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Hydrochloric Acid–Structure and Formula, Preparation, Properties, Uses, Practice Problems & FAQs

Hydrochloric Acid–Structure and Formula, Preparation, Properties, Uses, Practice Problems & FAQs

Just as you are reading this, some amount of hydrochloric acid is probably doing its eternal duty inside your belly! Because this is an acid that has vouched to give the process of digestion inside your stomach a kickstart! So next time when you have your favourite dessert or an extra cheesy pizza, rest assured that your stomach is all set to attack those delicacies with hydrochloric acid, once they reach the venue!

Yes, today we are about to learn the relationship that hydrogen and chlorine share with each other, which finally led to the formation of their combination into an independent compound called hydrogen chloride.


Interestingly this compound in its first state of formation is a gas and only when combined with water, this gas convert into an acid– “The Hydrochloric Acid”! Imagine how important this acid is to our lives! In fact, eating food and deriving energy out of it, itself would have been inhibited without the presence of this acid inside our stomach!

All thanks to the Almighty!! Hence it’s imminent that we learn about this particular compound called hydrochloric acid in order to understand its properties and ways of its functioning. 


  • Hydrochloric Acid– Introduction
  • Structure and Formula of Hydrogen Chloride
  • Preparation of Hydrochloric Acid
  • Physical Properties of Hydrochloric Acid
  • Chemical Properties of Hydrochloric Acid
  • Hydrochloric Acid in Stomach
  • Uses of Hydrochloric Acid
  • Practice Problems
  • Frequently Asked Questions

Hydrochloric Acid– Introduction

An aqueous solution of hydrogen chloride is hydrochloric acid, commonly referred to as muriatic acid. It has an unmistakably strong odour and is colourless. It is categorised as a powerful acid. In the digestive tracts of the majority of animal species, including humans, it is a part of the stomach acid. An essential laboratory reagent, concentrated hydrochloric acid is also a highly caustic and combustible industrial chemical.

The name hydrochloric acid is more frequently used by people of English heritage. However, languages like Spanish and German continue to use the phrase "acidum salis," which has historical connotations. The historical name's derivation dates back to the time when rock salt was used to make hydrochloric acid.

Hydrochloric acid is created when the gaseous hydrogen chloride is dissolved in water. It's a straightforward diatomic molecule. One covalent link holds the chlorine atom and hydrogen together. Since the chlorine atom is more electronegative than the hydrogen atom, the bond between hydrogen and chlorine is polar.

It has high acidity. It has no colour and is thick. It smells strongly unpleasant and is corrosive. It is widely utilised in both industries and as a reagent in laboratories. It is employed in the manufacturing of gelatin and the processing of leather. The molarity or concentration of HCl affects physical qualities such as density, melting point, pH, and boiling point.

Structure and Formula of Hydrogen Chloride

Hydrogen chloride is a polar compound formed by sharing an electron pair between hydrogen and chlorine. Because chlorine is more electronegative than hydrogen, HCl is a polar covalent molecule.


  • Therefore, hydrogen has a partial positive character, whereas chlorine attains a partial negative character. Given that the hydrogen and chlorine atoms share electrons, HCl exhibits covalent behaviour.
  • The electron pair prefers to move more toward the chlorine atom because it is more electronegative than hydrogen, making the bond polar. The molecule's dipole moment, which is equal to 1.03D, is also influenced by this polarity.
  • Even the covalent bond established has around 7 % ionic properties since the electronegativity difference between the two atoms is considerable but not high enough to form an ionic bond.
  • Hydrogen chloride's atoms are bonded at an angle of 180°.
  • When hydrogen chloride is dissolved in water, there will be strong dipole-dipole interaction between hydrogen chloride and water and hence is very soluble in water. Hydrogen chloride when solubilised in water it forms hydrochloric acid as now it can furnish hydronium ions.

HCl + H2O → [H3O]+ +Cl-

Preparation of Hydrochloric Acid

  • When hydrogen chloride gas is passed through the water at normal temperature, a liquid solution known as hydrochloric acid results. The substance's ability to release protons into the water makes it a potent acid. The acidity of acid increases with the amount of protons released.
  • Hydrogen and chlorine are used to make hydrochloric acid in industrial settings. These distinct elements are created as a by-product of the Chlor-alkali procedure. The Chlor-alkali process produces sodium hydroxide, including chlorine and hydrogen. This is done by electrolysis of a sodium chloride solution [NaCl (aq)] . 

