this important reaction class.