Hydrochloric acid

What is it?	Hydrochloric acid is a strong, corrosive chemical commonly used in industry.
Where is it found?	It can be found in stomachs as part of digestive fluid, and in factories for cleaning and processing.
Safety	It can burn skin and damage materials—always handle with care and protective gear.
Uses	Used to clean metals, make batteries, and help process food.

Hydrochloric acid, also known as muriatic acid or spirits of salt, is a watery solution made from a gas called hydrogen chloride. It looks clear and has a sharp, strong smell that can make your eyes water. Because it can break apart other substances very easily, we call it a strong acid.

Our bodies actually use hydrochloric acid to help digest food. It is part of the stomach acid that breaks down what we eat so our bodies can use it. Outside the body, hydrochloric acid is very useful too. Scientists use it in labs for experiments, and factories use it to make many everyday products. Even though it is powerful and must be handled with care, hydrochloric acid plays an important role in both nature and industry.

Etymology

Hydrochloric acid was once known as "spirits of salt" or "salt acid" because it was made from rock salt by a scientist named Johann Rudolph Glauber. In German, it is called "Salzsäure." Another old name for it was "marine acid air." The name "muriatic acid" comes from words meaning "pertaining to salt." The name "hydrochloric acid" was given by a French chemist, Joseph Louis Gay-Lussac, in 1814.

History

9th–10th century

In the early 900s, a scientist named Abu Bakr al-Razi mixed different salts and chemicals. He created a gas called hydrogen chloride and might have made hydrochloric acid, though he focused more on color changes from his experiments.

11th–13th century

Later scientists used al-Razi’s work. They heated metals with salts and made a chemical called mercury(II) chloride. They also found that heating certain salts could make strong acids, including hydrochloric acid.

14th–15th century

Aqua regia

Scientists discovered a special mix of acids called aqua regia, which can dissolve gold. It uses nitric acid and hydrochloric acid.

16th–17th century

In the late 1500s, recipes for making hydrochloric acid began to appear in books by scientists like Giovanni Battista Della Porta.

Industrial developments

During the Industrial Revolution, a process created a lot of hydrogen chloride as a by-product. Factories learned to capture this gas and dissolve it in water to make hydrochloric acid. Later, new methods were developed to produce it more efficiently.

Chemical properties

Hydrogen chloride is a gas that forms a bond between hydrogen and chlorine atoms. When mixed with water, it breaks apart completely, creating chloride ions and special water molecules called hydronium ions that connect with other water molecules.

Hydrochloric acid is a very strong acid, meaning it breaks apart almost entirely when placed in water, leaving almost no whole hydrogen chloride molecules behind.

Physical properties

Melting temperature as a function of HCl concentration in water

The physical features of hydrochloric acid, like how hot it boils and how cold it freezes, its thickness, and its sourness, change depending on how much hydrogen chloride it contains. These features range from being like water when there is very little hydrogen chloride to being much different when there is a lot, like over 40%.

Hydrochloric acid mixed with water can boil at a steady temperature when it contains 20.2% hydrogen chloride. There are also special points where the acid and water freeze together at different amounts of hydrogen chloride, from 68% down to none at all. These points involve special crystal forms of the acid mixed with water.

Mass fraction	Concentration		Density		pH	Viscosity	Specific heat	Vapor pressure	Boiling point	Melting point
kg HCl/kg	kg HCl/m³	mol/L	Baumé	kg/L		mPa·s	kJ/(kg·K)	kPa	°C	°C
10%	104.80	2.87	6.6	1.048	−0.5	1.16	3.47	1.95	103	−18
20%	219.60	6.02	13	1.098	−0.8	1.37	2.99	1.40	108	−59
30%	344.70	9.45	19	1.149	−1.0	1.70	2.60	2.13	90	−52
32%	370.88	10.17	20	1.159	−1.0	1.80	2.55	3.73	84	−43
34%	397.46	10.90	21	1.169	−1.0	1.90	2.50	7.24	71	−36
36%	424.44	11.64	22	1.179	−1.1	1.99	2.46	14.5	61	−30
38%	451.82	12.39	23	1.189	−1.1	2.10	2.43	28.3	48	−26
The reference temperature and pressure for the above table are 20 °C and 1 atmosphere (101.325 kPa). Vapor pressure values are taken from the International Critical Tables and refer to the total vapor pressure of the solution.

Production

Hydrochloric acid is made by mixing hydrogen chloride with water. Hydrogen chloride can be created in different ways, often as part of making other chemicals, like in the chloralkali process which also makes hydroxide, hydrogen, and chlorine.

Hydrogen chloride is formed by combining chlorine and hydrogen:

Cl₂ + H₂ → 2 HCl

Because this reaction gives off heat, the equipment used is called an HCl oven or HCl burner. The hydrogen chloride gas is then mixed with deionized water to make pure hydrochloric acid, which can be very clean and suitable for use in food.

Hydrochloric acid is made in different strengths for different uses. In industry, it is usually around 30% to 35% HCl. For household cleaning, weaker solutions are used, often between 10% and 12% strength. Different countries may sell it under different names and in different strengths.

Applications

Main article: Hydrogen chloride

Hydrochloric acid is a strong acid used in many jobs. It helps clean steel by taking off rust before the steel is shaped or coated. It can also dissolve metals and minerals to make useful products.

It is used to control how sour or bitter water is in factories, especially in making food and medicine. It is also used in labs to help measure how much of certain things there are in a mix. Plus, it helps clean things like bricks and kettles, and it is used in making some foods and food additives.

Presence in animals

Gastric acid is one of the main things our stomach makes. It is mostly made of hydrochloric acid and makes the stomach very acidic, with a pH of 1 to 2. This helps protect us from tiny harmful creatures and aids in breaking down food.

The stomach is safe from this strong acid because of a thick layer of mucus and other natural protections. If these protections fail, it can cause discomfort or health issues. Special medicines can help reduce too much acid in the stomach. Hydrochloric acid also helps in breaking down bone tissue in our bodies.

Safety

Hydrochloric acid is a strong acid that can harm living tissue and many materials, but it does not harm rubber. When handling it, people usually wear rubber gloves and other protective gear.

The vapors or mists from hydrochloric acid can be dangerous to breathe, so a special respirator can help. The acid in the air can also irritate the eyes, so protective goggles or a facemask may be needed.

Mass fraction	Classification	List of H-phrases
10% ≤ C	Causes skin irritation, Causes serious eye irritation,	H315, H319
C ≥ 10%	May cause respiratory irritation	H335
C ≥ 25%	Causes severe skin burns and eye damage	H314

Legal status

Hydrochloric acid is watched closely by international law because it can be used to make certain illegal substances. It is listed under a special agreement from 1988 made by many countries to stop the wrong use of drugs.