Which of the following is a clue of a chemical change?
a. iron changes color when heated
b. gas bubbles form in boiling water
c. balls of wax form when melted wax is poured into ice water
d. a gas forms when vinegar and baking soda are mixed

The changing of the colors of the iron when it is heated is an example of the physical change. When the iron is heated, the electrons gets excited and move to higher energy shells.

The formation of the gas bubbles in the boiling water is also an example of the physical change as no chemical reaction has taken place.

When the melted wax is poured into ice, the melted wax is formed. This is also an example of physical change, as just the state of the wax has changed.

When vinegar and baking soda are mixed, a chemical reaction takes place, which leads to the formation of the carbon-dioxide gas. This is an example of chemical change.

Hence, the answer is 'option D'.

the answer would be   10%