What is HCl? Understanding Hydrochloric Acid, Hydrogen Chloride, Chloride Salts

by Johnny Jacks
HCl has important applications in various industries such as water treatment, food production, and steel manufacturing. So what is HCl and what are its special properties? Let’s explore the content of the lecture on Hydrochloric Acid, Hydrogen Chloride, and Chloride Salts in the following article with Monkey.

What is the concept of HCl?

HCl is the general formula for hydrogen chloride gas and hydrochloric acid, where:

Hydrogen Chloride (HCl)

Definition: Hydrogen Chloride is a colorless gas with a pungent odor, highly soluble in water (1 liter of water can dissolve 500 liters of HCl at 20 degrees Celsius).

Molecular structure: H-Cl. Hydrogen Chloride is a covalent compound, and the molecule has a dipole.

Hydrochloric Acid (HCl)

Hydrogen Chloride (HCl) dissolves in water to form a strong acid – Hydrochloric Acid (HCl).

Definition: Hydrochloric Acid (HCl) is a colorless liquid with a pungent odor, with a maximum concentration of 37%.

The phenomenon of “smoke” from Hydrochloric Acid in moist air is caused by the escape of hydrogen chloride along with water vapor in the air, forming many small droplets of solution like fog.

Learn about Hydrogen Chloride

To study the solubility of HCl gas in water, an experiment is conducted: HCl gas is introduced into a test tube filled with water and the tube is covered with a rubber stopper. A straight glass tube is inserted through the rubber stopper, and the open end of the tube is immersed in a dish of water containing litmus solution.

Observations show that red-colored water jets appear in the dish through the glass tube. This phenomenon can be explained by the fact that hydrogen chloride gas dissolves readily in water, causing a rapid drop in pressure inside the test tube. The atmospheric pressure then forces the water into the tube to replace the HCl gas that has dissolved.

HCl gas is very soluble in water.

HCl gas is very soluble in water.

Thus, we can draw the following conclusions about Hydrogen Chloride:

  • HCl gas dissolves readily in water.
  • HCl gas is heavier than air (d= 36.5/29).
  • HCl gas does not turn litmus paper red. The resulting solution is Hydrochloric acid, which is capable of turning litmus paper red.
  • HCl gas does not react with CaCO3.

Learn more about hydrochloric acid HCl

Let’s learn more about Hydrochloric Acid (HCl) in detail in this section.

Physical properties of hydrochloric acid

Here are four typical physical properties of hydrochloric acid that you should remember:

  • Hydrochloric acid is a colorless solution with a pungent odor.
  • The most concentrated solution of HCl is achieved at a concentration of C% = 37% (at 20°C).
  • HCl solution dissolves in water, with a density of 1.19 (g/mL).
  • Concentrated HCl solution fumes in humid air.

Chemical properties of hydrochloric acid

Hydrochloric acid is a strong acid with the following typical acid properties:

Hydrochloric acid HCl turns litmus paper red.

Hydrochloric acid HCl reacts with metals that are more reactive than hydrogen to produce salt and hydrogen gas. For example, Fe + 2HCl → FeCl2 + H2 (g).

Note: Pb is more reactive than H but does not dissolve in HCl.

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Hydrochloric acid HCl reacts with metal oxides to produce salt and water. For example, Fe3O4 + 8HCl → 4H2O + FeCl2 + 2FeCl3.

Hydrochloric acid HCl reacts with bases to produce salt and water. For example, Fe(OH)2 + 2HCl → FeCl2 + 2H2O.

Hydrochloric acid HCl reacts with salts to produce new salts and new acids. For example, Na2CO3 + 2HCl → 2NaBr + H2O + CO2.

In addition to being a strong acid, HCl also exhibits oxidation-reduction properties:

Oxidation: HCl exhibits oxidation when it reacts with metals that are more reactive than hydrogen.

Reduction: This reduction property is due to the fact that in the HCl molecule, Chlorine has the lowest oxidation number of -1. When concentrated HCl solution reacts with strong oxidizing agents such as KMnO4, K2Cr2O7, MnO2, KClO3, etc., HCl is oxidized to Cl2 according to the equation: MnO2 + 4HCl → MnCl2 + Cl2 + 2H2O (+4) (-1) (+2) (0).

How is hydrochloric acid prepared?

Let’s explore 2 common methods of preparing HCl: In industry and in the laboratory.

