You find an unknown substance that cannot be separated by filtration, evaporation, or distillation. Would the explanation that this substance is an element be scientifically valid?

It's not enough.

Explanation:

To be able to separate mixtures into substances we have more methods than just distillation, filtration and evaporation. Certain mixtures cannot be filtered, however by decanting can be separated simpler substances from it. Centrifugation separates solids dispersed in liquids that cannot be separated by filtration. Also, it is necessary to try to separate the substance by the method of chromatography, crystallization, etc.

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