When water is heated on a stove, the temperature rises steadily until it reaches the boiling point, which is \(100^{\circ}\text{C}\) under standard conditions. Once the water begins to boil and transform into steam, the thermometer reading stops climbing despite the continuous addition of heat energy. This constant temperature raises a fundamental question: where is the continuous energy input going? The answer lies in how matter handles energy during a change of state.
The Distinction Between Heating and Phase Change
The energy absorbed by the water before it reaches its boiling point is known as sensible heat. This heat is responsible for increasing the kinetic energy of the water molecules, which is what a thermometer measures as a rise in temperature. As heat is added, the water molecules move faster, causing the temperature to climb up to the boiling point. When the water hits \(100^{\circ}\text{C}\), it has reached its maximum liquid temperature under atmospheric pressure.
At this point, any further energy added ceases to be sensible heat. The added energy no longer contributes to increasing the speed of the molecules, so the temperature remains fixed. Instead, the energy is dedicated to initiating the phase change from a liquid to a gas. This hidden energy, which changes the state of matter without changing its temperature, is known as latent heat.
The Molecular Work of Phase Change
The continuous energy supplied at the boiling point is used to separate the water molecules from each other. In the liquid state, water molecules are held together by attractive forces, primarily strong intermolecular forces called hydrogen bonds. These forces hold the liquid structure together and prevent the molecules from escaping. Energy is required to overcome these attractive forces.
The latent heat of vaporization is the precise energy required to break these bonds and allow the molecules to transition into the gaseous phase. For water, this separation requires approximately 40.8 kilojoules of energy for every mole of water vaporized. Once a molecule absorbs enough energy to break free, it escapes as steam. This bond-breaking process is purely energy-consuming and does not contribute to the molecules’ overall kinetic energy.
All incoming heat is absorbed by the molecules to perform the work of separation, which keeps the temperature of the remaining liquid water constant. Only after every molecule has absorbed enough latent heat will the substance fully convert to steam. Once all the water has converted to steam, any additional heat energy becomes sensible heat, causing the steam’s temperature to rise above \(100^{\circ}\text{C}\).
Why Latent Heat Matters in the Real World
The high energy requirement of the latent heat of vaporization has broad implications beyond a simple pot on a stove. One common application is evaporative cooling, which the human body utilizes through sweating. When sweat evaporates from the skin, it draws the necessary latent heat directly from the skin, effectively cooling the body. This process is highly efficient because a large amount of heat is removed for every small amount of water that changes state.
In industrial settings, this principle is utilized in refrigeration and air conditioning systems. These systems use specialized fluids, called refrigerants, which absorb a large amount of heat as they change from a liquid to a gas inside the cooling coils.
The substantial energy stored within steam is also harnessed in power generation. Steam engines and power plants use the heat energy absorbed by water during its phase change to drive turbines and generate electricity. The large energy density of steam makes it an effective medium for transferring heat and performing mechanical work.