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We heated rock salt for a chemistry lab and it changed from white crystally rock to brown rock and ash-like materiak on the bottom of the test tube. Also there was a sizzling\popping noise. Why did it change colour and why was there a noise? What caused this?

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"Rock salt" contains many impurities, in addition to its main ingredient, sodium chloride. Some trace chemicals in rock salt, such as lithium chloride or sodium carbonate, hold onto water, which might boil out, sizzling and popping, and if your specimen got hot enough, carbonates could decompose.

Even if you had used "table salt", rather than rock salt, chemicals are intentionally added, such as sodium ferrocyanide (to prevent caking), which has a yellowish color and potassium iodide (as a food supplement).

Had you heated pure sodium chloride, there should not have been a color change.

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I'm not too sure what you are asking. If you are refering to rock salt as in sodium chloride (NaCl), I too am at a loss at how you would have created such an effect.

I will assume that you have heated some random other compound since NaCl does not decompose/ react to heat so readily.

It is possible that the "rock salt" reacted to the heat and perhaps decomposed.

For example, CaCO3 (calcium carbonate) decomposes to CaO and Oxygen, perhaps forming the sizzling noise.

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