Acids are naturally found in nature. They are present in most of the food items. For example, citric acid and ascorbic acid (vitamin C) is commonly found in citrus fruits like oranges and lemons. Similarly, lactic acid is present in milk and malic acid in apples and tartaric acid in tamarind, and more. Besides this, carbonated beverages also contain phosphoric acid and vinegar contains acetic acid and in the human body, hydrochloric acid is present in the stomach that helps digest food.
Acids are also used for various domestic purposes. For example, in the kitchen, we use vinegar (acetic acid) and baking soda, we use boric acid for laundry and washing soda is used for cleaning. Acids are also used in industries and laboratories such as HCl and H2SO4.
What is an acid?
The word acid is derived from the Latin word acidus or acere which means 'sour'. Acids are ionic compounds or hydrogen-containing substances that produce protons (hydrogen ions) when they dissociate into ions in their aqueous solutions. They donate hydrogen ions and like to accept electrons. The higher the hydrogen ions produced by acid after dissociation, the higher is acidity, but lower is the pH of the solution.
Common definitions of acids:
Three theories have been introduced by three different scientists to define acids and bases. Let us see how they defined acids;
Types of acids
i) Based on dissociation
Based on their tendency to dissociate, an acid can be identified as a strong acid or weak acid. For example, an acid that dissociates or ionizes completely into ions is called a strong acid, e.g. hydrochloric acid. Whereas, an acid, which dissociates partially into ions in a solution is called a weak acid. For example, acetic acid. So, acids dissociate in their aqueous solution to form their constituent ions as shown below;
HCl (aq) → H+ + Cl-
H2SO4 → 2H+ + SO42-
CH3CO2H → H+ + CH3CO2-
ii) Based on their occurrence
Based on the occurrence, they can be of two types; natural and minerals acids.
How to identify acid in chemical reactions
The change in the number of hydrogen atoms in a chemical species or substance can tell you whether it is an acid or not. You can count the number of hydrogen atoms of a molecule before and after a chemical reaction. If it has less number of hydrogen atoms after the reaction, it is acid as an acid tends to donate hydrogen ions. For example, see the following reaction;
HCO3- + HOH → H2CO3 + OH-
Properties of Acids
Acids have lots of characteristic features or properties, some of which are listed below;
Uses of acids
Some common acids
HCl is a strong mineral acid as it ionizes or dissociates completely in water. Its chemical formula is HCl and is a highly corrosive acid. It has many industrial uses and is generally prepared by reacting HCl with water, which results in a highly pungent HCl solution in water as shown below;
HCl (g) + H2O (l) → H3O+ (aq) + Cl+ (aq)
So, HCl is needed to prepare chloride salts. It also has lots of uses such as it is used in the processing of steel, in the production of batteries, fireworks, and in the processing of sugar, to purify the table salt and to make gelatin. Besides this, it is widely used in titration to find the amount of bases.
It is also a strong mineral acid with high corrosive action. Its chemical or molecular formula is H2SO4. Sulphuric acid has a strong affinity for water. Its hydration is highly exothermic. So, it is used as a dehydrating agent. Especially concentrated sulphuric acid shows a strong dehydrating property. It removes water easily from compounds such as sugar and carbohydrates and produces heat, steam and carbon.
Besides this, when it reacts with most metals it generates hydrogen gas like a typical acid. It offers a wide range of uses such as for cleaning the domestic drain, as an electrolyte in batteries and also used in various cleaning agents. It is also a widely used chemical in chemical industries.
Nitric Acid (HNO3) is a strong corrosive acid. It is also a mineral acid that is generally used as a strong oxidizing agent and generally considered as a strong acid at an ambient temperature.
It can be formed by reacting water with nitrogen dioxide (NO2) as shown in the below chemical reaction;
3NO2 (g) + H2O (l) → 2HNO3 (aq) + NO (g)
Although nitric acid also reacts with metals, it depends on the concentration of the acid and the nature of the metal. Nitric acid in dilute form acts as a typical acid when it reacts with most of the metals. For example, it generates hydrogen gas when it reacts with magnesium, manganese or zinc. Some examples are shown below;
Mg + 2HNO3 → Mg (NO3)2 + H2
Zn + 2HNO3 → Zn (NO3)2 + H2
It is also a strong oxidizing agent. It may cause burns if comes in contact with the human body as it decomposes living tissue through acid hydrolysis with proteins and fats. It stains human skin yellow by reacting with the keratin. Besides this, it does not show acidic properties rather it exhibits oxidizing properties.
