Differences Between Acid and Base

Substances that donate their hydrogen ion (H⁺) (proton donor) and accept an electron from another are called acids. They have a pH below 7.0. On the other hand, substances that accept a proton and donate an electron are called bases. They have a pH above 7.0. Acids are sour, while bases are bitter.

Acids and bases are among the most important aspects of chemistry, and they also play a crucial role in other scientific fields. Many definitions distinguish substances as acids or bases, but the most widely accepted are the Arrhenius theory, the Brønsted-Lowry theory, and the Lewis acid/base theory. Together, acids and bases react to form salts.

Acids and bases are found everywhere, from the soaps used during showers to the citric acid or vinegar present in the kitchen. While they can sometimes be difficult to distinguish, certain theories help clarify their differences. These are discussed below, along with a brief description.

Comparative Table of Acids and Bases

Basis of Comparison	Acids	Bases
Arrhenius Concept	Acid is a substance that increases the concentration of H⁺ ions when dissolved in water.	Base is a substance that increases the concentration of OH⁻ ions when dissolved in water.
Brønsted-Lowry Concept	Acids are proton donors.	Bases are proton acceptors.
Lewis Concept	Species that accept a pair of electrons (an electrophile) and have vacant orbitals are known as Lewis acids.	Species that donate a pair of electrons (a nucleophile) and have a lone pair of electrons are known as Lewis bases.
Chemical Formula	Compounds with a chemical formula starting with H, e.g., HCl (hydrochloric acid), H₃BO₃ (boric acid), H₂CO₃ (carbonic acid). CH₃COOH (acetic acid) is an exception.	Compounds with a chemical formula ending in OH, e.g., KOH (potassium hydroxide), NaOH (sodium hydroxide).
pH Scale	Less than 7.	Greater than 7.
Physical Characteristics	Sour taste, burning sensation, sticky, reacts with metals to produce hydrogen gas.	Bitter taste, usually odorless (except ammonia), slippery, reacts with fats and oils.
Phenolphthalein Indicator	Remains colorless.	Turns pink.
Litmus Test	Turns blue litmus paper red.	Turns red litmus paper blue.
Strength	Depends on the concentration of hydronium ions.	Depends on the concentration of hydroxide ions.
Dissociation in Water	Acids dissociate to give free hydrogen ions (H⁺) when mixed with water.	Bases dissociate to give free hydroxide ions (OH⁻) when mixed with water.
Examples	Hydrochloric acid (HCl), sulfuric acid (H₂SO₄), nitric acid (HNO₃), carbonic acid (H₂CO₃).	Ammonium hydroxide (NH₄OH), calcium hydroxide (Ca(OH)₂), sodium hydroxide (NaOH).
Uses	Used as preservatives, fertilizers, in carbonated beverages, leather processing, household cleaning, soda making, food flavoring, etc.	Used in gastric medicine (antacids), soaps, detergents, cleaners, deodorants, non-hazardous alkalis to neutralize acidic wastewater, neutralizing soil acidity.

Definition of Acid

The word “acid” comes from the Latin word ‘acidus’ or ‘acere,’ meaning “sour.” An acid is a chemical substance that accepts electrons and donates hydrogen ions or protons. Most acids contain atoms bonded by hydrogen bonds, which dissociate in water to form cations and anions.

Acidity is measured by the presence of hydrogen ions; the higher the concentration of hydrogen ions, the higher the acidity and the lower the pH of the solution. It is measured on a scale between 1 and 7 (7 is neutral) on the pH meter scale.

Some acids are strong, while others are weak. Strong acids completely dissociate in water. For example, hydrochloric acid dissociates entirely into ions when dissolved in water. Acids that partially dissociate in water, and therefore the solution contains water, acid, and ions, are known as weak acids, such as acetic acid.

Acids are mainly defined in various ways, but the Arrhenius or Brønsted-Lowry acid definition is generally accepted. Although the Lewis acid is known as “Lewis acid,” these definitions do not include the same set of molecules.

