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In many tests for various radicals, a "<color> precipitate" or "<color> turbidity" is mentioned. For example, lead acetate with chromate ions gives a yellow precipitate, but hydrogen sulphide with potassium dichromate gives a yellow(or white) "turbidity".

In the reaction itself, the turbidity is noted as a precipitate with a down arrow, but I'm wondering if there is any difference between the two. It looks like turbidity refers to a colloid/suspension being formed, but I'm not sure.

I'd like to know:

  • The physical differences/distinguishing features between turbidity and precipitate, if any.
  • The observable differences/distinguishing features between the two, if any.
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My understanding is that turbidity is an adjective describing the presence or degree of particulate matter suspended in a liquid (see turbidimetry), whereas a precipitate is a particulate that can result in turbidity.

The turbidity generated by a precipitate is a function of its ability to form a stable suspension. If the precipitate simply drops to the bottom of a container or readily flocculates, the resultant turbidity will be low. The turbidity of a suspension is also going to be a function of the solvent and the nature and concentration of dissolved ions - a dramatic example of this is the rapid clarification of muddy freshwater at river deltas mixing with salt water.

If the word turbidity is being used as a noun, it is probably referring to a light-occluding suspension.

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