What is the main difference between an acid and a base?

Acid and Base are two very common terms in Chemistry and are used to differentiate between two large sets of compounds depending on their chemical behavior. In general, chemical compounds can be categorized into three main types based on dissolution; namely, Acids, Bases and Salts. Acids are chemical compounds that act as proton donors and can be easily recognized due to the possession of a releasable H+ ion. Bases are chemical compounds that act as proton acceptors and can be easily recognized in most cases, (inorganic bases) due to its possession of a releasable OH– ion. The main difference between acid and base is that acids act as proton donors whereas base act as proton acceptors. Upon the reaction of acids and bases in solutions, the H+ ions are neutralized by OH– ions, and they form water molecules together. And their ionic counterparts get together to form salts.

Inhaltsverzeichnis Show

What is an Acid
Colour Changes in Litmus Paper
Formation of Salts
Litmus Test and Other Reactions
Differences in Applications for acids and bases
Types & Examples of Acids and Bases

What is an Acid

According to chemical theories, an acid can be defined in many ways. According to the Arrhenius definition, an acid is a substance that increase the concentration of H+ ions when dissolved in water. According to the Brønsted-Lowry definition, an acid is a substance that can act as a proton donor. Therefore, the easier and quicker the compound could get deprotonated, the stronger its acidity is. According to the Lewis definition of acids, they are substances that can accept a pair of electrons. In this case, all metal cations and any electron deficient molecule can act as an acid. The more readily an acid can accept a pair of electrons, the higher its Lewis acidity is.

In general, acids are found at a pH value of less than 7. Strong acids have lower pH values. Acids react with bases to form salts. They are able to turn blue litmus paper into red. They also have a sour taste. (However it is not advisable to taste strong acids). Some common examples of acids are; Hydrochloric acid (HCl), Sulphuric acid (H2SO4), Acetic acid (CH3COOH), etc.

Sulphuric acid on a piece of towel

Bases can also be defined from different points of view using the definitions above. Therefore, an Arrhenius base is a substance that increases the concentration of OH– ions when dissolved in water. A Brønsted-Lowry base is a substance that can act as a proton acceptor. Therefore, the more readily a compound accepts protons, the higher its Brønsted-Lowry basicity is. According to the Lewis definition, bases are substances that can accept a pair of electrons. And the easier it is for a base to accept a pair of electrons, the stronger its Lewis basicity is.

In general, bases are found at a pH value of higher than 7. Strong bases have high pH values. They react with acids to form salts. They are able to turn red litmus paper into blue. They also taste slightly soapy. (However it is not advisable to taste strong bases). Some common examples of bases are; Sodium Hydroxide (NaOH), Calcium Hydroxide (Ca(OH)2), NH3, etc.

Sample of sodium hydroxide pellets

Definition

Acids are proton donors/ electron pair acceptors/ that are able to increase the H+ ion concentration in an aqueous solution.

Bases are proton acceptors/ electron pair donors/ that are able to increase the OH– ion concentration in an aqueous solution.

pH Value

Acids have a pH value <7.

Bases have a pH value >7.

Colour Changes in Litmus Paper

Acids turn blue litmus paper into red.

Bases turn red litmus paper into blue.

Formation of Salts

Acids react with bases to form salts in an aqueous medium.

Bases react with acids to form salts in an aqueous medium.

Taste

Acids taste sour.

Bases taste soapy.

Image Courtesy:

“Sulphuric acid on a piece of towel” by Toxic Walker – Own work. (CC BY-SA 3.0) via Commons

“SodiumHydroxide” by Walkerma – Own work. (Public Domain) via Commons

Bases are the chemical opposite of acids. Acids are defined as compounds that donate a hydrogen ion (H+) to another compound (called a base). Traditionally, an acid (from the Latin acidus or acere meaning sour) was any chemical compound that, when dissolved in water, gives a solution with a hydrogen ion activity greater than in pure water, i.e. a pH less than 7.0. Correspondingly, a base was any compound that, when dissolved in water, gives a solution with a hydrogen ion activity lower than that of pure water, i.e. a pH higher than 7.0 at standard conditions.

