In everyday chemistry, washing soda is chemically known as which compound?

Introduction / Context:
Many household products have common names that differ from their chemical names. Washing soda is one such substance that appears frequently in school chemistry, in cleaning products, and in water softening. This question tests your ability to link the familiar name washing soda with its correct chemical compound and to distinguish it from related substances such as baking soda and caustic soda.

Given Data / Assumptions:

• The common name given is washing soda.
• The options include several sodium and other metal salts.
• We assume the usual school level convention that washing soda refers to hydrated sodium carbonate.
• We do not need to consider hydrates in the option text, only the base compound name.

Concept / Approach:
In basic inorganic chemistry, certain compounds have traditional names due to long practical use. Washing soda is sodium carbonate, usually present as sodium carbonate decahydrate, written as Na2CO3·10H2O. Baking soda, on the other hand, is sodium bicarbonate, NaHCO3. Caustic soda is sodium hydroxide, NaOH. Identifying washing soda requires remembering this mapping between common names and formulas.

Step-by-Step Solution:
Step 1: Recall that washing soda is used in laundry, for water softening, and in some cleaning agents. Step 2: From standard chemistry, washing soda corresponds to sodium carbonate decahydrate, Na2CO3·10H2O, whose base salt is sodium carbonate. Step 3: Option A, sodium hydroxide, is commonly called caustic soda, not washing soda. Step 4: Option C, sodium bicarbonate, NaHCO3, is called baking soda, widely used in baking and as an antacid. Step 5: Potassium carbonate and calcium carbonate have their own uses, but they are not referred to as washing soda in standard nomenclature. Step 6: Therefore, the correct match for washing soda is sodium carbonate.

Verification / Alternative check:
You can verify by thinking about water softening. Washing soda removes hardness by reacting with calcium and magnesium ions, forming insoluble carbonates. The carbonate ion CO3^2 minus is essential in this process, which is present in sodium carbonate. In contrast, sodium bicarbonate is a mild base and is not primarily used as a washing agent. Textbook tables that list common names and formulas also confirm that washing soda is sodium carbonate decahydrate.

Why Other Options Are Wrong:
Sodium hydroxide (NaOH): Known as caustic soda, it is a very strong base and is not called washing soda.
Sodium bicarbonate (NaHCO3): Known as baking soda, used in baking and mild cleaning, but has different composition and properties.
Potassium carbonate (K2CO3): Sometimes called potash, with roles in glass and soap making, but not washing soda.
Calcium carbonate (CaCO3): Main component of limestone and chalk, again with different uses and not referred to as washing soda.

Common Pitfalls:
A common error is to confuse washing soda with baking soda simply because both names contain the word soda and both are sodium salts. Another mistake is to mix up caustic soda with washing soda, since both relate to cleaning. To avoid this, always remember the pairings: washing soda is sodium carbonate, baking soda is sodium bicarbonate, and caustic soda is sodium hydroxide. Learning these mappings thoroughly makes many other exam questions much easier.

Final Answer:
Washing soda is chemically sodium carbonate (Na2CO3).