Which of the following is evidence that a chemical change has occurred?

a. Water bubbling when it starts to boil.

b. Detergent removing grease from a dish.

c. Sugar turning black when it burns.

d. Oil and vinegar separating in salad
dressing

c. Sugar turning black when it burns.

Explanation:

Chemical change is defined as a change in which rearrangement of atoms take place and leads to formation of new substances.  

Physical change is defined as a change which leads to change in shape, size or state of the substance changes. No new substance is formed.

a)  Water bubbling when it starts to boil is a physical change as it accompanies only a change in phase.

b) Detergent removing grease from a dish is a physical change as no new substance is formed.

c) Sugar turning black when it burns is a chemical change as there occurs a chemical reaction between sugar and oxygen. The oxygen in the air reacts with the sugar and the chemical bonds are broken.

d) Oil and vinegar separating in salad  dressing is not a chemical change. It occurs because of difference in densities.