At the cathode, where water is converted to hydrogen gas as well as hydroxide ions. 

At cathode: 

2H2O (l)+2e-H2 (g)+2OH-(aq)

At anode:

2Cl-(aq) Cl2(g)+2e-

Hence in the Chlor-alkali process, during electrolysis:

Hydrogen gas is liberated at the cathode and chlorine gas is liberated at the anode. Sodium ions ,Na+(aq), from the dissolved sodium chloride and hydroxide ions, OH-(aq), from the water stay back and combine to produce sodium hydroxide solution, NaOH (aq). Further hydrogen and chlorine gas reacts to form hydrogen chloride gas, which then when dissolved in water at different concentrations forms the hydrochloric acid. 

  • Different concentrations of hydrochloric acid are offered for sale. However, the percentage of the acid utilised in factories and laboratories ranges from 28-36 % w/w.
  • Heating sodium chloride with concentrated H2SO4 produces hydrogen chloride on a commercial and laboratory scale. By passing the gas through concentrated sulfuric acid, the gas can be dried.

  • Gas can be produced at temperatures below and over 200° C when hydrogen chloride is prepared in a laboratory from sodium chloride. The lower temperature is desirable, though, as a higher temperature can result in a hard crust that will stick to the bottom of the flask and be challenging to remove.
  • Another method is the direct combination of hydrogen and chlorine to produce hydrogen chloride gas. The produced gas is dissolved in deionized water, to form pure hydrochloric acid.

Cl2 (g) + H2 (g) → 2HCl (g)

Physical Properties of Hydrochloric Acid

  • Hydrogen chloride gas is pungent and colourless. Hydrochloric acid is a transparent liquid.
  • When water and hydrogen chloride gas are combined, hydrochloric acid is created. Hydrochloric acid is a potent monoprotic acid because it easily releases one hydrogen ion per molecule into the aqueous solution.
  • Hydrochloric acid has a molecular weight of 36.46 g mol-1.
  • At room temperature, hydrogen chloride is a translucent to pale yellow liquid, and at concentrations of 25%, hydrogen chloride vapours can be seen.
  • The concentration of the acid (molarity) affects several of its physical properties, including the boiling point, melting point, specific heat, density, and vapour pressure of hydrochloric acid.
  • Strong acid hydrochloric acid has a pH that varies between 0.5 and 1.1 pH units depending on the concentration.
  • Hydrogen chloride is exceedingly inert and entirely dry as a gas. On solubilising in water, it forms hydrochloric acid

Chemical Properties of Hydrochloric Acid

  • The chlorides can be generated by the active metals and their oxides, hydroxides, and carbonates when it reacts with the hydrochloric acid.

Na2O (s)+ 2HCl (aq) 2NaCl (aq) + H2O (l)

K2CO3(aq) + 2HCl (aq) → 2KCl (s)+ H2O (l)+ CO2(g)

  • Reactions with metals in which hydrogen gas is displaced, reactions with simple (metal) oxides and hydroxides that are neutralised by the formation of a metal chloride and water, and reactions with weak acid salts in which the heavy acid is displaced are all examples of reactions with common strong acids that involve hydrochloric acid. 

Mg (s)+ 2HCl (aq) MgCl2(s) + H2(g)

Fe (s)+ 2HCl (aq) FeCl2(s) + H2(g

  • Chlorine gas can be released from the oxidation of HCl by potassium permanganate (KMnO4) or potassium dichromate (K2Cr2O7).

2KMnO4 (aq)+ 16HCl (aq) → 2KCl (aq)+ 2MnCl2 (aq)+ 5Cl2(g)  + 8H2O(l)

K2Cr2O7 (aq)+ 14HCl (aq) → 2KCl (aq)+ 2CrCl3 (s)+ 3Cl2 (g) + 7H2O(aq)

  • Carbon dioxide gas and sulphur dioxide gas are produced when hydrochloric acid combines with salts such as carbonates, hydrogen carbonates, sulphites, etc.

