Soda Ash Dense or sodium carbonate dense (also known as washing soda), Na2CO3 is a sodium salt of carbonic acid. It most commonly occurs as a crystalline heptahydrate, which readily effloresces to form a white powder, the monohydrate. Soda Ash Dense is domestically well known for its everyday use as a water softener. It has a cooling alkaline taste, and can be extracted from the ashes of many plants. It is synthetically produced in large quantities from table salt and limestone in a process known as the Solvay process.

The manufacture of glass is one of the most important uses of Soda Ash Dense. When it is combined with silica (SiO2) and calcium carbonate (CaCO3) and heated to very high temperatures, then cooled very rapidly, glass is produced. This type of glass is known as soda lime glass.

Soda Ash Dense is also used as a relatively strong base in various settings. For example, Soda Ash Dense is used as a pH regulator to maintain stable alkaline conditions necessary for the action of the majority of developing agents. It is a common additive in municipal pools used to neutralize the acidic effects of chlorine and raise pH.

In cooking, it is sometimes used in place of sodium hydroxide for lying, especially with German pretzels and lye rolls. These dishes are treated with a solution of an alkaline substance in order to change the pH of the surface of the food and thus improve browning.

In taxidermy, Soda Ash Dense added to boiling water will remove flesh from the skull or bones of trophies to create the "European skull mount" or for educational display in biological and historical studies.

In chemistry, it is often used as an electrolyte. This is because electrolytes are usually salt-based, and Soda Ash Dense acts as a very good conductor in the process of electrolysis. Additionally, unlike chloride ions which form chlorine gas, carbonate ions are not corrosive to the anodes. It is also used as a primary standard for acid-base titrations because it is solid and air-stable, making it easy to weigh accurately.

In domestic use, it is used as a water softener during laundry. It competes with the ions magnesium and calcium in hard water and prevents them from bonding with the detergent being used. Without using washing soda, additional detergent is needed to soak up the magnesium and calcium ions. Called Washing Soda, Soda crystals or Sal Soda in the detergent section of stores, it effectively removes oil, grease, and alcohol stains. Sodium carbonate is also used as a descaling agent in boilers such as found in coffee pots, espresso machines, etc.

In dyeing with fiber-reactive dyes, Soda Ash Dense is used to ensure proper chemical bonding of the dye with the fibers, typically before dyeing (for tie dyes), mixed with the dye (for dye painting), or after dyeing (for immersion dyeing).

Soda Ash Dense is used by the brick industry as a wetting agent to reduce the amount of water needed to extrude the clay.[citation needed]

In casting, it is referred to as "bonding agent" and is used to allow wet alginate to adhere to gelled alginate.

Soda Ash Dense is used in toothpastes, where it acts as a foaming agent, an abrasive, and to temporarily increase mouth pH.

Soda Ash Dense is used to create the photo process known as reticulation.