CDS (I) 2021 General Knowledge · Question 116 of 120

Correct — D, A – CO2 ; B – CaCO3. Two reactions run in sequence. Quicklime, calcium oxide, reacts vigorously with water and gives slaked lime, calcium hydroxide, releasing a great deal of heat as it does so: CaO + H₂O → Ca(OH)₂. That suspension is what is brushed on the wall. Over the next day or two the coating takes up carbon dioxide from the air and turns into calcium carbonate: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O. So component A is carbon dioxide and component B is calcium carbonate, which is the hard, shiny film that gives a whitewashed wall its finish. Calcium carbonate is the same substance as limestone and marble, which is why the surface sets firm rather than staying powdery.

• (a)A – O2 ; B – CaCO3 — Gets the product right and the reactant wrong. Oxygen does not react with slaked lime; the carbon in the calcium carbonate has to come from somewhere, and the only source in air is carbon dioxide.
• (b)A – CO2 ; B – Ca(OH)2 — Gets the reactant right and the product wrong. Calcium hydroxide is the slaked lime that was already on the wall — naming it as the product means nothing has changed.
• (c)A – O2 ; B – Ca(OH)2 — Wrong on both counts, and it has the wall reacting with oxygen to produce the very compound it started as.

Whitewashing is a two-step piece of chemistry that a student can follow in a courtyard. Calcium oxide, made by roasting limestone, reacts exothermically with water to give calcium hydroxide, which is only slightly soluble and forms a milky suspension. Painted on and left, that suspension slowly absorbs atmospheric carbon dioxide and hardens back into calcium carbonate — the reaction that also turns lime water milky in the school test for carbon dioxide.

The options are a two-by-two grid over the reactant and the product, so a candidate who can fix either half can eliminate two options at once. The quickest fix is the carbon atom: a product named CaCO₃ has to get its carbon from a carbon-bearing gas, so A must be CO₂. The second half then follows, since the point of the passage is that a thin new layer forms. It is worth noting the loop the chemistry makes — limestone is heated to make quicklime, quicklime is slaked to calcium hydroxide, and the coating turns back into calcium carbonate on the wall. The same reaction that hardens the whitewash is the one used every year to test a gas for being carbon dioxide.

• CaO + H₂O → Ca(OH)₂, an exothermic reaction that releases enough heat to make the water hiss.
• Ca(OH)₂ + CO₂ → CaCO₃ + H₂O is the slow reaction that hardens a whitewashed wall.
• Calcium carbonate is the substance of limestone and marble, and gives the finished coat its shine.
• The same reaction turns lime water milky and is the standard laboratory test for carbon dioxide.
• Quicklime is made industrially by heating limestone strongly, which drives off carbon dioxide.

• Naming oxygen as the gas because the process happens in air.
• Naming calcium hydroxide as the product, when it is what was applied in the first place.
• Confusing quicklime with slaked lime — the first is the oxide, the second the hydroxide.

As a fill-in-the-components item like this one, as a choose-the-correct-equation item, or as a statement set on quicklime and slaked lime.

• practice — not a real PYQ

  Lime water 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 hydroxide

  Answer(b) calcium carbonate — the insoluble white solid that clouds the solution, and the same compound that hardens a whitewashed wall.

• practice — not a real PYQ

  The reaction of quicklime with water is

  • (a)endothermic
  • (b)exothermic
  • (c)neither exothermic nor endothermic
  • (d)a displacement reaction

  Answer(b) exothermic — a large amount of heat is released, enough to make the water hiss and steam as the slaked lime forms.