Making sodium, potassium and ammonium salts
1 mark · EasyState why you cannot use the 'add excess solid, then filter' method to prepare sodium salts from acid and sodium hydroxide.
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Because all sodium compounds are soluble in water, so the excess solid dissolves and cannot be filtered off.
Also accept: Because sodium compounds are all soluble, so there is no solid left to filter.; Because any excess sodium hydroxide would just dissolve, so you can’t see or remove the extra.
Common mistakes: Referring to reaction speed instead of solubility.; Saying 'you cannot see the endpoint' (not the key reason for this step).
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Marking points
B1 · 1 mark · Correctly states that sodium compounds are soluble and the excess dissolves so cannot be filtered off.
Worked solution
All sodium salts are soluble in water, so any excess sodium hydroxide would dissolve instead of remaining as a solid. This means you cannot filter off the excess and cannot be certain all the acid has reacted.