Which of the following is the most likely sign that a chemical change has happened?
a) liquid rests on a table overnight, and some evaporate.
b) two liquids are mixed at room temperature and a gas forms.
c) salt is mixed into water, forming a solution.
d) a solid is crushed into very small pieces.

The correct option is B) two liquids are mixed at room temperature and a gas forms.

Explanation:

A chemical change can be described as a change which results in the formation of a new substance by the reactants. In the option B, two liquids react  to form a new product which is a gas. Hence, it is an example of chemical change.

Other options,like option D, is not a chemical change as by crushing a solid no new product is formed. Option C is also not correct because as when salt dissolves in water, no new product is made.  

d. temperate deciduous forest

c. heat could easily travel through the molecules in the air and escape/enter the flask. removing the air slows down the heat transfer.