What is the primary difference between washing soda and baking soda?

The primary difference lies in their chemical composition and primary uses. Washing soda is sodium carbonate decahydrate ($\text{Na}_2\text{CO}_3 \cdot 10\text{H}_2\text{O}$), used mainly for cleaning, laundry, and water softening due to its strong alkalinity. Baking soda is sodium bicarbonate ($\text{NaHCO}_3$), used in baking as a leavening agent (releasing $\text{CO}_2$ upon heating or reaction with acid) and as a mild antacid. Baking soda is less alkaline and decomposes at a lower temperature than washing soda.

Why is the Solvay process considered an economical method for producing sodium carbonate?

The Solvay process is economical primarily because it regenerates ammonia, one of its key reactants. Ammonium chloride, a byproduct, is reacted with calcium hydroxide (derived from limestone, another cheap raw material) to produce ammonia, which is then recycled back into the process. This minimizes the consumption of expensive ammonia and reduces waste, making the overall production cost-effective. The main byproduct, calcium chloride, has some industrial uses but is often discarded.

What is 'dead burnt plaster' and why is it undesirable?

Dead burnt plaster is anhydrous calcium sulfate ($\text{CaSO}_4$) formed when gypsum ($\text{CaSO}_4 \cdot 2\text{H}_2\text{O}$) is heated to a very high temperature (above $200^circ\text{C}$), much higher than the $120^circ\text{C}$ required for Plaster of Paris. At these high temperatures, all water of crystallization is expelled, and the resulting anhydrous $\text{CaSO}_4$ loses its ability to rehydrate and set into a hard mass. This makes it useless for applications requiring setting properties, such as surgical casts or construction materials.

Explain the 'slaking of lime' reaction.

Slaking of lime refers to the exothermic reaction between quicklime ($\text{CaO}$, calcium oxide) and water ($\text{H}_2\text{O}$) to form slaked lime ($\text{Ca(OH)}_2$, calcium hydroxide). The reaction is: $\text{CaO(s)} + \text{H}_2\text{O(l)} \rightarrow \text{Ca(OH)}_2\text{(s)}$. This process releases a significant amount of heat, causing the water to boil and the quicklime lumps to crumble into a fine powder. Slaked lime is sparingly soluble in water, forming lime water or milk of lime.

How is the alkalinity of sodium carbonate explained?

Sodium carbonate ($\text{Na}_2\text{CO}_3$) is a salt of a strong base ($\text{NaOH}$) and a weak acid ($\text{H}_2\text{CO}_3$). When dissolved in water, it undergoes hydrolysis. Specifically, the carbonate ion ($\text{CO}_3^{2-}$) reacts with water molecules to produce bicarbonate ions ($\text{HCO}_3^-$) and hydroxide ions ($\text{OH}^-$): $\text{CO}_3^{2-} + \text{H}_2\text{O} \rightleftharpoons \text{HCO}_3^- + \text{OH}^-$. The presence of these hydroxide ions in the solution is responsible for its alkaline nature, giving it a pH greater than 7.

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Some Important Compounds

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Version 1Updated 22 Mar 2026

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Some important compounds of s-block elements are those derived from Group 1 (alkali metals) and Group 2 (alkaline earth metals), primarily sodium, potassium, magnesium, and calcium. These compounds play crucial roles in industrial processes, biological systems, and everyday life. Their significance stems from the high reactivity of s-block metals, which readily form ionic compounds with characteri…

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The s-block elements, particularly sodium and calcium, form several compounds crucial for industry and daily life. Sodium carbonate ( $\text{Na}_2\text{CO}_3 \cdot 10\text{H}_2\text{O}$ ), or washing soda, is produced by the Solvay process and used in glass, soap, and water softening.

Sodium bicarbonate ( $\text{NaHCO}_3$ ), or baking soda, is an intermediate in the Solvay process, used as a leavening agent and antacid, decomposing to release $\text{CO}_2$ upon heating. Sodium hydroxide ( $\text{NaOH}$ ), or caustic soda, is made via the Castner-Kellner electrolytic process and is a strong base used in soap, paper, and petroleum refining.

Calcium oxide ( $\text{CaO}$ ), or quicklime, is formed by heating limestone ( $\text{CaCO}_3$ ) and reacts exothermically with water to form calcium hydroxide ( $\text{Ca(OH)}_2$ ), or slaked lime. Both are vital in cement, construction, and agriculture.

Calcium carbonate exists as limestone, marble, and chalk, decomposing to quicklime on heating. Plaster of Paris ( $\text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O}$ ) is prepared by carefully heating gypsum ( $\text{CaSO}_4 \cdot 2\text{H}_2\text{O}$ ) and is known for its setting property, used in medical casts and construction.

Understanding their preparation, properties, and uses is key for NEET.

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Key Concepts

Solvay Process Reaction Sequence

The Solvay process is a series of interconnected reactions. It starts with ammoniated brine reacting with…

Reactions of Slaked Lime with Carbon Dioxide

Slaked lime ( $\text{Ca(OH)}_2$ ) is used to detect $\text{CO}_2$ . When $\text{CO}_2$ is passed through lime…

Setting of Plaster of Paris

Plaster of Paris ( $\text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O}$ ) sets into a hard mass when mixed with…

Washing Soda: —

\text{Na}_2\text{CO}_3 \cdot 10\text{H}_2\text{O}

. Solvay process. Glass, soap, water softener. Efflorescent.

Baking Soda: —

\text{NaHCO}_3

. From Solvay. Antacid, baking. Decomposes to

\text{CO}_2

on heating.

Caustic Soda: —

\text{NaOH}

. Castner-Kellner process. Strong base. Soap, paper, bauxite purification.

Quicklime: —

\text{CaO}

. From

\text{CaCO}_3

calcination. Reacts with

\text{H}_2\text{O}

(slaking) to

\text{Ca(OH)}_2

Slaked Lime: —

\text{Ca(OH)}_2

. From

\text{CaO} + \text{H}_2\text{O}

. 'Milk of lime'. Turns milky with

\text{CO}_2

, clears with excess

\text{CO}_2

Limestone: —

\text{CaCO}_3

. Natural form. Decomposes to

\text{CaO} + \text{CO}_2

on heating.

Plaster of Paris (POP): —

\text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O}

. From gypsum (

\text{CaSO}_4 \cdot 2\text{H}_2\text{O}

) at

120^circ\text{C}

. Sets with

\text{H}_2\text{O}

Dead Burnt Plaster: — Anhydrous

\text{CaSO}_4

. Overheated gypsum, loses setting property.

Thermal Stability: —

\text{Li}_2\text{CO}_3

least stable alkali carbonate.

\text{BeCO}_3

least stable alkaline earth carbonate. Stability increases down Group 2.

For Calcium compounds: Quicklime Slakes Cool People.

Quicklime (

\text{CaO}

) reacts with water to Slake, forming Calcium hydroxide (

\text{Ca(OH)}_2

Plaster of Paris (

\text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O}

) is made from gypsum.

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