Bleaching = lighter color

Ok, so bleach usually works (I think) by oxidizing the cloth, hair, etc. So why does oxidizing stuff seem to always make it a lighter color?

Oxidation is a chemical change. If you oxidize most dyes, the new molecule you create tends to be colorless. Thus, as you oxidize something colored, it simply makes the colored portion that is left unoxidzed smaller, and thus lighter.

Jason R Remy

“No amount of legislation can solve America’s problems.”
– Jimmy Carter (1980)

Oxidizing things don’t always make them lighter. When wine or paper oxidizes it generally gets darker (specifically, more yellow or brown).

However, dyes are almost always things which are highly colored to start with (that’s why they were chosen as dyes), and any chemical change is going to tend to make them lighter or at least shift the color.

My whole chemistry education has been called into question by the glass isn’t liquid thread. However, here’s the way I learned it:

Colors are caused by the substance absorbing light. That is, certain photons cause the electrons to move into higher orbitals. Those photons are absorbed, the rest are reflected. For example if blue photons are absorbed, the material appears yellow. (I think I’ve got that right.) Oxidation means the removal of electrons. Without electrons, the substance can no longer absorb those photons, so all the light is reflected, and the substance appears to be white.

With paper, I think you’re confusing oxidation with combustion. Not fast combustion with flames and all, but combustion nevertheless. The molecules in the paper combine with oxygen to form different molecules, which may appear to be yellow or brown.

If I remember correctly (and there’s no guarantee that I do), oxidation occurs when the molecules being oxidized react with oxygen. The resulting oxide compound will have filled outer electron shells and will therefore be more stable. With paper, what’s occurring is oxidation (imo).

The argument about removal of electrons doesn’t sound right to me, but I can’t put my finger on why, and I left my chemistry book at home. I’ll work on it, though.

The Cat In The Hat

What? You don’t carry your chemistry book with you? Shame on you!

Are you saying that you don’t think electrons are actually removed during oxidation? Removing electrons is the definition of oxidation. The reverse – adding electrons – is called reduction. On the other hand, maybe what you don’t believe is that atoms without electrons reflect light, rather than absorb it. In either case, a quick session with your textbook should put your mind at rest.

Chemistry educations? combustion= rapid oxidation.

“Something inciteful that some one else once said”- Suhm Wonn (1397-1334)

Urr. Yes, loss of electrons is oxidation. But that’s just a definition.

In many cases, the electrons aren’t actually “lost” at all. They’re just somewhat less likely to be found near the “oxidized” part of the new molecule than they were before and somewhat more likely to be found near the oxygen (or whatever caused the oxidation).

I actually typed the rest of this in before, and decided it went into too much detail and got rid of it. Oh well, live and learn.

The reason that some compounds are colored is that they absorb electromagnetic radiation somewhere in the visible portion of the spectrum. Generally this is due to the electronic structure of the compound, just like Greg said.

If you have to guess whether a compound is colored or not, check to see if it has a system of conjugated double bonds (alternating single and double bonds). Compounds like this are often highly colored. This isn’t a foolproof method, but it’s a reasonable place to start guessing. The delocalization of electrons that occurs in such a system tends to shift the electronic absorption bands into the visible region.

Oxidizing these compounds can mess up this conjugation, and destroy or change the color.

Transition metal ions are also generally colored. Oxidation may change the color of these, but it doesn’t generally bleach them out. For example, “chromic acid” (potassium dichromate and sulfuric acid) undergoes a color shift from yellow-orange to bluish-green as it oxidizes. Both are highly colored, but the color is very, very different for the different oxidation states of chromium.

As the resident chemist around here, I feel compelled to comment… I really don’t mean to offend, but the above passage is 100% wrong. It was a noble effort, but the author has confused not two, but 3 seperate and unrelated issues.

  1. Objects appear colored because they reflect some of the light striking them and absorb other light. The absorbed energy can do a lot of things. The light energy could just cause the object in question to heat up a bit. Imperceptible to the touch, but most light that is “absorbed” is converted to heat energy.

  2. The light striking a colored surface is will, in all likelyhood, not cause the electrons to change orbitals. Electrons that change energetics then fall pack to their pervious energy state either phosphoresce or flouresce. These are special case scenarios only occuring in specific kinds of chemicals, not ordinary colors. Thus, if the striking light caused electrons to jump to a higher orbital then “relax” down again, you’d not have a colored shirt, you’d have a glow-in-the-dark watch dial.

  3. Oxidation is part of a greater concept called “oxidation-reduction” When we say something oxidized, it lost electrons * to a substance that was likewise reduced *. The electrons don’t disappear, they just move around a bit. Usually, this is the air, or other substances in the colored object (if we are talking about “bleaching”.) These two new compounds that are created may be colored, or they may not be. Since very few substances are vibrantly colored, the new compounds are usually white, or at best a murky yellow.

Jason R Remy

“No amount of legislation can solve America’s problems.”
– Jimmy Carter (1980)

“The light striking a colored surface is will, in all likelyhood, not cause the electrons
to change orbitals… Thus, if the striking light
caused electrons to jump to a higher orbital then “relax” down again, you’d not have a
colored shirt, you’d have a glow-in-the-dark watch dial.”

