Ask A Scientist© Chemistry Archive

name         Emma F.
status       educator
age          20s

Question -   I observered an experiment once where you poured clear
liquid into another jar of clear liquid and it changed colour.
Then they transferred the solution into another container of clear liquid
and it turned milky.
Do you know what the "liquids" are to make this experiment work? I
appreciate any help and/or ideas.
--------------------------
Emma,

A phenolphthalein (indicator) solution is clear as water. A dilute solution of
calcium hydroxide in water is also clear. When mixed, the result is a
beautiful pink-to-wine red solution. If that solution is then poured into a
sodium carbonate solution (also clear) the result is a pink-to red cloudy
solution.

To make all this work you would have to experiment with different solution
concentrations until a suitable combination is discovered.

Regards,
ProfHoff
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There are a number of ways of doing this, and there are many variations on
the theme, that goes like this: Start with an indicator solution that goes
from colorless to colored in a certain pH range. Prepare the second solution
of a colorless compound that has a pH higher than this range. Mixing the two
solutions will result in a colored solution because the pH is higher than
the pH of the color change. Then add a colorless solution of a salt, which
is insoluble at the high pH, or some lower pH. Poof! A precipitate falls
out. You can make the supernatant liquid colored or colorless by adjusting
the pH of the salt solution with a little acid.

For example: phenolphthalein turns colorless to pink at pH = 8.2 to 10.0;
thymolphthalein turns colorless to blue at pH = 9.4 to 10.6. Let one of
these be solution (1) prepared at pH = 7 (neutral). Make solution (2) about
1molar sodium bicarbonate. It too will be colorless. Mix solution (1) +
solution (2) and the mixture will turn pink or blue. Now add a third
solution (3), that forms an insoluble carbonate. A good choice is Ca(Cl)2,
and just a bit of acid to drop the pH to about neutral when you mix it with
solution (1) + (2). The pH will drop to about 7, Ca(CO3) will precipitate,
and the solution will return to its original colorless state.

If you look up the pH ranges of various indicators, the pH of various acids
and bases and the solubility of various metal salts you can invent many
variations on this theme. One variation would be to use Ag(NO3) [colorless,
soluble] with some indicator solution + HCl as solutions (1) and (2). When
mixed insoluble AgCl will precipitate and you can have or not have a colored
supernatant, depending upon your specific choices of indicator and pH of
solution (2).

You can find all the data you need in the CRC Handbook of Chemistry and
Physics, or Lange's Handbook of Chemistry.

Good Luck

Vince Calder
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