Hydrogen Chloride vs. Hydrochloric Acid

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  • 0:04 Compounds
  • 0:25 Chemical Formula
  • 0:54 HCl(g)
  • 1:23 HCl(aq)
  • 2:14 Properties
  • 3:08 Lesson Summary
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Lesson Transcript
Instructor: Matthew Bergstresser

Matthew has a Master of Arts degree in Physics Education

In chemistry, a substance can have the same chemical formula as another, but with completely different properties. In this lesson, we'll look at two compounds with the chemical formula HCl, and discover why they are so different.


Have you ever made lemonade from a powder? Have you ever just consumed the powder without mixing it with water? If so, I bet that is the last time you did it! There is a big difference between the powder and the powder mixed with water. The same goes for hydrogen chloride and hydrochloric acid. Let's learn about the similarities and differences between these separate versions of the same compound.

Chemical Formula

HCl is the chemical formula for both hydrogen chloride and hydrochloric acid. The major difference is the state they're in. Hydrogen chloride is a gas, and hydrochloric acid is an aqueous solution. Aqueous simply means it's dissolved in water. This is why it's important to include the state of the matter in parentheses. For the gaseous version, we write HCl(g), and for the acid version we write HCl(aq). Let's dig a little deeper about each of these states.


As we've discussed, hydrogen chloride is a gas. We can draw a Lewis dot structure of HCl(g), which shows the electron arrangement in the bond between hydrogen and chlorine. Diagram 1 shows the Lewis dot structure of hydrogen chloride gas.

Diagram 1. Lewis dot diagram of HCl(g).

The dots in Diagram 1 represent outer shell electrons. There are two electrons shared between hydrogen and chlorine, making this a covalent molecule. Something special happens when hydrogen chloride gas bubbles through water. Let's check it out.


A polar substance is electrically neutral overall, but one side of it is positively charged, and the other side is negatively charged. Diagram 2 shows water's polar nature.

Water is polar because the oxygen side is negative, and the hydrogen side is positive

When hydrogen chloride gas enters water, the water molecules grab the hydrogen atom in HCl(g) and pull it away from the chlorine atom. This is the dissolution process, which makes hydrochloric acid. Now we have an acidic solution because there are H3 O+1 ions present, which have the name hydronium. Diagram 3 shows the hydronium ion.

Diagram 3. Hydronium ion. The dotted bar just represents the bond going back into the screen. It is a way to represent a 3-D molecule.

Let's write the reaction.

HCl(g) + H2 O(l) → H3 O+1 + Cl-1

Diagram 4 shows how hydrogen chloride gas can be bubbled into water.

Diagram 4

Let's now go through the properties of hydrogen chloride gas and hydrochloric acid.

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