Phase Change Chemistry Example 4

Follow the full solution, then compare it with the other examples linked below.

Example 4

hard
A heating curve shows a substance warming from −20°C-20°\text{C} to 120°C120°\text{C}. It has a melting point of 0°C0°\text{C} and a boiling point of 100°C100°\text{C}. Explain why there are two flat (horizontal) sections on the curve, even though heat is continuously added.

Solution

  1. 1
    The first flat section occurs at 0°C0°\text{C} during melting. Energy is used to break the intermolecular bonds holding the solid lattice together, not to raise temperature.
  2. 2
    The second flat section occurs at 100°C100°\text{C} during boiling. Energy is used to separate liquid particles completely, overcoming remaining intermolecular forces.
  3. 3
    During both phase changes, temperature stays constant because all added energy goes into changing the arrangement of particles (potential energy) rather than increasing their kinetic energy.

Answer

Flat sections = phase changes where energy breaks intermolecular bonds without raising temperature.\text{Flat sections = phase changes where energy breaks intermolecular bonds without raising temperature.}
The heating curve is a key graph in thermodynamistry. The flat sections represent the latent heat of fusion and latent heat of vaporization — energy absorbed without a temperature change.

About Phase Change

A physical transition from one state of matter to another caused by adding or removing thermal energy, during which the temperature remains constant as energy.

Learn more about Phase Change →

More Phase Change Examples

Example 1 easy

Name the six phase changes and indicate whether each absorbs or releases energy.

Example 2 medium

Why does the temperature remain constant during a phase change, even though heat is being added?

Example 3 easy

What phase change occurs when dry ice ([formula]) goes directly from solid to gas?

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