chem 問題 緊急！ 後天 exam 答中全取 20 分

The answer is A.

All metal hydroxides are basic, and they can dissolve in (neutralize) acids to form salts and water. Therefore, all of A, B, C and D can dissolve in acids to form salts and water.
Zn(OH)2(s) + 2H+(aq) → Zn2+(aq) + 2H2O(l)
Cu(OH)2(s) + 2H+(aq) → Cu2+(aq) + 2H2O(l)
Fe(OH)2(s) + 2H+(aq) → Fe2+(aq) + 2H2O(l)
Ca(OH)2(s) + 2H+(aq) → Ca2+(aq) + 2H2O(l)

A few metal hydroxides, including zinc hydroxide (A), can dissolve in alkalis to form salts and water. These metal hydroxides are known as amphoteric hydroxide.
Zn(OH)2(s) + 2OH-(aq) → ZnO22-(aq) + 2H2O(l)

Calcium hydroxide can also dissolve in alkalis, but actually it only dissolve in the water of the alkali. There is no reaction. Therefore, no salt or water is formed.
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你都明白點解唔係B/C 咁我剩係解點解係A唔係D la.
答案D:因為Ca(OH)2 係soluble in water

同alkali混合後都係溶o係水 so唔叫有反應.

答案A:當Zn(OH)2 同excess alkali 混合
Zn(OH)2會變成soluble in water.