GRADE 8 · CHANGE IN STATE OF AGGREGATION
Boiling and vaporization
Boiling temperature, pressure and specific latent heat of vaporization.
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Understanding step by step
Water was already evaporating before it was heated. What new process occurs when vapour bubbles start growing in a saucepan? Why does this begin at a lower temperature in the mountains?
Vapour inside the liquid
In ordinary evaporation, molecules leave the water's upper surface. During boiling, vapour also forms inside the liquid in growing bubbles, each with its own liquid–vapour boundary. Boiling is vaporization involving the formation and growth of vapour bubbles within the liquid. Small bubbles of dissolved air can appear before boiling, so seeing any bubble at all is not sufficient evidence.
When can bubbles grow?
The surrounding water compresses a bubble. For sustained growth, the pressure inside must withstand this compression. Saturated vapour pressure increases with temperature, so heating makes growth possible. In a simple shallow-vessel model, boiling starts when vapour pressure equals the pressure above the liquid. At depth, the water column adds pressure, while surface tension matters for very small bubbles.
Why mountains are different
Predict: if the pressure above water is reduced, must it be heated as much? No: vapour can sustain bubbles at a lower pressure and therefore a lower temperature. Water thus boils below 100 °C in the mountains and above this temperature in a pressure cooker at higher pressure. Boiling does not imply one fixed temperature under all conditions. About 100 °C applies to pure water at ordinary atmospheric pressure.
Energy for the change of state
After boiling begins, the heater continues to supply energy. This energy separates the liquid's particles and increases the substance's volume, displacing its surroundings. The specific latent heat of vaporization L is the energy needed to turn 1 kg of liquid into vapour at the specified transition temperature, measured in J/kg. A larger L means more energy for the same mass. L can also change when boiling conditions change.
Accounting for the vapour mass
For liquid already at its boiling temperature, each kilogram vaporized requires L joules: Q = Lm. Doubling the vapour mass doubles Q; half the mass needs half the energy. If no vapour forms during a stage, its Lm contribution is zero. At constant pressure, a pure boiling liquid stays at approximately the same temperature: increasing the energy supply mainly speeds up vapour formation until the liquid has boiled away.
Returning to liquid
What happens in the reverse change? When vapour condenses under the same conditions, it releases energy of magnitude Lm: the transition energy that previously had to be supplied. If the resulting liquid then cools, it releases additional energy described by cmΔT, with a negative sign for the liquid. Do not combine these stages into a single formula: condensation changes the state; the subsequent cooling changes the temperature.
The starting temperature matters
Start with cold water and aim to turn only part of its mass into vapour. First, all the water being heated receives energy cmΔT. Then Lm is needed for the mass that actually vaporizes. Add these contributions; if all the water vaporizes, the masses in the two stages are equal. If some energy goes to the vessel and air, the source must supply more. Such losses may be neglected only within the model adopted for the problem.
A portion of vapour
Turn 0.2 kg of water at its boiling temperature into vapour. L = 2,3 МДж/кг. Find the heat required.
Worked example · 1
The water is already at its boiling temperature. Energy is therefore needed only to change 0.2 kg of liquid into vapour, with no preliminary heating.
Worked example · 2
Q = Lm = 2,3·0,2 = 0,46 МДж = 460 кДж. Unit check: МДж/кг · кг = МДж.
Worked example · 3
The mass is one fifth of a kilogram, so the energy is one fifth of 2.3 MJ. Constant temperature is consistent with this: the water's state and volume change.
Check the result
460 kJ. This energy turns the specified portion of boiling water into vapour under the given conditions.
Key idea · 1
Vapour bubbles grow when their internal pressure can withstand the surrounding pressure.
Key idea · 2
Boiling temperature depends on pressure, so 100 °C applies to specific conditions.
Key idea · 3
Q = Lm gives the energy to turn the required mass into vapour; account for preliminary heating and losses separately.
What comes next
Expanding vapour can push a piston or turbine blades. The next lesson connects thermal processes with mechanical work.
Basic Concepts
Boiling
Intense vaporization throughout a liquid, with vapour bubbles forming and growing within it.
Boiling begins when the saturated vapor pressure is equal to the external pressure.
Specific latent heat of vaporization
The energy required to convert a unit mass of liquid into vapor at the boiling point.
Condensing the same mass of vapour releases the same magnitude of energy.
Boiling point and pressure
The boiling point decreases as the external pressure decreases and increases as it increases.
High in the mountains, water boils below one hundred degrees, and in a pressure cooker, increased pressure allows it to heat up more.
Physical quantities
Specific latent heat of vaporization
LThe energy required to convert a unit mass of liquid into vapor during boiling.
Unit: joule per kilogram · \frac{\mathrm{Дж}}{\mathrm{кг}}
Lesson formulas
Heat of vaporization
Q = LmDuring boiling, the heat required equals the specific latent heat of vaporization multiplied by the mass of liquid vaporized.
Problems on this topic
Statements, answer options and the walkthrough.