2 edited body edited Dec 29 '16 at 16:57 Christian Wimber 1122 bronze badges In case a less abstract answer will help: A base, like $$\ce{NH_3}$$, is a base because it has a significant chance of picking up protons in water. You can almost think of it as a competition between the $$\ce{NH_3}$$'s and the $$\ce{OH^-}$$$$\ce{H_2O}$$'s to pick up the free protons. It is a weak base because it is not a certainty that all the $$\ce{NH_3}$$'s in a given sample will pick up a proton and hold onto it. In any given sample at any point in time, a certain portion of the $$\ce{NH_4^+}$$'s ($$\ce{NH_3}$$'s that won the proton) will give their protons up and a certain number of $$\ce{NH_3}$$'s will pick up new protons. Eventually, the system reaches a steady state (quantifiable with $$K_b$$). Our definition of an "acid" is just something that donates protons, as the $$\ce{NH_4^+}$$ does above. It's not that they're really different things: the $$\ce{NH_3}$$'s a base when its picking up protons and its an acid when it lets them go. In case a less abstract answer will help: A base, like $$\ce{NH_3}$$, is a base because it has a significant chance of picking up protons in water. You can almost think of it as a competition between the $$\ce{NH_3}$$'s and the $$\ce{OH^-}$$'s to pick up the free protons. It is a weak base because it is not a certainty that all the $$\ce{NH_3}$$'s in a given sample will pick up a proton and hold onto it. In any given sample at any point in time, a certain portion of the $$\ce{NH_4^+}$$'s ($$\ce{NH_3}$$'s that won the proton) will give their protons up and a certain number of $$\ce{NH_3}$$'s will pick up new protons. Eventually, the system reaches a steady state (quantifiable with $$K_b$$). Our definition of an "acid" is just something that donates protons, as the $$\ce{NH_4^+}$$ does above. It's not that they're really different things: the $$\ce{NH_3}$$'s a base when its picking up protons and its an acid when it lets them go. In case a less abstract answer will help: A base, like $$\ce{NH_3}$$, is a base because it has a significant chance of picking up protons in water. You can almost think of it as a competition between the $$\ce{NH_3}$$'s and the $$\ce{H_2O}$$'s to pick up the free protons. It is a weak base because it is not a certainty that all the $$\ce{NH_3}$$'s in a given sample will pick up a proton and hold onto it. In any given sample at any point in time, a certain portion of the $$\ce{NH_4^+}$$'s ($$\ce{NH_3}$$'s that won the proton) will give their protons up and a certain number of $$\ce{NH_3}$$'s will pick up new protons. Eventually, the system reaches a steady state (quantifiable with $$K_b$$). Our definition of an "acid" is just something that donates protons, as the $$\ce{NH_4^+}$$ does above. It's not that they're really different things: the $$\ce{NH_3}$$'s a base when its picking up protons and its an acid when it lets them go. 1 answered Dec 29 '16 at 16:03 Christian Wimber 1122 bronze badges In case a less abstract answer will help: A base, like $$\ce{NH_3}$$, is a base because it has a significant chance of picking up protons in water. You can almost think of it as a competition between the $$\ce{NH_3}$$'s and the $$\ce{OH^-}$$'s to pick up the free protons. It is a weak base because it is not a certainty that all the $$\ce{NH_3}$$'s in a given sample will pick up a proton and hold onto it. In any given sample at any point in time, a certain portion of the $$\ce{NH_4^+}$$'s ($$\ce{NH_3}$$'s that won the proton) will give their protons up and a certain number of $$\ce{NH_3}$$'s will pick up new protons. Eventually, the system reaches a steady state (quantifiable with $$K_b$$). Our definition of an "acid" is just something that donates protons, as the $$\ce{NH_4^+}$$ does above. It's not that they're really different things: the $$\ce{NH_3}$$'s a base when its picking up protons and its an acid when it lets them go.