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Iodine is a chemical element that is often associated with the color purple. This is because iodine crystals and iodine solutions like tincture of iodine have a distinctive purple color. However, iodine does not always have to be purple. The color of iodine depends on its physical state and concentration.
Is Solid Iodine Always Purple?
Solid iodine is usually purple. When iodine molecules come together to form iodine crystals, the crystals absorb visible light and reflect back purple light. This gives solid iodine like iodine crystals or resublimed iodine powder a very deep purple color.
However, not all solid iodine is purple. Amorphous solid iodine, which lacks a well-defined crystal structure, can appear black or brown. Impure solid iodine may also seem almost black. Additionally, solid iodine can form different colored crystalline structures depending on how it was produced. For example, iodine crystals grown by sublimation are purple while crystals grown from solution are transparent yellow or orange.
So while most common forms of solid iodine are distinctly purple, solid iodine can also take on other colors depending on its exact chemical structure and composition.
Is Liquid Iodine Always Purple?
Like solid iodine, liquid iodine also often has a purple color. The color of molecular iodine liquids like tincture of iodine comes from absorption of certain wavelengths of light. This gives an intense purple color.
However, the color of liquid iodine depends on its concentration. Dilute solutions of iodine appear yellow or brown. The more concentrated the iodine solution, the deeper purple it seems. A saturated solution of iodine in water has an extremely dark purple color.
Other solvents also influence iodine’s color. While tincture of iodine made with ethanol is purple, solutions of iodine made with other solvents like chloroform, carbon tetrachloride, or hexane are violet or pink.
Additionally, the pH of a solution affects iodine’s color. Acidic solutions are purple while basic solutions are brown. That’s why Lugol’s iodine, which contains iodine and iodide in water and potassium iodide, has a brown color.
So liquid iodine can have many colors besides purple depending on concentration, solvents, pH and other factors. A diluted iodine solution may be completely yellow.
Is Iodine Gas Always Purple?
Iodine gas is not purple. When iodine sublimes and becomes a gas, the iodine molecules are no longer close enough together to exhibit their characteristic purple color.
Instead, iodine gas generally appears violet when present at high concentrations. With lower concentrations of iodine vapor, the gas looks transparent and colorless.
So elemental iodine in its gaseous state does not have its familiar purple color. However, iodine gas can still be identified by its distinctive irritating odor.
Causes of Color Variations in Iodine
Iodine can appear purple, violet, pink, orange, yellow, brown or even colorless depending on several factors:
- Physical state – Solid iodine is usually purple. Liquid iodine varies in color based on solvent, concentration and pH. Gaseous iodine is transparent.
- Concentration – More concentrated iodine solutions are a deeper purple, while dilute solutions appear yellow or brown.
- Solvents – Iodine’s color differs in water, ethanol, chloroform, carbon tetrachloride, and other solvents.
- Acidity – Acidic iodine solutions are purple. Basic solutions of iodine are brown.
- Impurities – Impure iodine crystals may seem black or brown rather than purple.
- Crystal structure – The type of crystals affects iodine’s color, with purple sublimation crystals and yellow solution crystals.
So while many people associate iodine with its distinctive purple color, the color of iodine can vary dramatically based on physical, chemical and environmental factors.
Common Purple Iodine Forms
Here are some of the most familiar purple forms of iodine:
|Type of Iodine
|Dark purple solid crystals formed when iodine vapor is cooled and condensed. Often created by sublimation.
|Tincture of iodine
|A liquid solution of iodine in ethanol, used as an antiseptic. Deep purple color.
|Also called iodopovidone. An iodine solution complexed with povidone, used in medicine. Purple-brown color.
|Potassium iodide solution
|Dissolving potassium iodide in water forms an intense purple solution, unless very dilute.
So some of the most vividly purple forms of iodine include iodine crystals, concentrated ethanolic solutions, and potassium iodide solutions. But again, the purple color only appears under certain conditions.
Common Non-Purple Iodine Forms
Here are some examples where iodine appears yellow, brown, orange or colorless instead of its iconic purple:
|Type of Iodine
|Dilute iodine solutions
|Solutions with low iodine concentration are yellow or brown.
|Gaseous iodine vapor is transparent and colorless in low concentrations.
|An aqueous solution of iodine and potassium iodide, brown color.
|Iodine in nonpolar solvents
|Iodine in solvents like chloroform or carbon tetrachloride gives violet or pink colors.
|Basic iodine solutions
|Alkaline iodine solutions are brown rather than purple.
So under many common conditions involving solvents, concentration, pH and physical state, iodine appears yellow, brown, violet or colorless instead of its iconic purple.
While many people think of iodine as having a distinctive dark purple color, iodine can actually take on a wide variety of colors. The purple color appears under specific conditions when iodine molecules interact to absorb certain wavelengths of light. If iodine is too dilute, in a different physical state or solvent, or under alkaline conditions, it may appear yellow, brown, orange, violet, pink or even colorless rather than its familiar purple hue. So while purple iodine crystals, resublimed iodine powder, and concentrated iodine tinctures are very purple, iodine does not always have to be purple depending on its chemical environment.