Zinc chloride is made by dissolving zinc metal in hydrochloric acid. The reaction is straightforward, produces hydrogen gas as a byproduct, and yields a zinc chloride solution that can be used directly as soldering flux or evaporated down to a solid. The balanced equation is: Zn + 2 HCl → ZnCl₂ + H₂. For every gram of zinc, you need roughly two grams of concentrated (37%) hydrochloric acid, though using a slight excess of zinc ensures all the acid gets consumed.
The Direct Method: Zinc and Hydrochloric Acid
This is the simplest and most common approach. You add small pieces of zinc metal to hydrochloric acid in a glass or plastic container. The zinc dissolves, the solution bubbles as hydrogen gas escapes, and what remains is zinc chloride dissolved in water. The reaction is exothermic, meaning it gives off heat. In laboratory measurements, dissolving just 0.1 grams of zinc in 50 mL of dilute acid raises the temperature by about 1.2°C, so larger batches generate noticeable warmth.
The practical steps look like this:
- Start with the acid. Pour your hydrochloric acid into a heat-resistant glass container. Muriatic acid from a hardware store (typically 31–37% concentration) works, though it contains more impurities than laboratory-grade acid.
- Add zinc gradually. Drop in small pieces of zinc metal a little at a time. Adding it slowly controls the rate of bubbling and heat generation. The hydrogen gas produced is flammable, so work outdoors or in a well-ventilated area with no open flames.
- Use excess zinc. Keep adding zinc pieces until the bubbling stops completely and some undissolved zinc remains at the bottom. This guarantees all the hydrochloric acid has reacted, leaving a solution that is zinc chloride in water with no free acid.
- Filter. Pour the solution through a coffee filter or lab filter paper to remove the leftover zinc and any debris. The clear liquid is your zinc chloride solution.
Many people stop here. A filtered zinc chloride solution is exactly what you need for soldering flux, and it works well for other applications where an aqueous form is acceptable. Zinc chloride is extraordinarily soluble in water (432 grams per 100 mL at room temperature), so the solution can be very concentrated.
The Zinc Oxide Route
If you have zinc oxide powder instead of zinc metal, you can dissolve it directly in hydrochloric acid. The reaction is similar but doesn’t produce hydrogen gas, which makes it calmer and easier to manage. Use a weight ratio of roughly 1 part zinc oxide to 5 parts concentrated (37%) hydrochloric acid. Stir until the powder dissolves, then filter the solution to remove any undissolved material.
Zinc carbonate also works. When it reacts with hydrochloric acid, it produces carbon dioxide instead of hydrogen, so you’ll see fizzing but the gas isn’t flammable. Both alternatives give you the same end product: zinc chloride in solution.
Evaporating to Solid Zinc Chloride
Getting from a zinc chloride solution to a dry solid is the trickiest part of the process. Zinc chloride is hygroscopic, meaning it aggressively absorbs moisture from the air. Simply boiling off the water tends to produce a gummy, wet mass rather than a clean dry powder, and heating too aggressively in open air can cause partial decomposition into zinc oxide.
For a basic solid product, heat the solution gently until it thickens to a syrupy consistency with a density around 1.26 to 1.28 (noticeably heavier than water). At that point, increase heat gradually. Industrial processes bring the temperature to around 120–130°C for the initial evaporation, then raise it further to 320–360°C until the material becomes fully transparent and molten. After cooling for several hours, you get a solid mass of zinc chloride.
Producing truly anhydrous (water-free) zinc chloride requires more extreme measures. Industrial methods involve heating to 550–600°C under a flow of inert gas like nitrogen or argon, which carries away residual moisture and hydrochloric acid fumes. The temperature is then pushed to 750–900°C (above zinc chloride’s boiling point of 732°C) to vaporize the zinc chloride, which is collected as it condenses. This is not practical for most home or workshop settings.
Safety Considerations
Zinc chloride is corrosive. It causes severe skin burns and serious eye damage on contact, and its dust or fumes irritate the respiratory tract. The workplace exposure limit for zinc chloride fumes is just 1 mg/m³, which is an extremely small amount. Wear nitrile gloves, safety goggles (not just glasses), and work in a ventilated space.
The synthesis itself adds two more hazards. Hydrochloric acid fumes are irritating and corrosive to breathe, and the hydrogen gas produced during the zinc-metal method is flammable and can ignite or explode if it accumulates in an enclosed space. Always work outdoors or under a fume hood, and never near sparks or open flame.
Zinc chloride is also highly toxic to aquatic life, with long-lasting environmental effects. Never pour excess solution down a drain or into a storm sewer. Leftover solid should be collected in a sealed container. In many jurisdictions, zinc chloride waste is classified as hazardous and requires proper disposal through your local environmental agency.
Common Uses for Homemade Zinc Chloride
The most popular reason people make zinc chloride at home is soldering flux. A zinc chloride solution applied to metal surfaces before soldering cleans oxides and helps solder flow and bond. The solution you get after filtering (before any evaporation) is ready to use as flux. Some formulations add a small amount of ammonium chloride to improve performance, but straight zinc chloride solution works on its own for copper, steel, and galvanized surfaces.
Zinc chloride is also used as a wood preservative, a catalyst in certain chemical reactions, and in textile processing. For flux and wood treatment, the aqueous solution is all you need. Only specialized chemistry applications require the anhydrous solid form.