The diagram below was constructed to make four important points about acids and bases and their relative strengths.
 

1. Aqueous solutions of both weak and strong acids have pH lower than water (due to H₃O⁺ produced on reaction of the acid with water).
    
2. Aqueous solutions of both weak and strong bases have pH higher than water (due to H₃O⁺ in water being consumed by OH^– produced on reaction of the base with water).
    

   	aqueous solution no effect on litmus [H3O+] = [OH–] pH same as water		aqueous solution acid turns blue litmus red ([H3O+] > [OH–]) has a lower pH than H2O pH = –log[H3O+]
   acid	CH3OH not acidic	H2O	NH4+ weak acid	weak acid	H3O+	HCl strong acid
   conjugate base	CH3O– strong base	OH–	NH3 weak base	weak base	H2O	Cl– not basic
   	aqueous solution base turns red litmus blue ([H3O+] < [OH–]) has a higher pH than H2O				aqueous solution no effect on litmus [H3O+] = [OH–] pH same as water

    
3. If one member of an acid-base conjugate pair is strong, the other member does not react with water to change its pH (see dark blue and dark pink sections).
    
4. Both members of weak acid-base conjugate pairs react with water to change its pH (see the lighter blue and lighter pink regions of the diagram.