A soluble base is also called an alkali. A reaction between an acid and a base is called neutralization and this neutralization results in production of water and a salt. Volatile liquids (acids) when mixed with specific substances turn into salts. These substances form a concrete base and hence the name base was derived. Acids in general are H+ donors and Bases are H+ acceptors.

Acid versus Base comparison chart

	Acid	Base
Definition	Arrhenius Definition: An acid is any chemical compound which when dissolved in water gives a solution with a hydrogen ion activity greater than in pure water. Bronstead Lowry Definition: An acid is an substance which donates a proton.	Arrhenius Definition: A base is an aqueous substance that can accept hydrogen ions. Bronstead Lowry Definition: A base is any substance which accepts a proton.
pH (measure of concentration of hydrogen ions in a solution)	Less than 7.0.	Greater than 7.0 and could go up to 14 in case of stronger bases.
Physical characteristics	Depending on the temperature, acids can occur in solid, liquid or gaseous form. Taste sour.	Bases feel slippery because of the reaction of the base with the oils of your hand. Frequently solids except ammonia which is a gas. Taste bitter.
Strength	depends on concentration of the hydronium ions	depends on concentration of the hydroxide ions
Phenolphthalein	remains colorless	Makes it pink
Other Properties	Electrolytes, conduct electricity (because electrolytes), react with many metals.	Electrolytes, conduct electricity, ranges from insoluble to so soluble that they can react with water vapor.
Dissociation	Acids free hydrogen ions (H+) when mixed with water.	Bases free hydroxide ions (OH-) when mixed with water.
Chemical Formula	An acid has a chemical formula with H at the beginning of it. For example, HCl (Hydrochloric Acid). There is one exception to his rule, CH3COOH = Acetic Acid (vinegar)	A base has a chemical formula with OH at the end of it. For example, NaOH (Sodium Hydroxide).
Examples	Acetic acid i.e.CH3COOH and Sulfuric acid	Sodium Hydroxide (NAOH) and Ammonia (NH3)
Litmus test	Acids change litmus paper red.	Bases change litmus paper blue.

Bases have a slippery feel on fingers and taste bitter. They change litmus paper blue. Acids taste sour and create a stinging feeling on the mucous membranes. They change litmus paper red. They can react with bases to produce salts and water. They both conduct electricity depending on the dissociation of ions. Acids have a pH lesser than 7.0 and the lower it is, the stronger the acid becomes. Bases have a pH between 7 and 14. Higher the pH value, stronger will be the base. A pH level of 7 is a neutral substance which is water.

Litmus Test and Other Reactions

Litmus paper is made from dyes derived from lichens; it is water-soluble, meaning it can be fully dissolved in water. Acids turn blue litmus paper red, and bases turn red litmus paper blue. The following video shows how red and blue litmus paper reacts to ammonia, hydrochloric acid, water, and baking soda.

Strong acids have a corrosive effect on metals. They react with most of them to form hydrogen gas. Strong bases have a caustic effect on organic matter.

Differences in Applications for acids and bases

Acids are often used to remove rust from metals, as an electrolyte in batteries, for mineral processing, to produce fertilizers and gasoline and as additives in food and beverages. Bases are used primarily in cleaning as dishwashing and laundry detergents, oven cleaners and stain removers.

Types & Examples of Acids and Bases

Acids can be classified as Mineral acids, Sulfonic acids, Carboxylic acids, Vinylogous carboxylic acids and Nucleic acids. Some common acids include Hydrochloric acid (HCl), Sulfuric acid (H2SO4), Nitric Acid (HNO3), Acetic acid, Citric acid and Lactic acid amongst several others. Bases are of 2 types – a base and an alkali (a soluble base). Some common bases include Potassium Hydroxide (KOH), Sodium Hydroxide (NaOH) and Magnesium Hydroxide (Mg (OH)2).