Ask A Scientist Chemistry Archive

Question:  Why does  acid  make things change color ?
andrade

Answer 1:
The acid will affect the chemical that gives things their color.
Color is produced by a certain chemical reflecting that color from
white light (the stuff that comes from the sun).  The chemical will
absorb some colors and reflect others.  The acid then will modify that
chemical so it absorbs and reflects different colors from the original.
I hope this doesn't sound like double-talk!  These chemicals are usually
known as dyes, and there are a number of them to produce a wide variety
of colors.
-Joe Schultz

Answer 2:
Maybe you're asking why an acid makes an indicator change color.
Indicators are usually (usually naturally occurring) dyes which react 
with protons in solution to form either new compounds. The unreacted 
dye absorbs different wavelengths of light than the reacted dye, and so 
the two forms have different colors.
However, acids don't always change the color of compounds.
If you were to add a little vinegar (which is a weak acid) to a pinch 
of baking soda dissolved in water (a weak base) they would react and 
give a lot of carbon dioxide....which is colorless.
So there would be no color change. However, if you also add an indicator 
dye into the solution before your baking soda solution, it will change color 
when you add the vinegar.
An example of an indicator is the chemical which makes red cabbage red.
If you boil some red cabbage leaves, the solution you make is an
indicator. dye solution. Try it (with the help of an adult) and see!

-dr topper