Sodium hydroxide
General
Chemical formula	NaOH
Appearance	white solid
CAS number	1310-73-2
Physical
Molecular weight	40.0 amu
Melting point	596 K (323 °C)
Boiling point	1663 K (1390 °C)
Density	2.1 g/cm3
Crystal structure	?
Solubility	111 g/100 g of water
Thermochemistry
ΔfH0gas	-197.76 kJ/mol
ΔfH0liquid	-416.88 kJ/mol
ΔfH0solid	-425.93 kJ/mol
S0gas, 1 bar	228.47 J/mol·K
S0liquid, 1 bar	75.91 J/mol·K
S0solid	64.46 J/mol·K
Safety
Ingestion	May cause severe and permanent damage to the gastrointestinal system.
Inhalation	Irritation for low exposures, may be harmful or fatal in higher doses.
Skin	Dangerous. Symptoms range from mild irritation to severe ulcers.
Eyes	Dangerous. May cause burns and damage to cornea or conjuctiva .
SI units and standard conditions used unless otherwise stated. Disclaimer and references

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 unclogging stuck 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 inflammables (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 availalable 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 more used than potassium hydroxide because it dissolves in methanol much more easily and costs less, especially as less is needed for the same results.

Another alternative is sodium silicate.

Usage in cooking

Lye is also used for preparing various foodstuffs; examples are the Norwegian delicacy known as Lutefisk, which is basically cod jellied in lye, pretzels, hominy and German lye rolls. Olives are often soaked in lye.