It is a weak acid with the chemical formula H2CO3. The solution of carbon dioxide in water (carbonated water) is also called carbonic acid as they contain a small amount of carbonic acid. For example, When carbon dioxide dissolved in water produces carbonic acid and chemical equilibrium is created as shown below;
CO2 + H2O ↔ H2CO3
The reaction can be controlled to favour the reactants to produce CO2 from the solution that causes the formation of bubbles in carbonated beverages. Carbonic acid forms two types of salts, which are carbonates and bicarbonates.
It is the simplest carboxylic acid, which is also called methanoic acid. Its chemical formula is HCO2H. It is found naturally, e.g. in some ants. It was first isolated from ants and accordingly it was named after the Latin word 'formica' which means ant.
Formic acid is a liquid, which tastes sour and has a pungent smell. When it comes in contact with skin, it gives a stringing sensation. It is used in the processing of textiles and leather.
It is a weak acid with the chemical formula (C6H8O7). It is a tricarboxylic acid, which is an edible acid and naturally found in citrus fruits. In lesser amounts, it is also found in other fruits like tomatoes, cherries, strawberries and pineapple.
It is called citric acid as it is mostly found naturally in citrus fruits. In its purest form, it has a tangy and tart flavour. Being edible, it is used in foods and beverages to add flavour and as a preservative. It is also used in cleaning agents and as food supplements.
It is also known as aspirin. Its molecular formula is C9H8O4. It is a white, crystalline and weak acid, which is found in the leaves of the willow tree.
It has anti-inflammatory and antipyretic properties, which makes it a widely used medication to treat fever, pain and inflammation. Possibly, it is the most commonly used analgesic and antipyretic throughout the world. Besides this, this acid is also used as a medication to prevent platelet aggregation and blood clots stroke.
Acetic acid, which is also known as ethanoic acid, is an important carboxylic acid. It is a monocarboxylic acid that contains two carbons and one carboxylic acid group and its chemical formula or molecular formula is CH3COOH. Vinegar contains 5 to 10 % acetic acid, which gives vinegar its sour taste and pungent smell.
Acetic acid is generally found in a liquid state. In its purest form, it tends to crystalize below room temperature. Acetic acid is a colourless organic liquid with a pungent smell like vinegar. It has antibacterial and antifungal properties.
Hydrofluoric acid is also known as hydrogen fluoride and hydrogen monofluoride. Its chemical formula is HF. When it is dissolved in water, it is known as hydrofluoric acid.
It is a weak acid as it does not dissociate completely into ions in a solution. Although it is a weak acid, it is highly corrosive. It can eat glass and metals, so it is stored in plastic containers. If it comes in contact with skin, it may pass through the skin to damage the bones. Hydrofluoric acid is needed to produce fluorine compounds including Prozac and Teflon.
Oxalic acid is a weak acid. It is also known as ethanedioic acid and hydrogen oxalate. Its chemical formula is H2C2O4 and belongs to the family of carboxylic acids.
Oxalic acid is crystalline, colourless organic compound. It was named oxalic acid as it was first isolated from sorrel (Oxalis sp.). It is mostly found in green foods like green leafy vegetables. It is also produced by some microorganisms and animals. It is used in metal cleaners, in anti-rust products and as a bleaching agent for wood, leather, etc.
Phosphoric acid is also known as orthophosphoric acid, trihydrogen phosphate, and acidum phosphoricum. Its chemical formula is H3PO4. In its pure form it exists as a crystalline solid, whereas, in dilute form, it is an odourless and colourless liquid. It is made from mineral phosphorus, which is first converted to phosphorus pentoxide then treated again to convert it into phosphoric acid.
It is a strong mineral acid that is widely used in home cleaning and rust-preventive products. It is also used as a dental etchant. Besides this, it makes soft drinks tangy and is also used to give fruit-like flavour to food products. It also helps prevent the growth of bacteria in sugary solutions.