Why Is the Colour of Hydrogen Iodide So Fascinating?

Hydrogen iodide’s colour can be described as a dark purple colour with an almost black tint to it, though the exact colour will vary depending on the lighting and angle at which you are viewing it.

The colour of hydrogen iodide can also be described as reddish brown, but that may lead to confusion between it and iron (III) oxide, which can appear reddish brown as well under certain lighting conditions.

In reality, the colour of hydrogen iodide is closer to the colour of iron (II) oxide than iron (III) oxide.

The colour of hydrogen iodide is fascinating because it changes from purple to green when it reacts with an acid and vice versa. In order to understand why the colour of hydrogen iodide changes, it’s important to know what causes this change in the first place.

Hydrogen iodide is made up of two molecules, hydrogen and iodine, both of which have different properties depending on whether they are part of the solution or not.

Lewis Dot Structure of Hydrogen Iodide

 Hydrogen Iodide

Hydrogen iodide (H-I) is a molecule that consists of one atom each of hydrogen and iodine. It has a molecular weight of 33.04 g/mol, with an empirical formula H-I or HI.
The Iodine atom’s 7 electrons are arranged in three electron pairs, including two from the bond to the Hydrogen atom. There are four orbitals for these 7 electrons: 1s2 2s2 2p5 3s1 3p5.
The orbital corresponding to 3s1 and 3p5 orbitals have a high affinity for electrons, which is why they are filled first by the atomic orbitals.
The orbital corresponding to 2s2 2p5 does not have such a strong attraction for more electrons, which explains why it is empty at this point.
The electron configuration that results from combining these orbitals has a total of two electron pairs, with one in an antibonding orbital. Therefore, H-I has a neutral charge and is therefore classified as an ionic molecule.

The Polarity of Hydrogen Iodide

Color of Hydrogen Iodide

Hydrogen iodide is a compound that belongs to a family of organic compounds called heterocyclic amines. It is composed of one atom of hydrogen, one atom of iodine and one molecule of water.

The molecules are arranged in a linear fashion and are polar because there are some negative ions (from the H-I) and some positive ions (from the H-O). The bonds between these atoms have weak forces which make this compound unstable. In fact, it only lasts for about 10 seconds before decomposing into gaseous particles. In order to maintain its stability and prevent it from reacting with other substances, it needs to be stored under extremely cold conditions.

About Hydrogen Iodide (Colour of Hydrogen Iodide)

Color of Hydrogen Iodide

Iodine (I) is a chemical element with the symbol I and atomic number 53. The second-lightest of all elements, it occurs naturally in many minerals, including feldspars, apatite, and ferrates.
Iodine and its compounds are primarily used for chemical syntheses. These uses include photography, electrical engineering, and chemical analysis.
Hydrogen Iodide, or HI, is an inorganic compound that has a deep purple-pink colour. It’s solid and will dissolve in water, ethanol, ether, and acetone.
Hydrogen iodide has been used for many purposes including as a building block for organic compounds and as an oxidizing agent. The deep purple-pinkish hue that this chemical produces makes it very unique in comparison to other chemicals.

Solutions With Hydrogen Iodide

Hydrogen Iodide, H-I, has a very distinctive colour that sets it apart from other chemicals. Its name comes from two elements, Hydrogen and Iodine.
These two elements are naturally occurring and are found in nature, like underground or in the air. Hydrogen Iodide is not a naturally occurring chemical because it does not occur on its own in nature; it’s created when these two elements react to each other.
It can be made by adding iodine crystals into an acidified solution of ammonia (NH3) and hydrochloric acid (HCl).

The reaction between the ammonia and the hydrochloric acid produces hydrogen chloride gas which reacts with iodine crystals to create hydrogen iodide vapour.

Uses of Hydrogen Iodide

Color of Hydrogen Iodide

Hydrogen iodide gas (HID) has a deep purple-blue colour. When mixed with sulfuric acid, it produces a yellow liquid and when mixed with nitric acid, it produces a red-brown liquid.

It is being used in different industries 

  1. HID is used to create organic compounds and disinfectants.
  2. It’s an effective treatment for patients with chronic kidney disease.
  3. It prevents the oxidation of alcohol in industrial processes.
  4. Industrial uses include the manufacture of nylon and viscose rayon fibres.
  5. Its antiseptic properties make it useful in cleaning medical instruments such as scalpels, trocars, catheters, endoscopes, and dental instruments.
  6. The chemical can be made from sodium chloride or ammonium chloride.
  7. The salt mixture undergoes electrolysis in order to produce the chemical that is then heated until it turns into a vapour and collected in vacuum evaporators.
  8. A blue flame indicates the presence of hydrazine gas while potassium permanganate may be added to intensify this effect.
  9. Hydrogen iodide occurs naturally as a mineral in volcanic areas.

Uses in Organic Chemistry

It has been used in many organic reactions because it has strong oxidizing properties and can remove many functional groups from molecules.

  1. Hydrogen Iodide reacts with sodium sulfite to produce sulfur dioxide which causes irritation to the eyes and upper respiratory tract.
    The chemical equation for that reaction is
    H-I + Na2SO3 → 2NaHSO3 + HIO3.
  2. It has been used to oxidize alcohols into aldehydes or ketones, which could be further oxidized into carboxylic acids. The compound can also reduce carbonyls such as cyanohydrins and esters to their corresponding hydroxyl groups.
  3. Hydrogen Iodide can also be used as a reagent to produce aromatic rings in benzene. The reaction uses Hydrogen Iodide with propyne and produces 3,4-dihydro-2H-pyran, which can be further oxidized into quinoline.
    These reactions are possible because Hydrogen Iodide has a strong oxidizing property.
  4. It can easily remove hydroxyl, amine and sulfonic groups from molecules, and thus it works as an alkylating agent.

Safety, Hazards, Handling, Storage

  • Hydrogen iodide is a yellow-brown gas with a pungent odour.
  • It dissolves in water and can be stored in plastic containers or cylinders.
  • Hydrogen iodide should not come into contact with acids, bases, oxidizing agents, reducing agents, or organic matter due to the potential for explosion.
  • The chemical will also react violently with combustible materials like paper and oil.
  • It can also react explosively with water.
  • Inhalation of high levels of Hydrogen Iodide can cause irritation to airways and mucus membranes.
  • Ingestion may cause salivation, vomiting and diarrhoea.
  • Skin contact may cause irritation or burns.
  • Hydrogen Iodide has a low boiling point of -19 degrees Celsius and will readily evaporate. This makes storage in warm areas a potential hazard.
  • It also reacts with water to produce Iodine and Hydrogen gases, which are both highly flammable.
  • In addition to physical hazards, Hydrogen Iodide presents health risks and proper protective equipment should be worn when working with it. This includes a strong vapour mask, long sleeve clothing, gloves and safety glasses.


Learning about the properties and chemical formula for hydrogen iodide was interesting to me. There’s a lot more to chemistry than one might think.
I hope that now you are just as intrigued as I am about why hydrogen iodide has such an appealing shade of purple.

If you ever want to make a bottle yourself, or if you’re thinking about experimenting with different chemicals like me, be sure to be careful and use protective gear. You don’t want to end up in a hospital bed because you didn’t take proper precautions!


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