Getting It Clean and White

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The other day I splashed some bleach on my pants, and soon there was a white spot where the colour had been. My wife wondered whether the colour was permanently removed, or just hidden.

Bleaches contain substances called oxidizing agents which react with coloured dyes. This changes their chemical structure permanently, so that the dye molecule is no longer coloured.

There are two main types of bleaches used today. Common household bleach is chlorine bleach. It’s a solution of sodium hypochlorite and is a powerful oxidizing agent. It bleaches quickly at ordinary temperatures, but is hard on fabrics.

Many laundry bleaches use hydrogen peroxide which causes less fabric damage. However, it requires higher temperatures and longer times to permanently bleach out the colour. Thus, it works well in long, hot wash cycles. It’s usually put in the detergent as a substance called sodium perborate. This breaks down at temperatures above fifty degrees Celsius, thereby generating hydrogen peroxide.

One of the challenges for detergent manufacturers is to find catalysts to activate the peroxide so that it will work well in cold temperature washes. Other additives called optical bleaches are also used sometimes. They’re not real bleaches that remove colour, they’re rather brighteners.

So next time, unless you really like white spots on your pants, use that bleach carefully!