Which is a sign of a chemical change? a. ice cream left at room temperature melts. b. calcium chloride is dissolved in h2o and disappears. eliminate c. paint on a freshly painted wall dries and is no longer sticky. d. when h2s gas is bubbled into a lead acetate solution, a precipitate forms?

A chemical change is where new substances are formed, and usually involves either heat or electricity.
From the above 4 options, even though we don't know whether heat or electricity is involved, but we can see that option d is a chemical change because new substance is formed.
The new substance formed is the precipitate, meanwhile all other options does not show the formation of new substances.
Note that State changes is not a chemical change, since it can return to its own form. So a is not correct.
B only shows that it dissolves, but nothing new is formed.
And c is only about evaporation, again, nothing new is formed.
So the answer should be d.

Answer ozone (o3) is made of three atoms of oxygen, it is a molecule;