Preparing hydrochloric acid HCl in the laboratory

In the laboratory, HCl is prepared by the sulfate method – by reacting NaCl crystals with concentrated H2SO4 and heating, then absorbing it into water.

The reaction equation is as follows:

NaCl + H2SO4 → NaHSO4 + HCl (temperature < 250°C)

2NaCl + H2SO4 → Na2SO4 + 2HCl (temperature from 400°C upwards)

Production of hydrochloric acid in industry

In industry, HCl is prepared by burning H2 in Cl2 from the atmosphere. Both of these substances are products of the process of electrolysis of NaCl with a diaphragm.

H2 + Cl2 → 2HCl (temperature)

Applications of hydrochloric acid HCl

How is Hydrochloric acid HCl applied in daily life with the above physical and chemical properties?

In industry: 35% HCl is widely used in the steel, electroplating, chitin production, metallurgy, electric welding, rubber production, food processing (sugar, sweeteners, fish sauce), chemical industry and water treatment, power plants, mineral extraction industries, etc.

Removing rust on steel: This is the most practical and common application of HCl in daily life. Before the steel is put into production, ~18% HCl is used to remove rust from carbon steel, making them new again. In addition, people have invented the process of recycling HCl to save and improve the efficiency of rust removal on steel according to the equation:

4FeCl2 + 4H2O + O2 → 8 HCl + 2Fe2O3. Fe2O3 (iron oxide) is also a by-product that is widely used in the industry.

Use in the production of inorganic compounds: HCl is used in the production of inorganic compounds through many chemical reactions. Typical examples are water treatment chemicals such as FeCl3, PAC,… used as flocculants, used in wastewater treatment, drinking water, and paper production.

Production of organic compounds: HCl can also be used to create vinyl chloride and dichloroethane in PVC production. In addition, it is also applied in the production of bisphenol A, polycarbonate, ascorbic acid, activated carbon, and some products of the pharmaceutical industry.

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Control, neutralization of pH: In industries that require purity (production of pharmaceuticals, food, beverages, drinking water…), high-quality HCl solution is used to adjust the pH of water. For other industries, industrial Hydrochloric acid HCL is used to neutralize wastewater and treat swimming pool water.

Plastic regeneration by ion exchange: High-quality HCl is used to regenerate ion exchange resins in the plastic regeneration process. This is applied in all chemical industries, beverage production, and some food industries. HCl becomes popular to remove ions such as Na+, Ca2+ from water-containing solutions to produce demineralized water, by replacing Na+ with H+ and Ca2+ with 2 H+.

For organisms: Hydrochloric acid HCL in the stomach acts as an antimicrobial agent, preventing infection and supporting the digestive process. When the pH in the stomach is low, proteins in the stomach are denatured, creating conditions for enzymes to digest food more easily, and activating pepsinogen into the enzyme pepsin…

Overview of Chloride Salt

What is chloride salt?

What is chloride salt?

The salt of hydrochloric acid HCl is called chloride salt with the general formula: MCln.

Solubility: Most chloride salts dissolve in water except for PbCl2, AgCl, CuCl, and Hg2Cl2.

Applications: Chloride salt has many important applications such as: Used as potassium fertilizer (KCl), wood preservation because ZnCl2 has antibacterial properties, as a catalyst in organic synthesis (AlCl3), insecticide (BaCl2). NaCl is an important raw material in the chemical industry for producing hydrogen and chlorine, and is used as a food seasoning and preservative.

Identification: To identify chloride salt, we use AgNO3 reagent to produce white precipitate according to the equation: AgNO3 + NaCl → AgCl + NaNO3.

Common Chemical Reactions with HCl:

To remember the knowledge about hydrochloride, hydrochloric acid, and chloride salt, the following are the basic and most common chemical reactions with HCl.

HCl reacts with metals:

2HCl + Mg → MgCl2 + H2↑

Fe + 2HCl → FeCl2 + H2↑

6HCl + 2Al → 2AlCl3 + 3H2↑

Na + 2HCl → NaCl + H2↑

Zn + 2HCl → ZnCl2 + H2↑

HCl reacts with metal oxides:

Fe3O4 + 8HCl → 4H2O + FeCl2+ 2FeCl3

6HCl + Al2O3 → 2AlCl3 + 3H2O

MnO2 + HCl → MnCl2 + Cl2 + H2O

K2Cr2O7 + 14HCl → 2CrCl3 + 2KCl + 3Cl2 + 7H2O

KClO3 + 6HCl → 3Cl2 + KCl + 3H2O

HCl reacts with bases:

