Three principal theories about acid-base are,1. Arrhenius theory2. Bronsted-Lowry theory3. Lewis theoryAccording to the Arrhenius, acids give up hydrogen ion in aqueous medium and a base give up hydroxyl ion in its aqueous medium. Examples are aqueous solution of HCl, CH3COOH, PhCOOH, and H2SO4. These give up H+ in an aqueous medium. Aqueous solutions of NaOH, Ca (OH)2 are Arrhenius bases.Arrhenius theory has some limitations. This theory cannot explain the acidic nature of BF3 or basic behavior of NH3. This theory FAILS to give any explanation about acidity and basicity which does not produce H+ and OH- in their aqueous solutions.According to the Bronsted-Lowry, an acid releases proton and a base accepts proton. NH3 accepts proton and forms ammonium ion, so ammonia acts as a base.But this theory also has limitations. According to this theory acidic behavior of BF3 STILL cannot be explained.According to the Lewis theory, acids are those substances which accept electron pair and base are those substances which DONATES electron pairs. Lewis theory can explain the acidic nature of BF3. In BF3, B is TWO electrons short to fulfil its octet. So it can accept a pair of electrons from an electron donating substituent. So it is an electron pair acceptor compound and acts as an acid.Substances which have π electrons can donate electron pair to an electron deficient substance and acts as a base. Ethylene and acetylene is Lewis bases. AlCl3, PCl3 are Lewis acids.Hope you like the answer.................