An apple would be a mixture. While a mixture is a combination of elements and substances, a pure substance acts as an element itself, therefore, having the properties of only THAT certain element.
Dehydration is removal of water.
In alcohols dehydration is α-β elimination or 1,2 elimination, it means the hydroxyl group will be removed from α-carbon while the hydrogen will be removed from near by carbon.
In case of neopentyl alcohol there is no β hydrogen present on the β carbon [as shown in figure].
The only possible way for it to undergo dehydration is by rearrangement.
The process or mechanism can be understood as:
so the chief product is 2-methylbut-2-ene
The concentration of NaOH is 25.00% by mass, it means that 25.00% of the mass of the solution is of NaOH. Hence: