Introduction to Acids
Ever wondered why milk gets spoiled if you mix it with lime juice? Or why does a lemon tastes so sour. This happens because of the acids present in it. Acids and Bases are one of the essential topics of chemistry.
What are Acids?
An acid is a chemical substance that donates hydrogen ions or protons and/or accepts electrons. Most of the acids contain a hydrogen atom bonded which can release or dissociate to yield an anion and a cation in water. Higher the concentration of hydrogen ions produced by an acid, higher its acidity and lower the pH of the solution.
The word acid comes from a Latin word acidus or acere, which means “sour,”.
Common examples of acids are fruits like limes, lemons, grapefruit, oranges, etc. These fruits contain citric acid and due to which they taste tart or sour. Citric acid is a weak acid, but when mixed with water, it still produces hydrogen ions, that’s why the pH value of lemon juice is 2. Vinegar is another example of an acid. Vinegar consists of acetic acid. Have you ever wondered why your skin becomes swollen and red after an ant or a mosquito bite? This happens because these insects inject formic acid which causes such skin reactions. Sulfuric acid, nitric acid, hydrochloric acid, etc. are other common ones.
Arrhenius, Brønsted-Lowry, and Lewis Acids
A Lewis acid is a compound which can accept an electron pair to form a covalent bond. Some compounds which don’t contain hydrogen qualify as acids, including aluminium trichloride and boron trifluoride.
An acid is a material capable of acting as a proton donor. This is a less restrictive definition because solvents besides water are not excluded. Essentially, any compound that can be deprotonated is a Brønsted-Lowry acid, including typical acids, plus amines, and alcohol. This is the most widely used definition of an acid.
According to Arrhenius, an acid is a substance which increases the concentration of hydronium ions (H3O+) when added to water. You may also consider increasing the concentration of hydrogen ion (H+), as an alternative.
These are examples of types of acids and specific acids:
- Arrhenius acid
- Monoprotic acid
- Lewis acid
- Hydrochloric acid
- Sulfuric acid
- Hydrofluoric acid
- Acetic acid
- Stomach acid (which contains hydrochloric acid)
- Vinegar (which contains acetic acid)
- Citric acid (found in citrus fruits)
Classification of Acids
Acids are often classified based on source, the presence of Oxygen, strength, concentration and basicity.
Classification based on the source
This means that the acid is classified based on their source or origin. They are mainly of two types: Mineral acid and Organic acid.
- Mineral Acid: Mineral acid is procured from minerals. They are also known as inorganic acids. They do not contain carbon. E.g. H2SO4, HCl. HNO3, etc.
- Organic Acid: This is the acid obtained from organic materials such as plants and animals. E.g. Citric acid (Citrus fruits), Acetic acid (Vinegar), Oleic acid (Olive oil), etc.
Classification based on the presence of Oxygen
This means that the acids are classified based on the presence of Oxygen. These are of two types: Hydracids and Oxy-acids
- Hydracid: Those that include 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. E.g. HCl, HI, HBr, etc.
- Oxy-acid: Acids that consist of Oxygen in their composition is known as Oxy-acids. E.g. H2SO4, HNO3, etc.
Classification based on the strength of the acids
Acids produce hydrogen ions when mixed with water (H2O); the strength of an acid depends on the concentration of the hydrogen ions present in a solution. the lower number of hydrogen ions means that the acid is weak whereas, a greater number of hydrogen ions mean a greater strength of the acid. They are classified as :
- Weak Acids: An acid which doesn’t dissociate completely or dissociates negligibly in water is known as a weak acid. E.g. Those that which we usually consume on a daily basis, i.e. citric acid, acetic acid, etc
- Strong Acids: An acid which can be dissociated completely or almost completely in water is known as a strong acid. E.g. sulphuric acid, nitric acid, hydrochloric acid, etc
Classification based on its concentration
As we have studied that the concentration of the acid depends on the number of hydrogen ions that an acid produces in water. Based on this, the acid can be classified as :
- Diluted Acid: When an aqueous solution has a relatively low percentage of acid dissolved in it, then it is a dilute acid. E.g. dilute hydrochloric acid, dilute sulphuric acid, dilute nitric acid, etc.
- Concentrated Acid: When an aqueous solution has a relatively high percentage of acid dissolved in it, then it is a concentrated acid. E.g. concentrated hydrochloric acid, concentrated sulphuric acid, concentrated 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.
- 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.
- 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 two kinds of salts, i.e. the normal salt and a hydrogen salt
- 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.
Properties of Acids
- The properties of Acids are as follows;
- Acids change the colour of blue litmus to red
- They change the colour of Methyl Orange/Yellow to Pink
- Acidic substances convert Phenolphthalein from deep pink to colourless
- Are sour or tart in taste
- Acids produce carbon dioxide when reacted with carbonates.
- When reacting with metals, acids produce hydrogen gas
- They destroy the chemical property of bases
- The pH level of acids ranges from 0-6
- Acids lose their acidity when combined with alkalines
- Most acids are corrosive, which means that they tend to corrode or rust metals.