Introduction to Acids

Ever wondered why lemon tastes so sour? Or why milk gets spoiled if you mix it with lime juice. It is because of the acids present in it. Acids and Bases are one of the important topics of chemistry, Let’s find out more about them.

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What are Acids?

Acids are a molecule or other species which can donate a proton or accept an electron pair in reactions. The word “acid” is derived from the Latin word “acidus” which means sour. All acid elements have a few things in common i.e all are sour in taste, they turn blue litmus paper to red, and lose their acidity if they’re combined with alkaline substances. The pH level of acids ranges from 0 – 6.

Some common examples of acids are Citrus fruits such as lemons, limes, oranges, grapefruit, etc. All these fruits contain citric acid. Hence, they taste sour or tart. Citric acid is a weak acid but still, it produces hydrogen ions when mixed with water and that’s why the pH of lemon juice is 2. Another example of an acid is vinegar. Vinegar consists of acetic acid. Ever wondered why your skin becomes red and swollen after an ant bite or a mosquito bite? Because these insects inject formic acid which causes such skin reactions. Nitric acid, sulfuric acid, hydrochloric acid, etc. are other common ones.

Properties of Acids

The properties of Acids are as follows;

• Acids change the color of blue litmus to red
• They change the color of Methyl Orange/Yellow to Pink
• Acidic substances converts Phenolphthalein from deep pink to colorless
• Are sour or tart in taste
• The pH level of acids range from 0-6
• Acids lose their acidity when combined with alkalines
• They destroy the chemical property of bases
• When reacting with metals they produce hydrogen gas
• Acids produce carbon dioxide when reacted with carbonates.
• Most acids are corrosive in nature which means that they tend to corrode or rust metals.

Learn more about the Properties of Bases here.

Download Introduction to Acids Cheat Sheet PDF

Classification of Acids

Acids are often classified on the basis of source, the presence of oxygen, strength, concentration and basicity.

Classification based on the source

This means that the acid is classified on the basis of their source or origin. They are mainly of two types: Organic acid and Mineral acid.

• Organic Acid: This is the acid obtained from organic materials such as plants and animals. For e.g. Citric acid (Citrus fruits), Acetic acid (Vinegar), Oleic acid (Olive oil), etc.
• Mineral Acid: Mineral acid is procured from minerals. They are also known as inorganic acids. They do not contain carbon. For e.g.  H2SO4, HCl. HNO3, etc.

Classification based on the presence of Oxygen

This means that the acids are classified on the basis of the presence of oxygen. These are of two types: Oxy-acid and Hydracids.

• Oxy-acid: Acids that consist oxygen in their composition is known as Oxy-acids. For e.g. H2SO4, HNO3, etc.
• Hydracid: Those that consist hydrogen combined with other elements and do not contain any oxygen in their composition and do not contain any oxygen in their composition are called Hydracids. For e.g. HCl, HI, HBr, etc.

Classification based on the Strength of the acid

Acids produce hydrogen ions when mixed with H2O, the strength of an acid depends on its concentration of the hydrogen ions present in a solution. A greater number of hydrogen ions means greater strength of the acid whereas, lower number of hydrogen ions means that the acid is weak. They are classified as :

• Strong Acids: An acid which can be dissociated completely or almost completely in water is known as a strong acid. For e.g.  sulphuric acid, nitric acid, hydrochloric acid, etc

H+ + H2O <—-> H3O+

 HCl(aq)  <—-> H+(aq) + SO4-(aq)

 H2SO4(aq) <—> 2H+(aq) + SO4-(aq)

• Weak Acids: An acid which doesn’t dissociate completely or dissociates negligibly in water is known as a weak acid. For e.g. Those that which we usually consume on daily basis i.e. citric acid, acetic acid, etc

CH3COOH9(aq) <—–> CH3COO-(aq) + H+(aq)

HCOOH(aq) <—> HCOO-(aq) + H+(aq)

Classification based on its concentration

As we have studied above, the concentration of the acid depends on the number of hydrogen ions that it produces in water. Based on this the acid is classified as :

• Concentrated Acid: When an aqueous solution has a relatively high percentage of acid dissolved in it, then it is a concentrated acid. For e.g. concentrated hydrochloric acid, concentrated sulphuric acid, concentrated nitric acid, etc,
• Diluted Acid: When an aqueous solution has a relatively low percentage of acid dissolved in it, then it is a dilute acid. For e.g. dilute hydrochloric acid, dilute sulphuric acid, dilute nitric acid, etc.

Classification based on the basicity of the acid

Acid on dissociation in water produces hydrogen ion. The number of these hydrogen ions that can be replaced in an acid is the basicity of an acid.

• Monobasic Acid: A monobasic acid is an acid which has only one hydrogen ion. Therefore, these acids combine with one hydroxyl group of the base to form salt and water.  For e.g. HCl, HCOOH, HBr, etc

• Dibasic Acid: Dibasic acid is that which shares twp hydroxyl groups it is known as dibasic acid. Dibasic acid dissociates in 2 steps. They can provide 2 kinds of salts i.e. the normal salt and a hydrogen salt

H2SO4(aq) <—> H+(aq) + HSO-4(aq)

2NaOH(aq) + H2SO4(aq) <—-> Na2SO4(aq) + 2H2O(l)

• Tribasic Acid: Tribasic acids are those which can combine with three hydroxyl groups. They have three replaceable hydrogen ions, and they produce 3 types of salts. For e.g. H3PO4

NaOH(aq) + H3PO4(aq) <—> NaH2PO4(aq) + H2O(l)

2NaOH(aq) + H3PO4(aq) <—> Na2HPO4(aq) + 2H2O(l)

Learn more about the concept of Neutralization here.

Solved Example for You

Q: The characteristic properties of an acid is due to the presence of ___________

1. hydride ions
2. hydroxly ions
3. hydronium ions
4. oxide ions

Sol: The correct answer is option ‘c’. The acid in aqueous solution gives H+ ion which combines with water to form hydronium ions (H3O+). Hence, acid is characterised by the presence of hydronium ions.