Sodium hydroxide

Sodium hydroxide (NaOH), or caustic soda, is a strong solid base, white and odourless, sold as flakes or beads.

Characteristics

Strong base, very soluble in water; dissolving it releases a lot of heat.
Reacts with fats and oils to form soap (saponification).
Strongly raises the pH of solutions.
Corrosive to skin, eyes and metals such as aluminium, tin and zinc.

Role in the preparation: Acts as the alkali for saponifying oils when making soap at home and for adjusting pH.

Caution: Corrosive product: wear gloves, goggles and long-sleeved clothing, always add it slowly to water and never the reverse, and store it closed away from acids and children.

Sources: NIOSH Pocket Guide to Chemical Hazards - Sodium hydroxide (CDC) · Science ABC - Difference between soaps, shampoos and detergents

Documentary information: this is neither a recommendation for use nor a plant-protection or veterinary claim. Check your country's regulations before any use.