Question

# When washing soda is heated:A: $CO$ is releasedB: $CO + C{O_2}$ is releasedC: $C{O_2}$ is releasedD: Water vapour is released

Hint: Washing soda is sodium carbonate. It is also called soda ash or soda crystals. It is an inorganic compound and its formula varies for its hydrates. But all forms are white and water soluble.

Complete step by step solution:
As we know washing soda is sodium carbonate. Its molecular formula is $N{a_2}C{O_3}$ . But its formula varies for its hydrates. Hydrates are inorganic salts which have water molecules combined in a definite ratio as an integral part of the crystal. These are either bound to a metal center or crystallized with the metal complex. Sodium carbonate is obtained as three hydrates and one anhydrous salt. Anhydrous means from which water molecules are removed. These forms are as follows:
Sodium carbonate decahydrate $N{a_2}C{O_3}.10{H_2}O$
Sodium carbonate heptahydrate $N{a_2}C{O_3}.7{H_2}O$
Sodium carbonate monohydrate $N{a_2}C{O_3}.{H_2}O$. This is also known as crystal carbonate.
Anhydrous sodium carbonate is also known as calcined soda. It is formed when sodium carbonate hydrates are heated or when sodium hydrogen carbonate is heated.
In this question we have to find out the product which is released on heating washing soda. We know one form of sodium carbonate commonly known as washing soda is available as anhydrous form which is obtained on heating hydrated form. In anhydrous form no water molecule is present but in hydrated form it is present. So it is clear that on heating sodium carbonate water vapour is released. Reaction involved is as follows:
$N{a_2}C{O_3}.10{H_2}O\xrightarrow[\Delta ]{{ - 9{H_2}O}}N{a_2}C{O_3}.{H_2}O\xrightarrow[\Delta ]{{ - {H_2}O}}N{a_2}C{O_3}$

So the answer to this question is option D i.e. water vapour is released.

Note: Since we cannot find the absolute internal energy or enthalpy of gas so we deal with the change of internal energy of a system through different states. There are some processes carried out to change one state of the system to another. such as isothermal(Temperature constant), isobaric(Pressure constant), isochoric(Volume constant), reversible, irreversible etc.