2HCl + 2NaOH → 2NaCl + H2O

2HCl + Ca(OH)2 → CaCl2 + 2H2O

2HCl + Fe(OH)2 → FeCl2 + 2H2O

2HCl + Mg(OH)2 → MgCl2 + 2H2O

NH3 + HCl → NH4Cl (white smoke phenomenon)

HCl reacts with basic oxides:

MgO + 2HCl → MgCl2 + H2O

2HCl + CuO → CuCl2 + H2O

HCl reacts with salts:

CaCO3 + 2HCl → CaCl2 + H2O + CO2↑

K2CO3 + 2HCl → H2O + CO2 + 2KCl

AgNO3 + HCl → AgCl↓ + HNO3

Na2CO3 + 2HCl → 2NaCl + CO2 + H2O

Exercises on HCl with detailed explanations

After mastering the basic knowledge about hydrogen chloride, hydrochloric acid, and chloride salts, let’s refer to some exercises about HCl in Chemistry 10 Chapter 23 textbook. It is hoped that the following detailed and easy-to-understand solutions will help readers understand this lesson better.

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Exercise 1 Solution:

Given 20g mixture of Mg and Fe reacts with excess HCl to produce 1g H2 gas. What is the mass of chloride salt produced in the solution?

A. 40.5g B. 45.5g C. 55.5g D. 65.5g

Solution: The correct answer is C.

We have:

n(H2) = 1/2 = 0.5 mol.

Mg + 2HCl → MgCl2 + H2

Fe + 2HCl → FeCl2 + H2

n(Mg) = x; n(Fe) = y.

n(H2) = x + y = 0.5 mol.

mixture mass = 24x + 56y = 20g.

Solving the system of equations, we have x = y = 0.25 mol.

m(MgCl2) = 0.25 × 95 = 23.75g.

m(FeCl2) = 0.25 × 127 = 31.75g

Mass of chloride salt = 23.75 + 31.75 = 55.5g

Exercise 2:

List the physical properties of hydrogen chloride gas HCl.


Hydrogen chloride is a colorless gas with a pungent odor and is denser than air. It is highly soluble in water, forming an acidic solution. “At 20oC, one volume of water can dissolve 500 volumes of hydrogen chloride gas.

Exercise 3:

The following substances are present: concentrated sulfuric acid, water, solid potassium chloride. Write the reaction equations to prepare hydrochloric acid.


Chemical equations for the reaction to prepare hydrochloric acid are:

2NaCl (crystal) + H2SO4 (concentrated) → Na2SO4 + 2HCl (temperature)

2KCl + 2H2O → 2KOH + H2 + Cl2 (electrolysis of solution with diaphragm)

H2 + Cl2 → 2HCl (light)

Exercise 4:

Provide examples of chemical reactions of hydrochloric acid for illustration purposes.

a) These are oxidation-reduction reactions.

b) These are not oxidation-reduction reactions.


a) Examples that demonstrate hydrochloric acid as oxidation-reduction reactions are:

As a reducing agent:

MnO2 + 4HCl → MnCl2 + Cl2 + 2H2O

K2Cr2O7 + 14HCl –> 2CrCl3 + 2KCl + 3Cl2 + 7H2O

KClO3 + 6HCl –> 3Cl2 + KCl + 3H2O

As an oxidizing agent:

Mg + 2HCl → MgCl2 + H2↑

Mg + 2HCl → MgCl2 + H2↑

Fe + 2HCl → FeCl2 + H2↑

b) Examples that are not oxidation-reduction reactions of hydrochloric acid are:

CaCO3 + 2HCl → CaCl2 + CO2 + H2O.

CuO + 2HCl → CuCl2 + H2O.

Exercise 5:

How do the nature of the reactions in the sulfuric acid method and the synthesis method for preparing hydrochloric acid differ? What chemical properties of the reactants are used in these methods?


The nature of the sulfuric acid method is the use of an exchange reaction: NaCl + H2SO4 → NaHSO4 + HCl.

The nature of the synthesis method is the use of a combination reaction (oxidation-reduction reaction):

H2 + Cl2 → 2HCl.

Therefore, this article has helped you answer the question “What is HCl?” and provided important fundamental knowledge about hydrogen chloride, hydrochloric acid, and chloride salts. Goodheathplan hopes that this information will help you have a better understanding of HCl and be able to solve related exercises in the future.

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