Latent Heat
Definition and meaning of Latent Heat in chemistry.
Latent heat is the exact amount of thermal energy a substance absorbs or releases during a phase change. The overall temperature of the chemical stays completely flat while this physical change happens. This hidden heat goes entirely into changing the physical state of the matter.
In more detail
When you heat a solid or a liquid, the measured temperature normally goes right up. The added heat energy makes the chemical molecules move and vibrate much faster. However, this stops happening the moment the substance reaches its exact melting or boiling point.
At these specific transition points, the added thermal energy stops increasing the measurable temperature. Instead, the heat energy goes directly into breaking the strong attractive forces between the molecules. This physical process pulls the molecules apart to successfully form a liquid or a gas.
The word latent actually means hidden in everyday language. The heat seems hidden because a standard thermometer does not show any temperature change during the entire phase transition. There are two primary types of this hidden energy in physical chemistry.
The latent heat of fusion deals specifically with melting a solid or freezing a liquid. The latent heat of vaporization deals strictly with boiling a liquid or condensing a gas. Chemists use these exact heat values to design much better cooling systems and industrial chemical reactors.
Key facts
| Types of phase change | Melting, freezing, boiling, condensing |
|---|---|
| Temperature behavior | Stays completely constant during the change |
| Latent heat of fusion | Energy to melt or freeze a substance |
| Latent heat of vaporization | Energy to boil or condense a substance |
| Physical mechanism | Breaking or forming intermolecular bonds |
| Water vaporization energy | 2260 kilojoules per kilogram |
Think about vigorously boiling a large pot of pure liquid water on a hot stove. You keep adding lots of heat, but the water temperature stays stuck at exactly 100 degrees Celsius. The specific latent heat of vaporization for water is roughly 2260 kilojoules per kilogram. All the stove heat works exclusively to rip the water molecules apart to make gaseous steam. The temperature will only rise again after all the water boils away.
Frequently asked questions
Why doesn't the temperature go up when you add heat during boiling?
The added heat energy is completely busy breaking the bonds between the liquid molecules. It cannot make the molecules move faster until the bonds are broken.
Does freezing release or absorb latent heat?
Freezing releases latent heat into the surroundings. The molecules slow down and form bonds, which pushes heat out of the substance.
Which is larger, the heat of fusion or the heat of vaporization?
The heat of vaporization is almost always much larger. It takes far more energy to completely separate molecules into a gas than to just loosen them into a liquid.