關於acids and alkalis的簡單化學選擇題

1.
The answer is B.

In solid state, citric acid is molecular (C is incorrect) and contains no ions (D is incorrect). Each citric acid molecule contains 8 hydrogen atoms (A is incorrect), and only 3 of them can be ionized in water to give hydrogen ions. In the absence of water, solid citric acid cannot form hydrogen ions, and it thus cannot react with magnesium (B is correct).


2.
The answer is B.

A is incorrect.
Al reacts with dilute HCl and dissolves.
2Al(s) + 6H+(aq) → 2Al3+(aq) + 3H2(g)
When dilute NaOH is added to the resulting solution, a precipitate of aluminium hydroxide forms.
Al3+(aq) + 3OH-(aq) → Al(OH)3­(s)
Aluminium hydroxide precipitate is SOLUBLE in excess sodium hydroxide solution.

Al(OH)3(s) + OH-(aq) → [Al(OH)4]-(aq)

B is correct.
Ca reacts with dilute HCl and dissolves.
Ca(s) + 2H+(aq) → Ca2+(aq) + H2(g)
When dilute NaOH is added to the resulting solution, a precipitate of calcium hydroxide forms.
Ca2+(aq) + 2OH-(aq) → Ca(OH)2­(s)
Calcium hydroxide precipitate is INSOLUBLE in excess sodium hydroxide solution.


C is incorrect.
Cu does not react with dilute HCl, and thus it does not dissolve in dilute HCl.

D is incorrect.
Zn reacts with dilute HCl and dissolves.
Zn(s) + 2H+(aq) → Zn2+(aq) + H2(g)
When dilute NaOH is added to the resulting solution, a precipitate of zinc hydroxide forms.
Zn2+(aq) + 2OH-(aq) → Zn(OH)2­(s)
Zinc hydroxide precipitate is SOLUBLE in excess sodium hydroxide solution.

Zn(OH)2(s) + 2OH-(aq) → [Zn(OH)4]2-(aq)
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