Hydrogen chloride is a special chemical compound. Its chemical formula is HCl. It belongs to a group called hydrogen halides. At normal room temperature, it's a clear, colorless gas. When this gas touches the moisture in the air, it creates white fumes. These fumes are actually tiny droplets of hydrochloric acid. Both hydrogen chloride gas and hydrochloric acid are very important in many industries and technologies. Hydrochloric acid is simply hydrogen chloride dissolved in water.

Understanding Hydrogen Chloride
How Hydrogen Chloride is Made
Everyday Uses of Hydrogen Chloride
A Look Back: History of HCl
Staying Safe with Hydrogen Chloride
Images for kids
See also

Understanding Hydrogen Chloride

Hydrogen chloride is a diatomic molecule. This means it's made of two atoms. One is a hydrogen atom (H) and the other is a chlorine atom (Cl). These two atoms are joined by a strong connection called a polar covalent bond.

A Polar Molecule

The chlorine atom pulls electrons much more strongly than the hydrogen atom. This makes the bond "polar." Think of it like a magnet with two ends. One end of the molecule (the chlorine side) has a slightly negative charge. The other end (the hydrogen side) has a slightly positive charge. This "polarity" helps HCl dissolve easily in water.

How it Becomes an Acid

When hydrogen chloride gas touches water (H
2O), they react. They form hydronium ions ([H
3O]+
) and chloride ions (Cl−
). This reaction makes a solution called hydrochloric acid.

HCl + H
2O → [H
3O]+
+ Cl−

Hydrochloric acid is known as a strong acid. This means that almost all the HCl molecules break apart in water. They form ions, making the solution very acidic. Even without water, hydrogen chloride can act as an acid. For example, it can dissolve in other liquids like methanol.

HCl + CH
3OH → [CH
3OH
2]+
+ Cl−

Because it's an acid, hydrogen chloride can be corrosive. This is especially true when moisture is present.

What Hydrogen Chloride Looks Like

Hydrogen chloride is a colorless gas. It has a strong, sharp, and burning smell. When it gets very cold, it turns into a solid. This solid form has a special structure where the molecules link together in zigzag chains.

How much HCl dissolves (g/L) in common liquids
Temperature (°C)	0	20	30	50
Water	823	720	673	596
Methanol	513	470	430
Ethanol	454	410	381
Ether	356	249	195

This table shows how much hydrogen chloride can dissolve in different liquids at various temperatures. You can see it dissolves a lot in water!

Hydrochloric acid fumes turning pH paper red, showing they are acidic.

How Hydrogen Chloride is Made

Most of the hydrogen chloride made today is used to produce hydrochloric acid.

Making it from Basic Elements

One way to make hydrogen chloride is by directly combining two gases: chlorine gas (Cl
2) and hydrogen gas (H
2).

Cl
2 + H
2 → 2 HCl

This reaction creates heat, so the equipment used is often called an HCl "oven" or "burner." The hydrogen chloride gas produced is then absorbed into very pure water. This creates super clean hydrochloric acid, often used in the food industry. Blue light can also start this reaction.

Making it During Other Chemical Processes

Hydrogen chloride is often made as a side product when creating other important chemicals. These include plastics like Teflon and PVC. It's also made when creating CFCs (though many CFCs are now banned due to environmental concerns). In these processes, hydrogen atoms in organic compounds are replaced by chlorine atoms. The leftover hydrogen then combines with chlorine to form hydrogen chloride.

RH + Cl
2 → RCl + HCl

This hydrogen chloride can be used again or dissolved in water to make industrial-grade hydrochloric acid.

Making Small Amounts in the Lab

For school or lab experiments, small amounts of hydrogen chloride can be made. One way is to take hydrochloric acid and remove its water using sulfuric acid or calcium chloride. Another common method is reacting sulfuric acid with sodium chloride (table salt):

NaCl + H
2SO
4 → NaHSO
4 + HCl↑

This reaction happens at room temperature. If you heat it above 200 °C and there's still salt, another reaction can occur:

NaCl + NaHSO
4 → Na
2SO
4 + HCl↑

It's important that the chemicals used in these lab methods are dry. Hydrogen chloride can also be made by adding cold water slowly to certain reactive chlorine compounds, like phosphorus pentachloride (PCl
5).

PCl
5 + H
2O → POCl
3 + 2 HCl↑

Everyday Uses of Hydrogen Chloride

Most hydrogen chloride is used to make hydrochloric acid. It's also a key ingredient in making vinyl chloride. Vinyl chloride is the main building block for PVC, a plastic used in pipes, window frames, and many other products. Hydrogen chloride is also used to create trichlorosilane. This chemical is important for making very pure silicon, which is used in computer chips and solar panels.

Si + 3 HCl → HSiCl
3 + H
2

A Look Back: History of HCl

People have known about hydrogen chloride and hydrochloric acid for a very long time. Around the year 900, scientists like Jabir ibn Hayyan and Abu Bakr al-Razi were experimenting with sal ammoniac (a type of salt). They found that mixing it with other chemicals could produce hydrogen chloride gas. At first, they didn't realize how useful this gas was.

A big step forward was the discovery of aqua regia (royal water) around the 1300s. This powerful liquid could dissolve gold! It was made by adding sal ammoniac to nitric acid. This showed how important chemicals derived from hydrogen chloride could be.

Later, in the 17th century, Johann Rudolf Glauber used salt and sulfuric acid to make sodium sulfate. This process also released hydrogen chloride gas. In 1772, Joseph Priestley successfully prepared pure hydrogen chloride. By 1810, Humphry Davy proved that it was made of hydrogen and chlorine.

During the Industrial Revolution, there was a huge need for chemicals like soda ash. Nicolas Leblanc developed a way to make soda ash that produced hydrogen chloride as a byproduct. Initially, this gas was just released into the air. However, laws like the Alkali Act 1863 in Britain stopped this pollution. So, factories started dissolving the waste HCl gas in water, which led to the large-scale production of hydrochloric acid.

In the 20th century, hydrogen chloride was used to make important plastics. These included chloroprene (used in Neoprene rubber) and vinyl chloride (used in PVC). For example, to make vinyl chloride, HCl was added to acetylene (C
2H
2).

Staying Safe with Hydrogen Chloride

Hydrogen chloride is a dangerous chemical. When it touches water in your body, it forms corrosive hydrochloric acid.

Breathing it in: Inhaling the fumes can cause coughing, choking, and irritation in your nose, throat, and lungs. In serious cases, it can harm your lungs and even be deadly.
Skin contact: If it touches your skin, it can cause redness, pain, and severe chemical burns.
Eye contact: Getting hydrogen chloride in your eyes can cause severe burns and permanent eye damage.

Always be very careful around this chemical and follow all safety rules.

Images for kids

The structure of solid DCl, as determined by neutron diffraction of DCl powder at 77 K. DCl was used instead of HCl because the deuterium nucleus is easier to detect than the hydrogen nucleus. The extended linear structure is indicated by the dashed lines.
Side view of the solid DCl structure.
Infrared (IR) absorption spectrum of gaseous hydrogen chloride.
One doublet in the IR spectrum resulting from the isotopic composition of chlorine.
A chemical diagram showing the synthesis of chloroprene.