Hydrogen chloride (HCl) gas is produced by reacting a chloride salt with a strong acid, most commonly table salt (sodium chloride) with concentrated sulfuric acid. This is the classic laboratory method and the most accessible for small-scale generation. The gas itself is colorless, has a sharp, pungent smell, and is extremely soluble in water, dissolving at roughly 82 grams per 100 grams of water at 0 °C. That high solubility is what makes it easy to capture as hydrochloric acid but also means it readily attacks moist tissue, including your lungs, eyes, and skin.
The Salt and Sulfuric Acid Method
The most straightforward way to generate HCl gas uses two cheap, available reagents: sodium chloride (table salt or rock salt) and concentrated sulfuric acid. When you add sulfuric acid to salt, the reaction proceeds in two stages. At room temperature or with gentle heating, one mole of salt reacts with sulfuric acid to produce one mole of HCl gas and sodium bisulfate. At higher temperatures (above about 200 °C), the sodium bisulfate reacts with a second mole of salt to release another mole of HCl and leave behind sodium sulfate.
For most purposes, the first stage is sufficient. You place dry sodium chloride in a round-bottom flask or similar vessel, add concentrated sulfuric acid dropwise through a dropping funnel, and collect the gas that evolves. Gentle warming with a heating mantle or hot water bath speeds the reaction. The gas exits through tubing and can be directed into a collection vessel or bubbled into water to make hydrochloric acid solution.
This method works because sulfuric acid is a stronger, less volatile acid than HCl. It displaces the more volatile hydrogen chloride from the salt, and because HCl has a boiling point of roughly negative 85 °C, it leaves the reaction mixture as a gas almost immediately at room temperature.
Drying the Gas
HCl gas produced from the salt-acid reaction carries water vapor, especially if the sulfuric acid isn’t fully concentrated or if the salt is damp. For applications requiring dry HCl, the gas is passed through a drying column. Concentrated sulfuric acid is the standard desiccant for this purpose, typically held in a wash bottle or drying tower that the gas bubbles through. Sulfuric acid works here because it absorbs water aggressively without reacting with HCl gas itself.
Calcium chloride, another common lab drying agent, is not suitable. It reacts with HCl to form a complex, which removes the gas you’re trying to collect. Phosphorus pentoxide (a powerful solid desiccant) can be used in a drying tube as an alternative to sulfuric acid, though it’s more expensive and less convenient for continuous flow.
Other Generation Methods
Several other reactions produce HCl gas, though they’re less practical for intentional generation. Burning hydrogen gas in chlorine produces very pure HCl and is the basis of industrial “synthetic” hydrochloric acid production. This requires compressed gas cylinders of both hydrogen and chlorine and is not a small-scale method.
Many chlorinated compounds release HCl when they contact water. Phosphorus trichloride, for instance, reacts violently with water to produce phosphorous acid and three equivalents of HCl gas. The reaction is strongly exothermic (releasing about 15 kilocalories per mole) and difficult to control safely. Thionyl chloride and acetyl chloride similarly release HCl on contact with water or alcohols, which is why these reagents are used in organic chemistry to convert acids and alcohols to chlorides. These are sources of HCl gas, but they’re not practical or safe ways to generate it on purpose.
Collecting and Handling
HCl gas is denser than air (about 1.3 times heavier), so it can be collected by downward displacement. You invert a dry collection vessel and run the gas delivery tube to the bottom. The heavier HCl displaces the air upward and fills the vessel. You can test whether the vessel is full by holding a glass rod dipped in ammonia solution near the mouth. A white cloud of ammonium chloride smoke confirms HCl is present.
If your goal is to make hydrochloric acid solution rather than collect the pure gas, bubble the HCl through cold water using an inverted funnel at the end of the delivery tube. The inverted funnel prevents suck-back, a dangerous situation where water gets pulled backward into the hot reaction flask due to the gas dissolving so rapidly that it creates a vacuum in the tubing. This is one of the most common accidents in HCl generation setups, and using an inverted funnel with its rim just touching the water surface prevents it reliably.
Safety Essentials
HCl gas is corrosive and toxic. OSHA sets a ceiling exposure limit of 5 parts per million in workplace air, meaning concentrations should never exceed this level even briefly. At 50 ppm, the gas is considered immediately dangerous to life and health. You can smell HCl at concentrations well below 5 ppm, so if the odor is noticeable, your exposure is already approaching the safety limit.
Inhalation causes irritation of the nose, throat, and lungs. At higher concentrations it can trigger bronchospasm (airway constriction), coughing, and in severe cases pulmonary edema, where fluid accumulates in the lungs. Children are more vulnerable because their smaller airways swell shut more easily. Contact with skin or eyes causes burns proportional to the concentration and duration of exposure.
All work with HCl gas should happen in a functioning fume hood or outdoors with wind at your back. Wear chemical splash goggles (not just safety glasses), acid-resistant gloves, and a lab coat or long sleeves. Keep a supply of sodium bicarbonate (baking soda) solution nearby to neutralize any spills from the liquid reagents.
Neutralizing Excess Gas
Any HCl gas that isn’t collected needs to be neutralized before it reaches the atmosphere. The simplest method is to run the exit tube from your apparatus into a beaker or flask containing a dilute sodium hydroxide (caustic soda) solution, typically around 5 to 10 percent by weight. The HCl reacts with sodium hydroxide to form harmless sodium chloride and water. A sodium bicarbonate solution works too, though it fizzes as carbon dioxide is released, which can make the tubing connections messy.
In industrial settings, wet scrubbers use recirculating caustic solutions to strip HCl from exhaust streams, with the chemistry scaled up but fundamentally the same. For small-scale work, a simple bubbler flask with sodium hydroxide solution is adequate. Make sure the neutralizing solution doesn’t get depleted. If you’re generating a large volume of gas, check the pH of the scrubber solution periodically or use an indicator to confirm it’s still alkaline.