Asolution in a dish contains 4.0 grams of salt dissolved in 100 grams of water. if 50 grams of the water evaporate, this is evidence that the solution is a compound conserving mass an element a mixture

Answer is: a mixture.

Mixtures can be separeted y physical means, while compounds cannot.

Solution is homogeneous mixture composed of two or more substances.  

In aqueous solution, solvent is water and solute (in this example salt) is a substance dissolved in water.

Homogeneous solution of salt and water can be separated with heating (evaporating the water).

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they are all solids.