In our daily life, we use many compounds that scientists call acids. The orange or grapefruit juice you drink for breakfast contains citric acid (also known as vitamin C). When milk goes sour, it contains lactic acid. The vinegar used in salad dressing contains acetic acid.
What is acid?
An acid is any substance that tastes acidic in a water solution, turns blue litmus paper red, reacts with some metals to liberate hydrogen, reacts with bases to form salts, and promotes chemical reactions (acid catalysis).
Or a chemical that gives off hydrogen ions in water and forms salts by combining with certain metals. Acids have a sour taste and turn certain dyes red. Some acids produced by the body, such as gastric acid, can help the organs function properly.
An example of an acid is hydrochloric acid. Acidity is measured using a scale called the pH scale. On this scale, a value of 7 is neutral and a pH of less than 7 to 0 indicates increasing acidity.
Other examples of acids include the inorganic substances known as mineral acids— sulfuric, nitric, hydrochloric, and phosphoric acids—as well as the organic compounds belonging to the carboxylic acid, sulfonic acid, and phenol groups. Such substances contain one or more hydrogen atoms which are released in solution as positively charged hydrogen ions.
The Brønsted-Lowry theory and Lewis’s theory give broader definitions of acid, encompassing substances that typically show acidic behavior as pure compounds or dissolved in solvents other than water. Examples of non-aqueous acids are sulfur trioxide, aluminum chloride, and boron trifluoride.
Properties of acids
Due to the properties of their aqueous solutions, acids are an independent class of compounds. The properties of acid are as follows:
- Aqueous solutions of acids are electrolytes, which means they conduct electricity. Some acids are strong electrolytes because they ionize completely in water, yielding a great many ions. Other acids are weak electrolytes that, when dissolved in water, exist mainly in the non-ionized form.
- Acids are sour in taste. Lemons, vinegar, and sour candies all contain acids.
- Acids change the color of certain acid-base indicators. Two common indicators are litmus and phenolphthalein. Blue litmus turns red in the presence of an acid, while phenolphthalein turns colorless.
- Acids react with active metals to form hydrogen gas. Remember that an activity series is a list of metals in descending order of their reactivity. Metals that are above hydrogen in the activity series replace the hydrogen of an acid in a single exchange reaction, as shown below: Zn(s)+H2SO4(aq)→ZnSO4(aq)+H2(g)
- Acids react with bases to form a salt compound and water. When equal molar amounts of an acid and a base are combined, the acid is neutralized by the base. The products of this reaction are an ionic compound called salt and water.
- The acid reacts with carbonates and liberates carbon dioxide gas.
Examples of Acids
Following are 7 most common examples of acids:
- Hydrochloric acid (HCl)
- Sulphuric acid (H2SO4)
- Nitric acid (HNO3)
- Acetic acid (CH3COOH)
- Carbonic acid (H2CO3)
- Citric acid (C6H8O7)
- Acetylsalicylic acid (C6H4(OCOCH3) CO2H)
1. Hydrochloric acid (HCl)
Hydrochloric acid, also known as muriatic acid or spirits of salt, is an aqueous solution of hydrogen chloride with the chemical formula HCl. It is a colorless solution with a characteristic pungent odor.
Hydrochloric acid is classified as a strong acid. It is a component of gastric acid in the digestive system of most animal species, including humans. Hydrochloric acid is an important laboratory reagent and an industrial chemical.
Hydrochloric acid (HCl) is widely used to neutralize alkaline substances and as a bleaching agent in the food, textile, metal, and rubber industries. It is neutralized when it enters the soil and quickly hydrolyzes when exposed to water.
Hydrochloric acid is usually prepared by treating HCl with water.
HCl(g)+H2O(l)⟶H3O+(aq)+Cl−(aq)
Hydrochloric acid is the preferred acid for determining the amount of base in titration.
2. Sulphuric acid (H2SO4)
Sulfuric acid (H2SO4) is a strong acid with hygroscopic and oxidizing properties. Sulfuric acid is a mineral acid with the chemical formula H2SO4.
Sulfuric acid is also known as mattling acid or oil of vitriol. It has a strongly acidic nature and is corrosive. In higher concentrations, it acts as an oxidizing agent and dehydrating agent. It is a syrupy liquid that is odorless and colorless. It is water soluble and gives off heat when dissolved in water.
The main use of sulfuric acid is in fertilizer processing, e.g. lime superphosphate and ammonium sulfate. It is widely used in chemical processing, e.g. in the production of hydrochloric acid, nitric acid, sulfate salts, synthetic detergents, dyes and pigments, explosives, and medicines.
Concentrated sulfuric acid has very strong dehydrating properties, removing water (H2O) from other compounds, including sugars and other carbohydrates, and producing carbon, heat, and steam. Sulfuric acid reacts like a typical acid with most metals, producing hydrogen gas.
3. Nitric acid (HNO3)
Nitric acid is an inorganic compound with the formula HNO3. It is a highly corrosive mineral acid. The compound is colorless, but samples tend to turn yellowish over time due to the decomposition of nitrogen oxides.
Most commercially available nitric acid has a concentration of 68% in water. If the solution contains more than 86% HNO3, it is called fuming nitric acid.
