In Chemistry practical labs, one common method to collect a pure gas is by displacing water in an inverted jar — called Collection over Water (or Downward Displacement of Water). However, this method only works for certain gases.
Hydrogen (H₂), Oxygen (O₂), and Nitrogen (N₂) are the classic gases collected over water because they are nearly insoluble in it.
The 'Collection over Water' method works only when the gas is insoluble or sparingly soluble in water. If a gas dissolves in water, it will simply dissolve into the water rather than displacing it — making collection impossible.
Gases that are highly soluble in water cannot be collected this way:
CO₂ is slightly soluble in water (it forms carbonic acid), but it is **sparingly soluble enough** that it can be collected over water for short periods in a lab — though it is more accurately collected by downward displacement of air.
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