Chem: QA

anpanman SG

Solution Q forms a white ppt with a little aq. NaOH and the white ppt is insoluble in excess aq. NaOH.

When reacted with dilute HNO3 there is no effervescence

What is solution Q? My guess is either CaCO3 or CaCl2. The answer is CaCl2. I believe the clue to differentiate this 2 compounds lies in the word "no effervescence". well, what does that mean actually?

yiha093 SG

wronggggg--

UltimaOnline SG

Solution Q forms a white ppt with a little aq. NaOH and the white ppt is insoluble in excess aq. NaOH.

When reacted with dilute HNO3 there is no effervescence

"efferverscence" refers to the vigorous/rapid evolution/liberation of a gas from solution.

In acids, carbonate ions are protonated to form carbonic acid, which exists in equilibrium with carbon dioxide and water. Hence gaseous carbon dioxide is liberated.

For solution Q, the cation could be Ca2+ or Mg2+. The anion could be Cl- or NO3-. (of course, there are other possible candidates for both cation and anion, but the ones suggested above are the most commonly encountered for 'O' levels.)

Garrick_3658 SG

Simply speaking, effervescence means bubbles of gas. This crosses out CO3^2- which produces CO2 gas upon reaction with an acid.

The cation, as previously mentioned, can be Ca2+ or Mg2+ unless another differentiation test is carried out. I doubt it's in the O level syllabus though.

Unless nowadays questions like to be phrased with hidden tricks, then I assume that HNO3 has a reaction with the white ppt. Hence the anion is mostly likely not to be NO3- since there will be no net change if it is. This leaves Cl-.

UltimaOnline SG

Simply speaking, effervescence means bubbles of gas. This crosses out CO3^2- which produces CO2 gas upon reaction with an acid.

The cation, as previously mentioned, can be Ca2+ or Mg2+ unless another differentiation test is carried out. I doubt it's in the O level syllabus though.

The nitrate(V) ion remains a spectator. The purpose of the nitric(V) acid is to provide protons to react (in an Bronsted-Lowry acid-base neutralization reaction) with the hydroxide ions of the precipitate formed, eg. Ca(OH)2(s) or Mg(OH)2(s).