Depending on the amount of nitrogen dioxide present, fuming nitric acid is further characterized as red-fuming nitric acid at concentrations above 86% and white-fuming nitric acid at concentrations above 95%.
Nitric acid is used to make ammonium nitrate, a key ingredient in fertilizers. It is also used to make explosives such as nitroglycerin and trinitrotoluene (TNT) and to oxidize metals.
4. Acetic acid (CH3COOH)
Acetic acid is also known as ethanoic acid, ethylic acid, vinegar acid, and methane carboxylic acid. Acetic acid is a byproduct of fermentation and gives vinegar its distinctive smell. Vinegar is about 4-6% acetic acid in water.
More concentrated solutions are found in laboratory use, and pure acetic acid containing only traces of water is called glacial acetic acid. Dilute solutions such as vinegar can encounter the skin without harm, but more concentrated solutions will cause skin burns. Glacial acetic acid can cause skin burns and permanent eye damage, and it can rust metal.
Acetic acid is used in the manufacture of acetic anhydride, vinyl acetate monomer, cellulose acetate, chloroacetic acid, acetic acid esters, plastics, dyes, photographic chemicals, insecticides, and rubber.
Other commercial uses include the manufacture of vitamins, hormones, antibiotics, and organic chemicals, and as a food additive. It is also used in various textile printing processes.
5. Carbonic acid (H2CO3)
Carbonic acid is an organic compound with the chemical formula H2CO3. The molecule rapidly converts to water and carbon dioxide in the presence of water; However, contrary to popular belief, it is quite stable in the absence of water at room temperature.
The mutual conversion of carbon dioxide and carbonic acid is related to the respiratory cycle of animals and the acidity of natural waters.
Carbonic acid is commonly used in the production of soft drinks, artificially carbonated sparkling wines, and other carbonated beverages. Carbonic acid salts are called bicarbonates (or carbonates of hydrogen) and carbonates.
6. Citric acid (C6H8O7)
Citric acid is an organic compound with the chemical formula (C6H8O7). It is a colorless weak organic acid. It occurs naturally in citrus fruits. In biochemistry, it is an intermediate in the citric acid cycle found in the metabolism of all aerobic organisms.
More than two million tons of citric acid are produced annually. It is widely used as an acidifier, flavoring agent, and chelating agent.
Citric acid was first extracted from lemon juice by a Swedish researcher in 1784.
The odorless and colorless compound was made from lemon juice until researchers discovered in the early 20th century that it can also be made from the black mold Aspergillus niger, which produces citric acid when it feeds on sugar.
Because of its acidic sour taste, citric acid is primarily used as an acidifier, flavoring agent, and preservative, especially in soft drinks and sweets. It is also used to stabilize or preserve medicines and as a disinfectant.
7. Acetylsalicylic acid (C6H4(OCOCH3) CO2H)
Acetylsalicylic acid (also known as aspirin) is a medication used to treat pain, fever, and inflammation. Aspirin, in the form of willow leaves, has been used for its health benefits for at least 2,400 years. Aspirin, an acetyl derivative of salicylic acid, is a white, crystalline, weakly acidic substance.
Aspirin is also used long-term to prevent further heart attacks, ischemic strokes, and blood clots in people at high risk. In the case of pain or fever, the effect typically sets in within 30 minutes. Aspirin works in a similar way to other NSAIDs, but it also suppresses normal platelet function.
Examples of Binary Acids
A binary connection consists of two elements. Binary acids have the prefix hydro before the full name of the nonmetallic element. They have the suffix -ic. Examples include hydrochloric acid, and hydrofluoric acid includes:
- Hydrofluoric Acid – HF (Hydrogen fluoride)
- Hydrochloric Acid – HCl
- Hydrobromic Acid – HBr
- Hydroiodic Acid – HI
- Hydrosulfuric Acid – H2S (Hydrogen sulfide)
Examples of Ternary Acids
Ternary acids usually contain hydrogen, a nonmetal, and oxygen. The name of the most common form of the acid consists of the nonmetallic root name with the suffix -ic. The acid containing one less oxygen atom than the most common form is denoted by the suffix -ous.
An acid that contains one less oxygen atom than the -ous acid has the prefix hypo- and the ending -ous. The acid containing one more oxygen than the most common acid has the prefix per- and the suffix -ic.
- Nitric Acid – HNO3
- Nitrous Acid – HNO2
- Hypochlorous Acid – HClO
- Chlorous Acid – HClO2
- Chloric Acid – HClO3
- Perchloric Acid – HClO4
- Sulfuric Acid – H2SO4
- Sulfurous Acid – H2SO3
- Phosphoric Acid – H3PO4
- Phosphorous Acid – H3PO3
- Carbonic Acid – H2CO3
- Acetic Acid – HC2H3O2
- Oxalic Acid – H2C2O4
- Boric Acid – H3BO3
- Silicic Acid – H2SiO3
Strong and Weak Acids
Acids can be identified as either strong or weak depending on how completely they dissociate into their ions in water. A strong acid such as hydrochloric acid completely breaks down into its ions in water. A weak acid only partially dissociates into its ions, so the solution contains water, ions, and the acid (e.g., acetic acid).