During white-washing of walls, slaked lime reacts slowly with carbon dioxide in air to form a thin layer of calcium carbonate on the walls. Which of the following reactions represents this correctly?
- (a)CaO (s) + CO₂ (g) → CaCO₃ (s)
- (b)CaO (l) + CO₂ (g) → CaCO₃ (s)
- (c)Ca(OH)₂ (l) + CO₂ (l) → CaCO₃ (s) + H₂O (l)
- (d)Ca(OH)₂ (aq) + CO₂ (g) → CaCO₃ (s) + H₂O (l)
Correct — D, Ca(OH)₂ (aq) + CO₂ (g) → CaCO₃ (s) + H₂O (l). Whitewashing starts with quicklime, calcium oxide, which is slaked with water to give calcium hydroxide — slaked lime — and it is a solution of that slaked lime, not the quicklime itself, that is brushed on the wall. Over the next two or three days the calcium hydroxide on the wall reacts slowly with the small amount of carbon dioxide in the air to leave a thin film of calcium carbonate, and it is this film that gives the shiny white finish. Water is the other product, which is why the equation must have something on the right besides the carbonate. Every state symbol in option (d) is right as well: the slaked lime is in aqueous solution, the carbon dioxide comes out of the air as a gas, the calcium carbonate deposited is a solid and the water is liquid. Incidentally the same compound, CaCO₃, is what marble is made of.
- (a)CaO (s) + CO₂ (g) → CaCO₃ (s) — Starts from quicklime rather than slaked lime, so it describes a different reaction from the one in the stem. It is a real combination reaction, and it is the reverse of the thermal decomposition of limestone in a lime kiln — but it is not what happens on a whitewashed wall.
- (b)CaO (l) + CO₂ (g) → CaCO₃ (s) — The same wrong starting material with a wrong state symbol added. Calcium oxide is a solid at ordinary temperatures — it melts only above 2,500 °C — so writing it as a liquid is doubly incorrect.
- (c)Ca(OH)₂ (l) + CO₂ (l) → CaCO₃ (s) + H₂O (l) — The right chemistry with the wrong physical states, which is precisely what the examiner is testing. Slaked lime on a wall is in aqueous solution, so it should be (aq) and not (l), and the carbon dioxide is drawn from the air, so it is a gas and not a liquid.
Three reactions of calcium sit next to one another in the school syllabus and are constantly interchanged. Calcium carbonate heated in a kiln decomposes to calcium oxide and carbon dioxide. Calcium oxide reacts vigorously with water, releasing a great deal of heat, to give calcium hydroxide — this is slaking. Calcium hydroxide in solution then absorbs carbon dioxide to give calcium carbonate again, which is both the whitewashing reaction and the reason lime water turns milky when carbon dioxide is bubbled through it.
The four options are not four different chemistries; they are one chemistry printed four ways, and only one of them survives inspection on both counts — the correct reactants and the correct physical states. So read in two passes. First check the left-hand side: the stem says slaked lime, so any option beginning with CaO is out, which removes (a) and (b) at once. Then check the state symbols on what is left, and (c) fails because it calls an aqueous solution a liquid and a gas a liquid. That leaves (d). Candidates lose this mark by treating (aq) and (l) as interchangeable; the exam clearly does not, and it has asked essentially this same whitewashing reaction in three separate papers.
- Quicklime is calcium oxide, CaO; slaked lime is calcium hydroxide, Ca(OH)₂; the whitewash film is calcium carbonate, CaCO₃.
- Slaking, CaO + H₂O → Ca(OH)₂, is strongly exothermic — the reaction vessel becomes hot to the touch.
- The whitewashing reaction, Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l), takes two to three days to give its shiny finish.
- The same reaction is the lime-water test: carbon dioxide bubbled through lime water turns it milky by precipitating calcium carbonate.
- Marble, limestone and chalk are all forms of calcium carbonate.
- Writing quicklime where the stem says slaked lime; the two are one water molecule apart and the exam checks which one you start from.
- Treating (aq) and (l) as the same thing. A solution of a solid in water is aqueous, not liquid.
- Forgetting that water appears as a product; an equation with only calcium carbonate on the right cannot balance from calcium hydroxide.
As an equation to pick, as a fill-in-the-component item naming reactant A and product B, or as a statements item about the quicklime-and-water reaction.
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 wall and the same two or three days, asked as a fill-in-the-blank instead of an equation. Whoever can name the gas taken from the air and the solid left behind has already answered both items.
CDS_GK_2023_I_Q122023Which 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
Answer(b) 2 and 4
The trap in this paper's options, isolated as a statement six months earlier. Statement 3 there is wrong for exactly the reason options (a) and (b) are wrong here — it is the slaked lime, not the quicklime, that takes up carbon dioxide on the wall.
- practice — not a real PYQ
When carbon dioxide is passed through lime water, the lime water turns milky. The white precipitate formed is
- (a)calcium oxide
- (b)calcium hydroxide
- (c)calcium carbonate
- (d)calcium bicarbonate
Answer(c) calcium carbonate — the same reaction as whitewashing, Ca(OH)₂ + CO₂ → CaCO₃ + H₂O. Passing excess carbon dioxide then dissolves the precipitate as calcium bicarbonate and the milkiness disappears.
- practice — not a real PYQ
The reaction CaO(s) + H₂O(l) → Ca(OH)₂(aq) is best described as
- (a)an endothermic decomposition reaction
- (b)an exothermic combination reaction
- (c)a displacement reaction
- (d)a double displacement reaction
Answer(b) an exothermic combination reaction — two reactants give a single product, and the slaking of quicklime releases so much heat that the vessel becomes hot.