Q18

The fact that the pH of the resulting solution changes over time suggests that some reaction involving acids and bases or carbonates have occurred.

Because solid R is gradually added to aqueous solution S, the substance initially present must have been substance S. This is the moment when R has not been added yet.

Since the initial pH of the solution is close to 0, substance S must have been something acidic, like hydrochloric acid.

This means that substance R can be something basic or a random carbonate to neutralise the solution.

However, note the pH at the end. At the neutralisation/equivalence point, the pH drastically goes up to 7, but all of a sudden the pH stops at 7. We wish for the pH to go beyond 7, but then it just stays there.

This is indicative of an oxide which does not dissolve in water (if an oxide dissolves in water, it would have formed soluble hydroxides which are alkaline and have a pH above 7). Remember, a base does not exhibit alkaline properties unless successfully dissolved in water.

So, R is likely an insoluble metal oxide and S is likely hydrochloric acid.

Q19

This is basically a question of knowing the type of oxide possessed by an oxide (acidic, basic, amphoteric, neutral). You must memorise the list for this as there is not much explanation for this at Sec 3 and Sec 4 level.

Copper (II) oxide is a neutral oxide, meaning it does not react well with acids and bases. Sodium hydroxide is no exception to this.

Nitrogen dioxide and phosphorus (V) oxide are acidic oxides (they form nitric acid and phosphoric acid when dissolved in water), meaning that they react well with bases but not acids.

Zinc oxide is an amphoteric oxide, meaning it reacts well with both acids and bases. Sodium hydroxide is no exception.

So, copper (II) oxide is unlikely to react with aqueous sodium hydroxide.

Copper(II) oxide is a basic oxide, not neutral oxide.

Neutral oxides include N2O , CO, NO MnO2, and even water (H2O)

For qn 18, it should be specified that metal oxides react with water / dissolve to form a base and not oxides as a whole

i dont really understand the part about insoluble oxide

From the options , R is either a soluble metal oxide or insoluble metal oxide.

If R is a soluble metal oxide, it will dissolve in water / react with it to form basic/alkaline solutions of the metal hydroxide.

Eg. Lithium oxide, Li2O reacts with water to form lithium hydroxide, which is basic/alkaline

Li2O + H2O →2 LiOH

This LiOH can dissociate to form the Li+ ion and OH- ions.

LiOH → Li+ + OH-


This would mean after the acid S is fully neutralised by reaction with R, adding more R will just result in a alkaline/basic solution with pH above 7.


But since the pH stays constant at around 7 after the sharp increase (whereby the acid S has been fully neutralised by reaction with R), this suggests that the metal oxide R is insoluble in water and did not dissolve to form a basic/solution.

So adding more R after acid S has been fully neutralised does not further change the pH of the solution.

ohh thx for helping me :)

Wrote the copper (II) oxide one wrongly, I was thinking of something else when I wrote “neutral oxide”

For the O-level syllabus, just remember that all Group I oxides and calcium oxide is soluble in water. These dissolve to produce an alkali (from soluble bases). All other metal oxides are insoluble in water, and thus referred to as insoluble bases. Factually, barium oxide also dissolves in water - good to know for school questions, but the O-level syllabus focusses on the first 20 elements and selected transition elements (Fe, Cu) and others such as Zn, Ag, Au and Pb.