Na2CO3(s) + 2HCl (aq) → 2NaCl (aq)+ H2O (l)+ CO2(g)

NaHCO3(s) + HCl (aq) → NaCl (aq) + H2O (l)+ CO2(g)

Na2SO3 (s)+ 2HCl (aq) → 2NaCl (aq) + H2O (l)+ SO2 (g)

  • Aqua regia is a 3:1 by volume mixture of concentrated HCl and concentrated HNO3. Metals like gold and platinum can be dissolved by it to create their soluble chlorides.

Hydrochloric Acid in Stomach

Hydrochloric acid, a number of enzymes, and a mucus layer that shields your stomach lining make up stomach secretions. The body uses hydrochloric acid to help digest and absorb nutrients like calcium. Additionally, it gets rid of stomach viruses and germs, shielding your body from infection. Hydrochloric acid (HCl), potassium chloride (KCl), and sodium chloride (NaCl) makes up stomach acid. The stomach contain 5,000 parts per million, or about , 0.5 % of hydrochloric acid.

The pH range of a healthy stomach is usually 1.0 to 2.0. Normally, the stomach's low fluid content keeps bacteria out. But at the same time, these pH values almost put battery acid, which can melt steel, in the same class as stomach acid.

Uses of Hydrochloric Acid

  • Vinyl chloride, the monomer of PVC, the third most used plastic in the world, is produced industrially using hydrochloric acid.
  • Other organic compounds like dichloroethane and bisphenol-A are also made using HCl.
  • Metal surfaces can be cleaned of stains using a diluted hydrochloric acid solution. Although the acid is caustic in nature, rust can be removed from iron surfaces using diluted doses of the acid.
  • The pool tile cleaning process uses diluted hydrochloric acid as another application.
  • In the food industry, gelatin is also made using hydrochloric acid.
  • Hydrochloric acid is widely utilised in the rubber sector, dye processing, and battery manufacturing.

Practice Problems

Q 1. The molecule of hydrochloric acid is:

a. Triatomic
b. Monoatomic
c. Diatomic
d. Tetratomic 

Answer: (C)
HCl consists of two atoms, hydrogen and chlorine. So it is a diatomic molecule. So the correct answer is option C.

Q 2. Hydrochloric acid can be neutralised by:

a. NaOH
b. Na2CO3
c. NaCl
d. Na2S

Answer: (A)
Hydrochloric acid can be neutralised only by a base. Sodium hydroxide is a base and all others are salt. So option A is the correct answer. Base reacts with the acid to produce salt and water.

HCl (aq)+ NaOH(aq) NaCl (aq)+H2O(aq)

Q 3. HCl dissociates in:

a. Benzene
b. Water
c. Carbon tetrachloride
d. Carbon disulphide

Answer: (B)
Hydrochloric acid is a polar covalent compound. Hence it dissolves in polar substances like water. There is a dipole-dipole interaction between oppositely charged H+ and Cl- of HCl and H+ and OH- ions of water. All other given options are non-polar in nature. So option B is the correct answer.

Q 4. The smell of hydrochloric acid is:

a. Pungent
b. Fresh
c. Fruity
d. Floral

Answer: (B)
Hydrochloric acid has a pungent characteristic smell. So option A is the correct answer.

Frequently Asked Questions-FAQs

Q.1. How is hydrochloric acid used in the pickling of steel?
The removal of rust or iron oxide scale from iron or steel before undergoing further processing, such as extrusion, rolling, galvanising, and other procedures, is one of the most significant uses of hydrochloric acid. 

Q.2. How is hydrochloric acid produced in the stomach?
HCl is generated by the abdomen parietal cells. This channel exchanges hydrogen ions from the parietal cell within stomach potassium ions using ATP energy. As a result, the lumen of the stomach contains both hydrogen and chloride ions.

Q.3. Is hydrochloric acid inflammable?
Hydrochloric acid is not flammable, however, it is very much corrosive and quite poisonous. Chlorine gas, which is very toxic and caustic, and extremely flammable hydrogen gas are formed over the course of a severe fire. Hydrogen gas produced by contact with common metals is particularly flammable.

Q.4. What can be the impacts of HCl on humans?
The eyes, skin, and mucous membranes are all corrosive to hydrochloric acid. Acute (short-term) inhalation exposure may result in pulmonary edema, ocular irritation, and inflammation of the respiratory tract, nose, and eyes in people.

Related Topics

Allotropy and Allotropes of Phosphorus


Phosphorus halides

Nitric Acid


Alkali Metals

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