While we often notice energy as a change in temperature, such as water heating up on a stove, a distinct form of energy transfer occurs without any temperature alteration. This hidden energy, known as latent heat, plays a significant role when substances change their physical state.
Defining Latent Heat
Latent heat refers to the energy absorbed or released by a substance during a change in its physical state, also known as a phase transition, without a corresponding change in temperature. The term “latent” means hidden, reflecting that this energy does not manifest as a measurable temperature change.
Instead, this energy is used to rearrange the substance’s molecular structure, either breaking or forming intermolecular bonds. For instance, when ice melts, the added heat energy is used to weaken the bonds holding the water molecules in a rigid solid structure, allowing them to move freely as a liquid, while the temperature remains at 0°C.
The Role of Phase Changes
Latent heat is specifically involved in the transitions between solid, liquid, and gas states. The latent heat of fusion is the energy absorbed when a substance melts from a solid to a liquid, or released when it freezes from a liquid to a solid, at a constant temperature. For example, when ice at 0°C melts into water at 0°C, it absorbs approximately 334 kilojoules of energy per kilogram. This energy breaks some of the bonds within the ice’s crystalline structure, enabling the molecules to move more freely.
Another is the latent heat of vaporization, which is the energy absorbed when a liquid turns into a gas (vaporization) or released when a gas condenses back into a liquid, all at a constant temperature. Water, for instance, requires about 2260 kilojoules per kilogram to transform from liquid water at 100°C into steam at the same temperature. This energy overcomes the intermolecular forces holding liquid molecules together, allowing them to escape into the gaseous state.
The latent heat of sublimation describes the energy absorbed when a solid directly transforms into a gas, or released when a gas directly transforms into a solid (deposition), without passing through the liquid phase. Dry ice (solid carbon dioxide) provides a common example; it sublimes directly into gaseous carbon dioxide at room temperature, absorbing energy from its surroundings. In all these phase changes, the added or removed energy changes the state of matter rather than its temperature.
Latent Heat Versus Sensible Heat
The distinction between latent heat and sensible heat is based on their effect on temperature. Sensible heat is the energy transfer that causes a change in the temperature of a substance, measurable with a thermometer. When you heat a pot of water on a stove, the water’s temperature rises from room temperature up to its boiling point; this measurable increase in temperature is due to sensible heat. The energy added increases the kinetic energy of the water molecules, making them move faster.
In contrast, latent heat involves a change in the state of a substance without any change in its temperature. For example, once that same pot of water reaches 100°C, continued heating does not raise its temperature further but instead converts the liquid water into steam. This energy is absorbed as latent heat of vaporization, breaking the bonds between water molecules to form a gas. Both sensible and latent heat are forms of energy transfer, but they manifest differently: sensible heat changes temperature, while latent heat changes phase.
Latent Heat in Daily Life
Latent heat plays a significant role in many everyday phenomena, often influencing comfort and natural processes. When humans sweat, water evaporates from the skin. This evaporation absorbs latent heat, providing a cooling effect. Similarly, refrigeration and air conditioning systems utilize refrigerants that absorb latent heat from the indoor environment as they evaporate, then release that heat outdoors as they condense, effectively moving heat out of a space.
Steam provides an example of latent heat’s impact. Steam at 100°C causes more severe burns than boiling water at the same temperature because it carries a large amount of latent heat of vaporization. When steam touches cooler skin, it condenses back into liquid water, releasing this large amount of latent heat directly onto the skin, leading to intense tissue damage.
Latent heat also drives large-scale weather phenomena. Cloud formation occurs when water vapor cools and condenses into liquid droplets or ice crystals. This condensation process releases latent heat into the surrounding air, warming it and causing it to rise further, which can intensify storms and contribute to atmospheric circulation. The melting of snow and ice in spring absorbs latent heat from the environment, influencing regional temperatures and water cycles.