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?

• The molecules are leaving the liquid phase.

• The solid and liquid phases coexist in equilibrium.

• The vapor pressure of the substance is decreasing.

• The average intermolecular distance is decreasing.

• The temperature of the substance is increasing.

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

• is lost to the surroundings very quickly

• is used in overcoming the intermolecular attractions in the solid

• is used in breaking the bonds within the molecules of the solid

• causes the nonbonding electrons in the molecules to move to lower energy levels

• causes evaporation of the liquid, which has a cooling effect

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?

• The specific heat capacity of the gaseous ethanol is less than the specific heat capacity of liquid ethanol.

• The specific heat capacity of the gaseous ethanol is greater than the specific heat capacity of liquid ethanol.

• The heat of vaporization of ethanol is less than the heat of fusion of ethanol.

• The heat of vaporization of ethanol is greater than the heat of fusion of ethanol.

• The temperature of the copper changed more than the temperature of the water.

• The temperature of the water changed more than the temperature of the copper.

• The temperature of the water and the copper changed by the same amount.

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?

• The metal temperature changed more than the water temperature did; therefore the metal lost more thermal energy than the water gained.

• The metal temperature changed more than the water temperature did, but the metal lost the same amount of thermal energy as the water gained.

• The metal temperature changed more than the water temperature did; therefore the heat capacity of the metal must be greater than the heat capacity of the water.

• The final temperature is less than the average starting temperature of the metal and the water; therefore the total energy of the metal and water decreased.

• Y = 2X, because the volume of HCl(aq) used in trial 2 is twice the volume used in trial 1.

• Y = X, because the number of moles of acid and base reacting with each other is the same in both trials.

• The relationship between X and Y cannot be predicted.

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

• 22°C

• 27°C

• 30°C

• 33°C

• 38°C