Lavoisier's Mercury Trap

Why it matters

Lavoisier heated mercury in a sealed bell jar for twelve days, watching the mercury level rise as the metal turned red. He proved that only one-fifth of air is 'active' enough to support combustion.

The idea in plain words

Balance check: confirm that two mercury atoms combine with one oxygen molecule to form two units of red mercury oxide.

Predict first

Why did the mercury level rise to exactly one-fifth of the jar's volume?

Only oxygen reacts with mercury under these conditions. Since oxygen makes up 1/5th of air, its removal leaves a partial vacuum that draws the mercury up by that fraction.

• Mercury simply expanded when heated, pushing the level up
• Oxygen (1/5th of air) reacted with mercury and was removed from the gas phase — correct
• Nitrogen dissolved into the mercury
• The air turned into water vapor

What you do

Heat the silvery mercury. Watch it transform into red powder as it consumes the invisible oxygen.

Check yourself

What is the chemical equation for the reaction observed?

The balanced equation is 2Hg + O₂ → 2HgO, representing a combination reaction.

What visual change confirms the reaction occurred?

Mercury (silvery) combines with oxygen to form Mercury(II) oxide (red).

What fraction of air is 'inactive' (does not react with mercury) in this experiment?

Nitrogen makes up 4/5th of air and is inactive in this reaction, leaving the volume unchanged.

• Hg + O₂ → HgO₂
• 2Hg + O₂ → 2HgO — correct
• Hg + O → HgO
• Silvery liquid turns into red powder — correct
• Red powder turns into silvery liquid
• The liquid bubbles vigorously
• 1/5th
• 4/5th — correct
• None of it