Science (086) — CBSE Class 10 board question

(i) NaOH by Chlor-Alkali Process:

When electricity is passed through aqueous NaCl solution (brine), it decomposes to give NaOH, chlorine gas, and hydrogen gas:

$$2\text{NaCl(aq)} + 2\text{H}_2\text{O(l)} \xrightarrow{\text{electricity}} 2\text{NaOH(aq)} + \text{Cl}_2\text{(g)} + \text{H}_2\text{(g)}$$

(ii) Baking Soda (NaHCO₃):

Baking soda is prepared by passing CO₂ through a concentrated solution of sodium chloride saturated with ammonia (Solvay process):

$$\text{NaCl} + \text{H}_2\text{O} + \text{CO}_2 + \text{NH}_3 \rightarrow \text{NH}_4\text{Cl} + \text{NaHCO}_3$$

(iii) Plaster of Paris (CaSO₄·½H₂O):

It is prepared by heating gypsum (CaSO₄·2H₂O) at 373 K:

$$\text{CaSO}_4 \cdot 2\text{H}_2\text{O} \xrightarrow{373\text{ K}} \text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O} + \frac{3}{2}\text{H}_2\text{O}$$

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• These three equations are directly from the CBSE Class 10 Chapter on Acids, Bases and Salts.
• Chlor-alkali process is so named because products are chlorine (chlor) and alkali (NaOH). The electrolysis equation must be balanced and include water as a reactant.
• Baking soda: The Solvay process equation is standard; NaHCO₃ precipitates out as it is less soluble.
• Plaster of Paris: Remember it is heated at exactly 373 K (not higher), and the formula CaSO₄·½H₂O (hemihydrate) must be written correctly — examiners check this.