Sodium hydroxide (NaOH), also known as caustic soda or lye in North America, is a caustic metallic base used in industry (mostly as a strong chemical base) in the manufacture of paper, textiles, and detergents.

Sodium hydroxide is occasionally used in the home as an agent for unblocking drains, but it is highly caustic and has a high danger of causing chemical burns, permanent injury or scarring, and blindness, due to its high reactivity. Therefore, it should be stored separately.

When sodium hydroxide reacts with water and fluids, it can become hot enough to cause fires. For this reason, it is important to have the proper type of chemical fire extinguisher on hand before working with sodium hydroxide. Store NaOH in an airtight container to prevent it from absorbing water and CO2 from the air. It can create enough heat to ignite flammables (such as alcohols), so add slowly in biodiesel processors.

Sodium hydroxide is manufactured by electrolysis of an aqueous solution of sodium chloride. It is a by-product of the process that is used to make chlorine.

Both NaOH and KOH are commonly called "lye" in North America, which can lead to some confusion. However, most commercially available lye is NaOH.

Lye is also a main ingredient in the making of soap. NaOH is now most commonly used for this, but traditionally KOH was used because it was easier to obtain.

Sodium hydroxide solution will leave a yellow stain on fabric and paper.

Biodiesel

For biodiesel, sodium hydroxide is used as a catalyst. This only works with anhydrous sodium hydroxide, because water and lye would turn biodiesel into soap (saponification).

It is used more often than potassium hydroxide because it dissolves in methanol much more easily and costs less, especially as a smaller quantity is needed for the same results.

Another alternative is sodium silicate.

Food preparation

Food uses of lye include washing or chemical peeling of fruits and vegetables, chocolate and cocoa processing, caramel color production, poultry scalding, soft drink processing, and thickening ice cream. Olives are often soaked in lye to soften them, while pretzels and German lye rolls are glazed with a lye solution before baking to make them crisp.

Lye is used to make the Scandinavian delicacy known as Lutefisk (from "lutfisk", which directly translated to English means "lye fish"; basically cod jellied in lye). Hominy is dried maize kernels reconstituted by soaking in lye-water.