Description of experiment
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Needed compounds:
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sulphuric acid : H2SO4
sodium hydroxide : NaOH
vanadium pentoxide : V2O5
Class:
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elem=V
condensation
acid/base
Summary:
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Vanadium (V) builds different kinds of poly-vanadates, with colors ranging
from colorless to deep orange/red.
Description:
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Add some solid NaOH and solid V2O5 to some water: The NaOH quickly dissolves,
the V2O5 does not dissolve.
Boil gently: Now the V2O5 also dissolves. The resulting liquid becomes clear
and colorless when a large excess amount of NaOH is used. With less NaOH,
the liquid becomes pale yellow/brown and clear.
Add a slight excess amount of dilute sulfuric acid (appr. 15% by weight):
The liquid turns intense orange/red, but on shaking it turns intense yellow.
On cooling down, an orange precipitate is formed, which floats through the
liquid as orange/red flakes.
Add a little more sulfuric acid (appr. 15% by weight): The precipitate
dissolves, the liquid becomes clear again and it becomes yellow. The yellow
color is due to the presence of [VO2]+ and condensation products.