Sodium carbonate (washing soda) is used in:

Sodium carbonate (washing soda) is used in the glass, soap, and paper industry. 

While prepare paper from pulp, the pulp is processed number of times in mill. Due to which some important chemicals are lost from the pulp. Washing soda and caustic soda is added to the pulp to compensate for chemicals that are being lost in the pulp.

Washing soda is used as an ingredient for manufacturing glass.In glass industry a mixture of washing soda, silicon dioxide and calcium carbonate are heated at a very high temperature. Thereafter, it is cooled down very quickly which results in the formation of glass.

Washing Soda is used as cleansing agent for domestic purposes like washing clothes. Also, it is used as a component of many dry soap powders.