A student dissolves table salt into a glass of water, then evaporates the water away in a warm oven. What's left behind is white salt crystals that look, taste, and behave exactly like the salt they started with. What does this experiment show about salt dissolved in water?
- It is a compound, because the salt and water have chemically combined into a brand-new substance
- It is a single element, because the dissolved mixture looks like one uniform liquid all the way through
- It is a mixture, because the salt and water have kept their own separate properties and can be separated again by a physical method like evaporation
- It is a solution but not a mixture, because dissolving one substance in another counts as a chemical change
Why C? And why not the others?
Correct answer: C. It is a mixture, because the salt and water have kept their own separate properties and can be separated again by a physical method like evaporation
Because plain evaporation - a physical process that involves no chemical reaction - is enough to recover the original salt unchanged, this shows the salt and water never bonded chemically; they were only mingled together, which is the definition of a mixture, and mixtures can always be separated again by physical means such as evaporation, filtration, or distillation. The option calling it a compound is wrong because in a true compound, like sodium chloride's own bond between sodium and chlorine atoms, the original substances cannot be recovered by a simple physical process - forming or breaking a compound requires a chemical reaction, which evaporation is not. The option calling it a single element is wrong because an element is made of only one type of atom, whereas this glass clearly contains two distinct substances (salt and water) that were mixed together, not one pure elemental substance. The option claiming dissolving is a chemical change is wrong because dissolving does not create or break any chemical bonds between the salt and water molecules - it is a physical process, which is exactly why the salt could be fully recovered afterward with its properties unchanged.
Source: BBC Bitesize GCSE Chemistry: Elements, compounds and mixtures