Washing soda is a highly alkaline chemical compound which can be used to remove stubborn stains from laundry. It also has numerous uses around the house, and it is used in a range of industrial applications as well. Washing soda should not be confused with washing powder, which is a powdered soap used as a detergent; it is also not the same thing as baking soda, although the two compounds are closely related.

The chemical formula for washing soda is Na₂CO₃, and it is also known as sodium carbonate. It is a salt of carbonic acid, a chemical which produces a wide range of salts collectively known as carbonates. One common source of washing soda is the ashes of plants; for this reason, it is sometimes called soda ash. Sodium carbonate can also be created from sodium chloride, also known as table salt.

In laundry, washing soda accomplishes several things. The high alkalinity of washing soda helps it act as a solvent to remove a range of stains, and unlike bleach, washing soda does not usually stain. It is also used in detergent mixtures to treat hard water; the washing soda binds to the minerals which make water hard, allowing detergent to foam properly so that clothing will come out clean, without any residue. Sodium carbonate is also used by some textile artists, since it helps dyes adhere to fabric, resulting in deeper penetration and a longer lasting color.

Around the house, washing soda can be used to descale things like coffee machines and bathroom tiles which may accumulate mineral deposits as a result of exposure to hard water. It can also be used to strip floors of wax so that they can be refinished, and for other touch cleaning jobs like scrubbing the stove. You should wear gloves when cleaning with washing soda, because it can cause skin irritation. Incidentally, the best way to treat some types of chemical burn is with baking soda, also called sodium bicarbonate, as it is a buffer and it will neutralize both acids and alkalis. To treat a hydrofluoric acid burn, apply baking soda mixed with water to the site of the burn for several minutes, flush the wound with water, and seek medical attention.

Many markets carry washing soda, typically with other laundry products. Some companies make mixed detergents with washing soda which are specifically formulated for hard water, and you can also find washing soda on your own. Since sodium carbonate can be dangerous in large quantities, make sure to keep washing soda out of the reach of children and pets, and clearly label the container to indicate that it is caustic. It can be harmful to the eyes, cause irritation to the lungs if inhaled, and may cause abdominal pain or vomiting if large doses are swallowed.