Properties, Preparations, and Applications of Sodium Compounds

Sodium Bicarbonate ($NaHCO_3$)

Also known as baking soda; a white crystalline solid sparingly soluble in water that produces a weakly basic solution via anionic hydrolysis and forms as an intermediate in the Solvay ammonia soda process.

Thermal Decomposition of Sodium Bicarbonate

Upon heating, solid $NaHCO_3$ decomposes to yield anhydrous sodium carbonate, water, and carbon dioxide: $2NaHCO_3 \rightarrow Na_2CO_3 + H_2O + CO_2\text{ (g)}$.

Indicator Behavior of $NaHCO_3$ Solution

Due to weak basicity from anionic hydrolysis, an aqueous solution of sodium bicarbonate turns methyl orange yellow but remains colourless with phenolphthalein.

Standard Baking Powder Composition

A leavening mixture consisting of sodium bicarbonate ($NaHCO_3$), primary calcium phosphate ($Ca(H_2PO_4)_2$) as the acidic component, and starch as an inert filler.

Role of Sodium Aluminium Sulphate ($NaAl(SO_4)_2$)

Used at approximately $20\%$ in combination baking powder as a retarding agent to slow down the reaction rate and ensure a slower, more even liberation of $CO_2$ gas.

Hygroscopic Behavior of Commercial Salt

Pure sodium chloride ($NaCl$) is neither deliquescent nor hygroscopic; commercial salt absorbs atmospheric moisture solely due to the presence of deliquescent impurities like $CaCl_2$ and $MgCl_2$.

Purification of $NaCl$ via Common Ion Effect

Passing hydrogen chloride ($HCl$) gas into a saturated solution of crude sodium chloride dramatically increases $Cl^-$ concentration, causing pure $NaCl$ to salt out while the more soluble $CaCl_2$ and $MgCl_2$ remain dissolved.

Salt Cake

Anhydrous sodium sulphate ($Na_2SO_4$), obtained as an industrial by-product of hydrochloric acid manufacturing via the reaction $2NaCl + H_2SO_4 \rightarrow Na_2SO_4 + 2HCl$.

Glauber's Salt ($Na_2SO_4\cdot 10H_2O$)

Sodium sulphate decahydrate, which crystallizes out when an aqueous solution of sodium sulphate is cooled below $32^\circ\text{C}$. Cooling below $12^\circ\text{C}$ yields the heptahydrate ($Na_2SO_4\cdot 7H_2O$).

Efflorescence of Glauber's Salt

When exposed to dry air, Glauber's salt spontaneously loses its water of crystallization to convert into anhydrous sodium sulphate: $Na_2SO_4\cdot 10H_2O \rightarrow Na_2SO_4 + 10H_2O$.