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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
source | link

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.