Hydrogen chloride is represented by the chemical formula HCl. This means that a molecule of hydrogen chloride contains one atom of hydrogen and one atom of chlorine. At room temperature (about 77°F [25°C]) and at a pressure of one atmosphere, hydrogen chloride exists as a gas. Consequently it is generally stored under pressure in metal containers. A much more convenient way to use hydrogen chloride is by dissolving it in water to form a solution. Hydrogen chloride is very soluble in water, the latter dissolving hundreds of times its own volume of hydrogen chloride gas. The resulting solution is known as hydrochloric acid and this also is generally given the chemical formula HCl. Commercial hydrochloric acid usually contains 28-35% hydrogen chloride by weight, and is generally referred to as concentrated hydrochloric acid. When smaller amounts of hydrogen chloride are dissolved in water, the solution is known as dilute hydrochloric acid.
Hydrogen chloride is a colorless, nonflammable gas with an acrid odor. The gas condenses to a liquid at -121°F (-85°C) and freezes into a solid at -173.2°F (-114°C). Hydrochloric acid is a colorless, fuming liquid having an irritating odor. Both hydrogen chloride and hydrochloric acid are corrosive, and so must be treated with great care. Both substances strongly irritate the eyes and are highly toxic if inhaled or ingested. Exposure to hydrogen chloride vapor can damage the nasal passages and produce coughing, pneumonia, headaches and rapid throbbing of the heart, and death can occur from exposure to levels in air greater than about 0.2%. Concentrated hydrochloric acid solutions cause burns and inflammation of the skin. Chemists always wear protective rubber gloves and safety glasses when using either hydrogen chloride or hydrochloric acid, and generally work in a well ventilated area to reduce exposure to fumes.
While dry hydrogen chloride gas is fairly unreactive, moist hydrogen chloride gas (and hydrochloric acid solutions) react with many metals. Consequently, dry hydrogen chloride gas can be stored in metal containers, whereas solutions of highly corrosive hydrochloric acid must be handled in acid-proof materials such as ceramics or glass. When hydrochloric acid reacts with metals, hydrogen gas and compounds known as metal chlorides are usually generated. Metal chlorides are formed when a metal displaces the hydrogen from the hydrogen chloride. For example, zinc metal dissolves in hydrochloric acid to form hydrogen gas and zinc chloride. Both moist hydrogen chloride and hydrochloric acid also react with many compounds including metal oxides, hydroxides, and carbonates. These are all examples of basic compounds, which neutralize hydrochloric acid, and form metal chlorides.
Obviously, hydrochloric acid is acidic. Like most acids, hydrogen chloride forms hydrogen ions in water. These are positively charged atoms of hydrogen that are very reactive and are responsible for all acids behaving in much the same way. Because all the hydrogen atoms in hydrogen chloride are converted into hydrogen ions, hydrochloric acid is called a strong acid. Nitric and sulfuric acids are other examples of strong acids.