Phase Change (Chapter 7, Sections 7.6 – 7.9)

Question-and-answer flashcards covering definitions, processes, energy changes, and numerical examples of phase changes and latent heat.

Why does the temperature of an ice-water mixture stay at 0 °C as long as ice is present?

The absorbed heat is used as latent heat to melt the ice rather than to raise the temperature.

Name the three common phases of matter.

Solid, liquid, and gas (vapor).

What do we call the heat required to melt a specific amount of a solid at its melting point?

Heat of fusion (a type of latent heat).

At what temperature does pure water melt and freeze at 1 atm pressure?

0 °C.

What happens to temperature during a phase change at constant pressure?

It remains constant until the transition is complete.

What is sublimation? Give an example.

Direct change from solid to gas; e.g., solid naphthalene (moth balls) turning into vapor.

What is the reverse process of sublimation?

Deposition (gas directly to solid).

Define heat of crystallization.

Heat released when a specific amount of liquid solidifies at a given temperature; numerically equal to the heat of fusion.

Does melting increase or decrease the order of particles?

It decreases their order (from orderly crystal lattice to less-ordered liquid).

Is vaporization a cooling or warming process for the remaining liquid?

Cooling, because high-energy molecules leave, lowering the average kinetic energy.

State the main difference between evaporation and vaporization (boiling).

Evaporation is slow and surface-only; vaporization is rapid and occurs throughout the liquid with bubble formation.

Why does evaporation cool a liquid?

Fast, high-energy molecules escape, leaving behind slower ones and reducing average kinetic energy and temperature.

What is the heat of vaporization?

Heat required to convert a specific amount of liquid to vapor at a given temperature.

What is the heat of condensation and how does it compare with heat of vaporization?

Heat released when vapor becomes liquid; equal in magnitude but opposite in sign to the heat of vaporization.

Is condensation a warming or cooling process for the surroundings?

Warming, because it releases heat.

What term describes the heat needed to change the phase of 1 kg of a substance at constant temperature and pressure?

Latent heat.

In what units is latent heat usually expressed?

Joules per kilogram (J/kg) or calories per kilogram (cal/kg).

At what temperature does liquid water boil at 1 atm?

100 °C.

How much heat is required to melt 2 kg of ice at 0 °C? (Latent heat of fusion = 33.5 × 10⁴ J/kg)

6.7 × 10⁵ J.

During condensation, what happens to the kinetic energy of gas molecules that strike the liquid surface?

It is transferred to the liquid, increasing its temperature (warming effect).

What general rule relates energy changes to order during phase transitions?

Changes toward a more random state absorb energy; changes toward a more ordered state release energy.