Definition of Acids and Bases and their Properties

How do you identify an acid and a base?  You may say that acid has sour taste. Yes your are right!  There are several examples of acids that you are familiar, like for example ascorbic acid (vitamin c), citric acid, acetic acid in vinegar and lactic acid in milk.  But take note not all acids can be tasted.  You need to use acid-base indicators to identify if a substance is an acid or base. Base, on the other hand is said to be bitter in taste and slippery to touch like detergents.  Below are definitions of acids and bases from different chemists and some of the properties of acid and bases that you can use to identify the difference between the two.

Definition of Acids and Bases

Arrhenius Definition of Acids and Bases

Swedish chemist Svante Arrhenius define acids as substances that produce H+ ions in aqueous solution while bases are substances that produces OH- ions in aqueous solutions.

Let us look at the example below:

The equation above hydrochloric acid when dissolved in water is ionized to form H+ ions and Cl- ions.

The above equation shows that NaOH when dissolved in water is ionized to produce Na+ ions and OH- ions.

Its the H+ ions that makes a substance an acid while  OH- ions make the substance a base. 

Bronsted-Lowry Acids and Bases Definition

Danish chemist Johannes Bronsted  and English chemist Thomas Martin Lowry , define acids as substances that donate protons (hydrogen ions) while bases are substances that accept protons (hydrogen ions).  Their definition is quite broad compared to Arrhenius definition, Arrhenius acids and bases are only limited to aqueous solution, whereas in Bronsted-Lowry definition of acids and bases includes substances capable of donating proton while acids includes any substances capable of accepting proton.

The above equation shows the ionization of hydrochloric acid in water.  But this is not only what happens in the solution the H+ ions or the proton is being attracted to the oxygen of water molecule forming a hydrated proton H3O+, called hydronium ion. Therefore, the ionization of hydrochloric acid in water look like this:

The above equation shows that HCl is the Bronsted-Lowry acid, since it donated proton or H+ ion to H2O, while H2O is the Bronsted-Lowry base since it accepted protons from HCl.  

Let us have an additional example.  Identify if Bronsted-Lowry acid or base:

Given a) HBr and b) NO2-.  

a) HBr is an example of Bronsted-Lowry acid since its capable of donating proton in once dissolve in water.

b) NO2- is an example of Bronsted-Lowry base since it has capability to accepts H+ ion or proton to form HNO2.

General Properties of Acids and Bases

Acids

• Acids have sour taste.
• Acids change blue litmus paper to red.
• Acids react with other metals (higher in position in the activity series) to produce hydrogen gas.
• Acids also react with carbonates and bicarbonates (like NaCO3, NaHCO3)  to produce carbon dioxide gas.
• Aqueous acids are electrical conductor.

Bases

• Bases have bitter taste.
• Bases are slippery to touch.
• Bases change red litmus paper to blue.
• Bases are electrical conductor