This raises another interesting point. If things are colored by the fact that they remove some frequencies of the light that was hitting them, then where does the absorbed energy go? If it wasn’t absorbed by the electrons momentarily jumping to higher orbitals, then does the molecule as a whole vibrate faster, converting the light energy to heat? I mean, I know that happens generally, but the problem is that it doesn’t seem to me that this process would tend to absorb only certain frequencies, leaving distinct colors (green grass, etc.)

No offense taken, but you are making me waste part of my Sunday doing research to prove you wrong. Despite my flippant comment before, I not only don’t carry my Chem Text with me, I actually think I threw it out a long time ago. Therefore, I was forced to do some Internet research.

Here’s a site with a good nutshell view: especially if you click on the “More Information” Link. Here we see:

That’s pretty much what I said. (I didn’t mean the electrons “disappear”, just they are no longer part of the dye. Sorry if I confused you there.) There is nothing in this link about orbitals, but let’s say I’m down to 50% wrong, and continue the research …

Now at we find that colors are caused by electrons absorbing certain frequencies of light, which causes them to “vibrate” at higher energies. Through contact with the other molecules, the electrons convert this vibrational motion into thermal energy, so the absorbed photon is not reemitted. Now I learned (and regurgitated) that the photons would cause the electrons to move into higher orbitals, and when they dropped back down they would emit non-visible light (like heat.)

So to sum up:

  1. Bleaching removes electrons
  2. Without electrons the dye no longer absorbs light.
  3. There is some doubt as to the exact mechanism by which light is absorbed in the first place.

(In fact, I found a reference to a book called The Physics and Chemistry of Color: The 15 Causes of Color by Kurt Nassau. If I were Cecil, I would buy and read this book, but I’m not, so I won’t. Even so, I would rank me at 25% wrong, with a possibility of 100% right.

Don’t worry though. I’m sure you’re a very good chemist. It’s not like, “How does bleach work?” comes up every day.

Damn! I so wanted to be the resident chemist.

Well, to add insult to injury; many bleaches claim to get clothes “whiter than white”. This is done by adding a fluoresent compound which attaches to the fibers. Many white clothes already have these compounds in them, but they get washed out over time. If you want to see this phenomenon, wear a brand new white t-shirt to your favorite strip club or disco.

“If you stick your finger in a pie, whatever is in the pie will be on your finger, and whatever is on your finger will be in the pie…unless you wear a rubber glove”----some demented old lady

Thus, the energy of the absorbed light is converted into thermal-kinetic energy, and not, as you first stated:

This is “electronic” energy, light absorbed by my green T-shirt in no way causes the electrons to change orbitals. It causes an increase in kinetic energy of the entire atom: Thermal energy.


This is entirely correct, but:

This is not correct. What this loss of electrons does is that it causes the substance to absorb light in different wavelengths than the previous substance. (I’ll use the light-as-wave model since it makes explaining this easier than the light-as-photon model). If the new substance created reflects all light in the visible spectrum, it will appear white. It is entirely possible, but rarer, that bleaching will cause a change to a different but equally vibrant color. If the new substance after the bleaching has occured still reflects only some of the wavelengths in the visible range, it will appear as a different color. Dyes that, when bleached, change from one distinct color to another are used as redox indicators and are used in exactly the same way that acid-base indicators are used. (Potassium Permangate is a classic redox indicator due to its color change from bright purple to muddy brown Manganese Oxide. Likewise, Potassium Dichromate(orange)/Potassium Chromate(green) makes an ideal redox indicator) Still, since chemically speaking, color is a very rare thing, your most likely to create a substance that is not as vibrantly colored as the one you started with. Also, you can “bleach” one white substance to another white substance, but that is hardly very interesting.

Jason R Remy

“No amount of legislation can solve America’s problems.”
– Jimmy Carter (1980)

I always wondered why white items of clothing glowes so brightly.I have a jacket with white stripes down the sleeves that glow VERY brightly in blacklight. I noticed that the teeth and the whites of the eyes will glow a bit (in a crowded room, you can see many pairs of eyes and lots of glowing smiles :)). Do they contain flourescent substances? Also, how does the sun bleach colors? How does that work? (To get back on topic kind of…)

I checked three PChem texts which happened to be handy (Adamson, Barrow, and Moore), as well as Structure and Spectra of Molecules by Richards and Scott. All of them agree that electronic transitions are usually in the visible/UV range, while vibrational transitions are in the IR range.

Can anyone cite one real authority (high school web sites don’t count) that believes that something OTHER than transitions between electronic energy levels (aka “orbitals”) is responsible for absorption in the visible band?

When a molecule absorbs a photon, it can go not only into a higher electronic energy level, but to an excited vibrational state within the excited electronic state. If the vibrational state is high enough (up in the “continuum” region), it can actually cause chemical bonds to break, resulting in a new compound which is not colored.

There’s another phenomenon called “hole burning” which is a kind of bleaching: some dyes, when bathed in a laser beam, develop a temporary “hole” in their absorption spectra at the frequency of the laser, but that doesn’t generally affect the visual color that much.