• Arrhenius Concept: A substance that increases the concentration of hydrogen ions (H⁺) when added to water is called an acid.
• Brønsted-Lowry Concept: In this case, an acid is a proton donor. This theory defines substances without dissolving them in water, making it widely used and accepted.
• Lewis Acid: Certain compounds that do not contain hydrogen atoms are still considered acids, such as boron trifluoride and aluminum trichloride. Thus, a compound that accepts a pair of electrons to form a covalent bond is known as a Lewis acid.

Properties of Acids

• Corrosive (‘burns’ your skin).
• Has a pH below 7.
• Turns blue litmus paper red.
• Reacts with metals to produce hydrogen gas.
• Reacts with bases to produce salt and water.
• Reacts with carbonates to form carbon dioxide, water, and salt.
• Sour taste.
• Dissociates hydrogen ions (H⁺) when dissolved in water.

Importance

Biologically, nucleic acids like DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) contain genetic information and other hereditary material passed from one generation to the next. Amino acids are also vital, as they help produce proteins. Fatty acids and their derivatives, which are carboxylic acids, also play an important role.

Even hydrochloric acid, part of the gastric acid secreted in the stomach of animals, helps hydrolyze proteins and polysaccharides. Acids are also useful in defense mechanisms, such as ants that produce formic acid, while octopuses produce a black acid called melanin.

Other acids, such as lactic acid, vinegar, sulfuric acid, and citric acid, found in nature, are known for their various important uses.

Definition of Base

Bases donate electrons and accept hydrogen ions or protons. Bases are chemical substances that are the exact opposite of acids. In water, a base reduces the concentration of the hydronium ion (H₃O⁺), whereas an acid increases the concentration. However, some strong acids also act as bases. Bases are measured between the range of 7-14 on the pH meter scale.

There is often confusion between bases and alkalis. Many bases do not dissolve in water, but if a base dissolves in water, it is called an alkali. When a base reacts with an acid in an aqueous solution and the solution becomes neutral, it is called a neutralization reaction.

For example, sodium hydroxide is both a base and an alkali because it neutralizes acids in any acid-base reaction and is soluble in water. On the other hand, copper oxide is a base but not an alkali because it neutralizes acid in an aqueous solution but does not dissolve in water.

A strong base is a chemical compound that deprotonates or removes a proton (H⁺) from a molecule of a very weak acid in an acid-base reaction. Alkali metal and alkaline earth metal hydroxides, such as sodium hydroxide and calcium hydroxide, are examples of strong bases. A weak base is a substance that does not ionize completely in an aqueous solution, or its protonation is incomplete.

• Arrhenius Concept: A substance that produces hydroxide ions (OH⁻) in an aqueous solution is called a base. For example, sodium hydroxide (NaOH) dissociates in water to give Na⁺ and OH⁻ ions. Substances like LiOH, Ba(OH)₂, and NaOH can be considered Arrhenius bases. But this theory was limited to substances containing hydroxide in their formula and was only applicable in aqueous solutions. Due to this limitation, another concept called the Brønsted-Lowry theory emerged.
• Brønsted-Lowry Concept: According to this theory, a substance that can accept hydrogen ions (H⁺) or protons is known as a base.
• Lewis Base: One of the most accepted concepts after the Brønsted-Lowry theory. An atom, molecule, or ion with a free pair of electrons can be considered a Lewis base since these bases are nucleophilic. This means that with the help of a lone pair, they attack the positive charge of the molecule. NH₃ is a Lewis base. In other words, a substance like the OH⁻ ion, which can donate a pair of non-bonding electrons, is called a Lewis base or electron-pair donor.

Properties of Bases

• Corrosive (‘burns’ your skin).
• Has a pH above 7.
• Turns red litmus paper blue.
• Bitter taste.
• Dissociates hydroxide ions (OH⁻) when dissolved in water.
• Reacts with acids to form salt and water.
• Slippery or soapy feel to the touch.

Importance

Bases have a bitter taste and are slippery to the touch. Alkalis are bases that can be dissolved in water. Some strong bases like sodium hydroxide, potassium hydroxide, and sodium hydroxide are highly corrosive to the skin and can burn you. Ammonium hydroxide (household ammonia) is used in cleaning products, while magnesium hydroxide is used as an antacid to treat indigestion. Sodium bicarbonate is also used as an antacid, and sodium carbonate is used to manufacture glass.