Which of the following statements with regard to the reaction given below are correct? CaO(s) + H₂O(l) → Ca(OH)₂ (aq) + Heat 1. Quicklime is used for whitewashing of walls. 2. The solution of slaked lime is used for whitewashing of walls. 3. CaO reacts slowly with CO₂ in air to form a thin layer of CaCO₃ on walls. 4. Calcium hydroxide called ‘slaked lime’ is an inorganic compound. Select the correct answer using the code given below.
- (a)1 and 4 only
- (b)2 and 4
- (c)2 and 3
- (d)1, 3 and 4
Correct — B, 2 and 4. The reaction printed in the stem is quicklime taking up water to give slaked lime, with heat released. Statement 2 is the one that matches practice: the wall is coated with a suspension of slaked lime, calcium hydroxide, in water, and that is what whitewash is. Statement 4 is a plain naming fact — calcium hydroxide, the common name of which is slaked lime, is an inorganic compound, having no carbon-hydrogen framework. Statement 1 names the wrong substance, since quicklime is the starting material and is slaked before it goes anywhere near a wall. Statement 3 also names the wrong substance: it is the slaked lime on the wall, not the calcium oxide, that reacts slowly with atmospheric carbon dioxide to leave the thin shiny film of calcium carbonate.
- (a)1 and 4 only — Keeps the true statement 4 but pairs it with statement 1, which puts quicklime on the wall. Quicklime is caustic and reacts violently with water, so it is slaked first; the reaction in the stem is that very step.
- (c)2 and 3 — Gets the whitewashing right and then hands the carbon-dioxide reaction to the wrong compound. What is spread on the wall after slaking is Ca(OH)2, and it is Ca(OH)2 plus CO2 that gives CaCO3 and water.
- (d)1, 3 and 4 — Combines both of the substitutions the item is testing, keeping calcium oxide in both statement 1 and statement 3 while dropping the only accurate account of whitewashing.
Whitewashing is a two-stage chemistry spread over a day. Quicklime, calcium oxide, is first slaked with water to give calcium hydroxide, and that reaction gives out enough heat to make the water hiss — the equation the paper prints. The calcium hydroxide is then suspended in water and brushed onto the wall. Over the next two or three days it reacts with carbon dioxide from the air to form calcium carbonate, and it is that hard, thin carbonate film that gives a whitewashed wall its shine. Written out, Ca(OH)2 plus CO2 gives CaCO3 plus H2O — the same reaction that turns limewater milky in a school test for carbon dioxide.
Both false statements are made false by a single substitution, calcium oxide standing where calcium hydroxide belongs, and that is the habit the item is drilling: at each stage, name the compound actually present. Quicklime exists only before the water is added; from the moment it is slaked, everything that happens on the wall involves the hydroxide. It is fair to note that calcium oxide does react with carbon dioxide in its own right to give calcium carbonate; what makes statement 3 wrong is the claim that this is what happens on a whitewashed wall, where no quicklime remains. Statement 4 is included as a free mark, and it is worth reading carefully rather than dismissing: inorganic simply means the compound is not built on a carbon-hydrogen skeleton, and calcium hydroxide is not.
- Quicklime is calcium oxide, CaO; slaked lime is calcium hydroxide, Ca(OH)2; the change from one to the other is exothermic.
- Whitewash is a suspension of slaked lime in water, and the paint name for it is limewash.
- The shine appears over two to three days as Ca(OH)2 on the wall reacts with atmospheric CO2 to form a thin film of CaCO3.
- The same reaction is the classroom test for carbon dioxide — the gas turns limewater milky by precipitating calcium carbonate.
- Slaked lime goes by several trade names, among them hydrated lime and builders' lime.
Track which compound is actually present at each stage and both false statements give themselves away.
- Letting quicklime stand in for slaked lime, which is what makes both false statements here false.
- Assuming the shine on a whitewashed wall is the lime itself rather than the carbonate that forms on it.
- Skipping statement 4 as trivial and losing the mark it was put there to give.
As a statements item on the whitewashing sequence, as a fill-in-the-compounds item naming the gas and the film formed, or as a straight equation-recognition item.
CDS_GK_2021_I_Q1162021Calcium oxide reacts vigorously with water to produce slaked lime whose solution is used for white-washing walls. This slaked lime reacts with component (A) in air to form a thin layer of component (B) to give a shiny finish. What are the components (A) and (B)?
- (a) A – O2 ; B – CaCO3
- (b) A – CO2 ; B – Ca(OH)2
- (c) A – O2 ; B – Ca(OH)2
- (d) A – CO2 ; B – CaCO3
Answer(d) A – CO2 ; B – CaCO3
The same sequence spelled out correctly two years earlier. There the paper states that it is the slaked lime that meets carbon dioxide and leaves calcium carbonate; here it quietly writes CaO in place of Ca(OH)2 and asks whether you notice.
- practice — not a real PYQ
The heat evolved when quicklime is added to water makes the reaction an example of
- (a)an endothermic reaction
- (b)an exothermic reaction
- (c)a displacement reaction
- (d)a reduction reaction
Answer(b) an exothermic reaction — slaking releases enough heat to make the water hiss and steam, which is why the stem writes 'Heat' on the product side.
- practice — not a real PYQ
Limewater turns milky when carbon dioxide is passed through it because of the formation of
- (a)calcium oxide
- (b)calcium carbonate
- (c)calcium bicarbonate
- (d)calcium chloride
Answer(b) calcium carbonate — the insoluble carbonate precipitates out, and the same reaction gives a whitewashed wall its shiny finish.