For a classroom demonstration, a chemistry teacher puts samples of two different pure solid powders in a beaker. The teacher places the beaker on a small wooden board with a wet surface, then stirs the contents of the beaker. After a short time the students observe that the bottom of the beaker is frozen to the wood surface. The teacher asks the students to make a claim about the observation and to justify their claims. Which of the following is the best claim and justification based on the students’ observation? a
An exothermic chemical change occurred because heat flowed from the contents of the beaker to the room.
b
An exothermic physical change occurred because heat flowed from the contents of the beaker and the water on the board to the room.
c
An endothermic physical change occurred because the freezing of water is an endothermic process.
d
An endothermic chemical change occurred because the temperature of the beaker and the water on the board decreased as heat was absorbed by the reaction.