Which of the following is an example of a ball of clay undergoing a physical change? I. The ball is thrown into the air
II. The ball is broken into two separate halves
lll. The ball is rolled along the ground
lV. The ball is flattened into a pancake like shape

Which best describes the activation energy of a chemical reaction? a. the combined energy of all the reactants b. the amount of energy required for a reaction to occur c. the difference in energy between products and reactants d. the potential energy stored in the bonds of reactants and products