Question

Which statement is best supported by the information in the heating curve?

a-At segment T-U, the substance changes from a liquid to a gas and does not change temperature.
b-At segment T-U, the substance changes from a gas to a liquid and changes temperature.
c-At segment Q-R, the substance releases heat and increases in temperature.
d-At segment Q-R, the substance absorbs heat and decreases in temperature.

Answer 1

When a substance changes phase, that is it goes from either a solid to a liquid or liquid to gas, the energy, it requires energy to do so. The potential energy stored in the interatomics forces between molecules needs to be overcome by the kinetic energy the motion of the particles before the substance can change phase.That is why there is no rise in temprature, as all energy applied is consumed in conversion of states.

When something changes phase from solid to liquid, or from liquid to gas, it takes energy to break the intermolecular interactions. These interactions between the water molecules are what make it solid. When you have ice, these interactions are strongest, which is why ice is hard. Then when you have water, the interactions are not as strong, and although the water still "stays together" it is now a liquid and moves and flows freely. Then when all the interactions are broken, it become a gas, or steam, and now none of the water molecules are attached to any other molecules. Whenever it goes through a phase change like this, the energy goes into breaking up these interactions, and so the temperature stays constant until all the interactions are broken. Once all the ice is melted, or all the water has turned to steam, then any added heat will act to raise them temperature again