acid

Acids are molecules that 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 an alkaline substance. The pH level of acids ranges from 0-6.

Some common examples of acids are citrus fruits such as lemons, limes, oranges, grapefruits, 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. Do you know why your skin becomes red and swollen after an ant bite or a mosquito bite? It is because these insects inject formic acid which causes such skin reactions. Other common acids are nitric acid (HNO₃), sulfuric acid (H₂SO₄), hydrochloric acid (HCl), etc.

Scientists use something called a pH scale to measure how acidic or basic a liquid is. A pH is a number from 0 to 14.

• From 0 to 7 are acids, with 0 being the strongest.
• From 7 to 14 are bases with 14 being the strongest base.
• If a liquid has a pH of 7, it's neutral. This would be something like distilled water.

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 convert 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 alkaline
• 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

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

Organic acid – This is the acid obtained from organic materials such as plants and animals. For example, citric acid (citrus fruits), acetic acid (vinegar), oleic acid (olive oil), etc.

Mineral acid – This is the acid procured from minerals. They are also known as inorganic acids. They do not contain carbon. For example, H₂SO₄, HCl, HNO₃ etc.

Classification based on the presence of oxygen

Oxy-acid – Acids that contain oxygen in their composition is known as oxy-acids. For example, H₂SO₄, HNO_3, etc.

Hydra-acid – Those that contain 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 example, 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, a lower number of hydrogen ions means that the acid is weak.

Strong Acids: An acid that can be dissociated completely or almost completely in water is known as a strong acid. For example, sulfuric acid, nitric acid, hydrochloric acid, etc.

Weak Acids: An acid that doesn’t dissociate completely or dissociates negligibly in water is known as a weak acid. For example, those that are usually consumed on a daily basis like citric acid, acetic acid, etc.

Classification based on the concentration

The concentration of the acid depends on the number of hydrogen ions that it produces in water.

Concentrated acid – When an aqueous solution has a relatively high percentage of acid dissolves in it, then it is a concentrated acid. For example, concentrated hydrochloric acid, concentrated sulfuric 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 example, dilute hydrochloric acid, dilute sulfuric acid, dilute nitric acid, etc.

Classification based on the basicity of the acid

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

• Monobasic acid – A monobasic acid is an acid that has only one hydrogen ion. Therefore, these acids combine with one hydroxyl group of the base to form salt and water. For example, HCl, HCOOH, HBr, etc.
• Dibasic acid – A dibasic acid is that which shares two hydroxyl groups it is known as dibasic acid. Dibasic acid dissociates in two steps. They can provide two kinds of salts i.e. the normal salt and hydrogen salt.
• Tribasic acid – Tribasic acids are those which can combine with three hydroxyl groups. They have three replaceable hydrogen ions, and they produce three types of salts. For example, H₃PO₄