Acids

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;

• Arrhenius acid: As per Arrhenius theory, an acid is a substance that produces or increases the concentration of hydrogen ions (H⁺) in a solution after its dissociation.
• Bronsted-Lowry acid: According to Bronsted-Lowry theory, it is a substance that donates protons.
• Lewis acid: This Lewis theory says that a substance that can accept a pair of electrons is an acid.

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⁺ + SO₄^2-

CH3CO2H → H⁺ + CH₃CO₂^-

ii) Based on their occurrence

Based on the occurrence, they can be of two types; natural and minerals acids.

• Natural acids: They are found naturally such as in fruits and in the body, e.g., lactic acid, citric acid, tartaric acid are found in fruits and hydrochloric acid is present in the stomach.
• Mineral acids: These acids are prepared from minerals. For example, hydrochloric acid (HCl) and nitric acid (HNO₃), etc.

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;

• They have a sour taste in an aqueous medium.
• They are soluble in water.
• They can be harmful if tasted or consumed.
• They are good conductors of electricity, especially strong acids. For example, electrolytes are aqueous solutions of acids.
• Their pH value is less than 7 which ranges from 0 to 6.
• They change the blue colour of litmus to red.
• They produce hydrogen gas when they react with active metals.
• Acid when reacts with the base forms a neutral substance. In this reaction, they produce a salt compound and water. It occurs mostly when a strong acid reacts with a strong base.
• They are generally highly corrosive and thus can rust metals easily.
• They release hydrogen ions when added to water.
• Acids change the orange or yellow colour of Methyl Orange to pink.
• They make the pink coloured Phenolphthalein colourless.
• Some weak acids such as carbonic acid do not react with metals.
• The metals located above the hydrogen in the metal activity series form salt and hydrogen on reacting with dilute acids.
• They form sulphur dioxide along with salt and water when they react with sulphites and bisulphites.
• Their classification is based on their strength, sources, concentration, etc.
• Minerals acids are colourless liquids. However, sulphuric acid sometimes appears yellow due to impurities. Some of the organic acids may be white solids.

Uses of acids

• Vinegar (a solution of acetic acid) is used as a food preservative.
• Citric acid is found in lemon and orange juices. It is also used as a food preservative.
• Sulphuric acid is used in batteries. Such batteries are used in automobiles to start the engine.
• Sulphuric acid and nitric acid are widely used in industries in the production of dyes, paints, fertilizers, and more.
• Phosphoric acid is one of the main ingredients in many soft drinks.

Some common acids

Hydrochloric Acid

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.

Sulphuric Acid

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

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.

Carbonic Acid

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.

Formic Acid

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.

Citric Acid

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.

Acetylsalicylic Acid

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

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

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

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

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.