Blue clouds

Blue clouds appear when crystals of orange smoke touch the solution surface.

Scientific name: Dichromate ions react with hydrogen peroxide in acidified solution forming blue CrO5.

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Chemistry experiment 36 - Blue clouds of a chromium peroxo complex

by koen2all

Safety

Wear eye protection goggles and gloves. The second part of the experiment should be performed in a fume hood.

Always follow general safety recommendations. Please note that conducting chemistry experiments you must comply with the relevant legal procedures in your country.

Reaction formula

K2Cr2O7 + H2SO4 + H2O2 → 2CrO5 + K2SO4 + 5H2O

2CrO5 + 7H2O2 + 3H2SO4 → Cr2(SO4)3 + 10H2O + 7O2

K2Cr2O7 + 4NaCl + 6H2SO4 → 2CrO2Cl2 + 2KHSO4 + 4NaHSO4 +3H2O

CrO2Cl2 + 2H2O → H2CrO4 + 2HCl

H2CrO4 + H2SO4 + H2O2 → 2CrO5 + H2SO4 + 5H2O

Step-by-step instruction

WARNING! This experiment is dangerous! You should NOT perform this at home. ONLY carry out this experiment, if you are a trained chemist, and you understand local safety and legal requirements, which are required to perform such experiments

  1. Prepare water solution of hydrogen peroxide and sulfuric acid.
  2. Add a few drops of concentrated components to the flask with [200 ml] of water.
  3. Mix them carefully.
  4. Add some crystals of potassium dichromate to the solution.
  5. Observe the formation of blue clouds and oxygen bubbles.
  6. For the second part of the experiment we need to obtain chromil chloride. This part should be performed in a fume hood.
  7. Put some potassium dichromate and sodium chloride into the beaker.
  8. Add some concentrated sulfuric acid.
  9. The orange vapor of chromil chloride is produced.
  10. Take the flask with fresh water solution of hydrogen peroxide and sulfuric acid and pour the gas to it. Chromil chloride is more dense then air, so it can be poured out of the beaker easily.
  11. When orange vapor touches the solution surface it turns to blue clouds.

Scientific background

In this experiment the formation of blue chromium(VI) peroxide(CrO5) is observed. When potassium dichromate (K2Cr2O7) crystals are added to the flask, they react with potassium dichromate as follows:

K2Cr2O7 + H2SO4 + H2O2 → 2 CrO5 + K2SO4 + 5 H2O

Chromium(VI) peroxide(CrO5) is an unstable substance and reacts again with release of oxygen gas.

2CrO5 + 7H2O2 + 3H2SO4 → Cr2(SO4)3 + 10H2O + 7O2

In the second part of the experiment the orange chromil chloride vapor is produced when potassium dichromate and sodium chloride are mixed with concentrated sulfuric acid:

K2Cr2O7 + 4NaCl + 6H2SO4 → 2CrO2Cl2 + 2KHSO4 + 4NaHSO4 +3H2O

When the gas reaches the solution surface it reacts with water forming dichromate ions. Dichromate ions in turn react with hydrogen peroxide and sulfuric acid producing chromium peroxide.

CrO2Cl2 + 2H2O → H2CrO4 + 2HCl

H2CrO4 + H2SO4 + H2O2→ 2 CrO5 + H2SO4 + 5H2O

Danger:
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Difficulty:

Published on 09 March 2015

  • Fire
  • Heating with fire
  • Explosion
  • Poisoned gas
  • Organic
  • Electricity
  • Solution
  • Oxidation reduction
  • Color change
  • Precipitate
  • Gassing
  • Catalyst