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Acids and Bases are a group of chemical compounds with specific chemical properties. These are defined in terms of 2 theories :

• Arrhenius theory
• Lowry-Brønsted theory

Arrhenius theory

I call this the two gun theory.
The ACID shoots out an H⁺.
The BASE shoots out an OH^-.

Acid
An acid is a substance that produces hydrogen ions (H⁺);
also called hydronium ions (H₃O⁺) when it dissolves in water.

or more detailed :

Base
A base is a substance that produces hydroxide ions (OH^-) when it dissolves in water.

Lowry-Brønsted theory

I call this the one gun and target theory.
The acid is the gun.
The base is the target.
The acid (gun) shoots out an H⁺ at a base (target).

An acid is a proton (H⁺ ion) donor.
A base is a proton (H⁺ ion) acceptor.

The Lowry-Brønsted theory leads to what is called conjugate acid-base pairs.
When the acid donates an H⁺ to a base, the acid itself becomes its own conjugate base.
And the base, upon receiving the H⁺, becomes it's own conjugate acid.

Example 1
Identify the Lowry-Brønsted conjugate acid-base pairs.

The actual reacting compounds are HCl (acid) and NH₃ (base).
What the acid HCl becomes, is called it's own conjugate base Cl^-.
What the base NH₃ becomes, is called it's own conjugate acid NH₄⁺.

Solution

So there are TWO conjugate acid-base pairs: acid-base 1 and acid-base 2.

Example 2
Identify the Lowry-Brønsted conjugate acid-base pairs.

Solution

In the Lowry-Brønsted theory, there is no mention of water needing to be present.
Further, a base does not need to have an OH^- present. A compound just needs to be able to accept an H⁺ to be called a base.

Acid Base 1 : Theories

Arrhenius theory

Lowry-Brønsted theory