Phase Changes

The graph shows the temperature of a pure substance as it is heated  at a constant rate in an open vessel at 1.0 atm pressure. The substance changes from the solid to the liquid to the gas phase. At what time is the substance at its normal freezing point?

Which of the following best describes what is happening to the substance between $$t_4$$   and $$t_5$$  ?

The cooling curve above shows how the temperature of a sample varies with time as the sample goes through phase changes. The sample starts as a gas, and heat is removed at a constant rate. At what time does the sample contain the most liquid?

At 1.0 atm a sample of ice is heated to liquid water and then to water vapor. The heating curve is shown in the graph. What is the sign for the changes in enthalpy for the process corresponding to segment X, going from left to right?

A molecular solid coexists with its liquid phase at its melting point. The solid-liquid mixture is heated, but the temperature does not change while the solid is melting. The best explanation for this phenomenon is that the heat absorbed by the mixture

The heating curve for a sample of pure ethanol is provided above. The temperature was recorded as a 50.0 g sample of solid ethanol was heated at a constant rate. Which of the following explains why the slope of segment T is greater than the slope of segment R?

A 10. g cube of copper at a temperature T1 is placed in an insulated cup containing 10. g of water at a temperature $$T_2$$  . If $$T_1$$   > $$T_2$$  , which of the following is true of the system when it has attained thermal equilibrium? (The specific heat of copper is 0.385 J/(g·°C) and the specific heat of water is 4.18 J/(g·°C).)

A 100 g sample of a metal was heated to 100°C and then quickly transferred to an insulated container holding 100 g of water at 22°C. The temperature of the water rose to reach a final temperature of 35°C. Which of the following can be concluded?

NaOH(aq) + HCl(aq) $$\rightarrow$$  NaCl(aq) + H₂O(l)

A student conducted an experiment to determine $$\Delta H_{rxn}^{\degree}$$  for the reaction between HCl(aq) and NaOH(aq). The student ran two trials using the volumes of HCl(aq) and NaOH(aq) indicated in the table above, and determined the amount of heat released. Which of the following best explains the relationship between X and Y?

In an insulated cup of negligible heat capacity, 50. g of water at 40.°C is mixed with 30. g of water at 20.° C. The final temperature of the mixture is closest to