NDA Physics · Heat and Thermodynamics

Phase Change, Boiling, Evaporation, and Cooling

When a substance changes phase it absorbs or releases latent heat at constant temperature; boiling happens when vapour pressure equals atmospheric pressure, so changing the pressure changes the boiling point.

All Heat and Thermodynamics notes NDA Physics strategy

Why this matters

About 7 PYQs, mostly EASY recall with a couple of MODERATE conceptual ones. The recurring facts: the definition of latent heat of vaporization, the pressure-boiling-point link (pressure cooker cooks faster, water boils cooler on a mountain), and evaporation versus boiling. One MODERATE trap is Newton's law of cooling — knowing exactly which situations it applies to (gentle convective cooling, not phase change or furnace radiation).

Concept 1 of 4

Latent heat of fusion and vaporization

Intuition

While a substance changes state — solid melting to liquid, or liquid boiling to vapour — its temperature stays FLAT even though heat is still pouring in. That heat goes entirely into breaking the bonds between molecules, not into raising the temperature. That hidden heat is the latent heat.

Definition

Latent heat is the heat per unit mass absorbed or released during a phase change with no change of temperature ( $Q = mL$ ). Two kinds:

Latent heat of fusion — heat to change unit mass of solid to liquid (melting) at the melting point.
Latent heat of vaporization — heat to change unit mass of liquid to vapour (boiling) at the boiling point, with no temperature change.

The temperature plateau during melting/boiling is the signature of latent heat.

Latent heat

Q = mL

Qheat absorbed or released at constant temperature
mmass changing phase
Lspecific latent heat (fusion or vaporization)

Worked example

How much heat changes 50 g of water at 100°C into steam at 100°C? (Latent heat of vaporization = 540 cal/g.)

Practice this conceptself-check · 4 quick reps

From the bank · past-year question

Example 1Heat and ThermodynamicsEASY

The amount of heat required to change a liquid to gaseous state without any change in temperature is known as

[Q83 · Apr · 2017]

Latent heat is absorbed at CONSTANT temperature

The defining phrase the NDA hunts for is 'without changing the temperature'. During a phase change the thermometer reading is flat while heat is still flowing — the heat breaks molecular bonds instead of warming the substance.

Drill 3 more on latent heat of fusion and vaporization

Concept 2 of 4

Boiling point depends on pressure

Intuition

A liquid boils when its vapour pressure climbs up to equal the surrounding (atmospheric) pressure — at that point bubbles of vapour can form throughout the liquid. So if you RAISE the external pressure, the liquid needs a higher temperature to reach that pressure: boiling point goes up (pressure cooker). LOWER the pressure and it boils cooler (high altitude).

Definition

Boiling point is the temperature at which a liquid's vapour pressure equals the external (atmospheric) pressure in an open vessel. Consequences:

Higher pressure → higher boiling point. A pressure cooker seals in steam, raising the pressure, so water boils above 100°C and food cooks faster.
Lower pressure → lower boiling point. At high altitude the air pressure is lower, so water boils below 100°C — cooking takes longer.

Boiling condition

P_{\text{vapour}} = P_{\text{atmospheric}}

$P_{\text{vapour}}$ saturated vapour pressure of the liquid
$P_{\text{atmospheric}}$ external (atmospheric) pressure

Worked example

Explain why a pressure cooker cooks food faster than an open pan.

Practice this conceptself-check · 4 quick reps

From the bank · past-year question

Example 2Heat and ThermodynamicsEASY

The temperature at which vapour pressure of the liquid in an open vessel becomes equal to the atmospheric pressure is called

[Q65 · Sep · 2025]

Boiling is vapour pressure = atmospheric, not 'less than'

Distractors offer 'vapour pressure becomes less than' or 'greater than' atmospheric. Boiling is the exact EQUALITY: the liquid's vapour pressure rises to meet the external pressure. Pick the 'equal to' option.

Drill 4 more on boiling point depends on pressure

Concept 3 of 4

Evaporation versus boiling

Intuition

Evaporation and boiling both turn liquid into vapour, but they are different. Evaporation is a slow, surface-only process that happens at ALL temperatures; boiling is a rapid, throughout-the-liquid process that happens only at the boiling point. Evaporation speeds up with higher temperature, larger surface area, dry air, and a breeze.

Definition

Evaporation — a surface phenomenon in which fast-moving molecules escape from the liquid surface at any temperature. It is faster when:

the temperature is higher,
the surface area is larger,
the surrounding air is drier and moving (wind).

Boiling — a bulk phenomenon (bubbles form throughout) occurring only at the boiling point, where vapour pressure equals atmospheric pressure. Evaporation cools the liquid left behind (it carries away the most energetic molecules).

Worked example

Under what conditions does evaporation from a liquid surface happen most rapidly?

Practice this conceptself-check · 4 quick reps

From the bank · past-year question

Example 3Heat and ThermodynamicsEASY

Evaporation from the surface of a given liquid takes place more rapidly when

[Q53 · Apr · 2022]

Evaporation happens at ALL temperatures; boiling does not

A common confusion is that a liquid only evaporates when heated. Evaporation occurs at any temperature (wet clothes dry in the shade). Boiling is the one that needs the specific boiling-point temperature.

Concept 4 of 4

Newton's law of cooling

Intuition

A hot body cools faster when it is much hotter than its surroundings and slows down as it approaches room temperature — that is why a cup of coffee cools quickly at first, then lingers. Newton's law of cooling captures this: the rate of cooling is proportional to the temperature difference with the surroundings. It applies only to gentle convective cooling over a small temperature difference, not to phase changes or furnace-hot radiation.

Definition

Newton's law of cooling: the rate of loss of heat of a body is proportional to the difference between its temperature and that of its surroundings (for a small temperature difference, by convection):

-\frac{d\theta}{dt} \propto (\theta - \theta_0).

It applies to ordinary convective cooling — e.g. a hot drink cooling on a table. It does NOT apply during a phase change (melting ice, boiling water) or to a body radiating at furnace temperatures (very large temperature difference).

Newton's law of cooling

-\frac{d\theta}{dt} \propto (\theta - \theta_0)

$\theta$ temperature of the body
$\theta_0$ temperature of the surroundings
$\frac{d\theta}{dt}$ rate of change of temperature with time

Worked example

Of these four cases, where does Newton's law of cooling apply? (1) ice melting in a glass of water, (2) water boiling in an open container, (3) a metal rod heated in a furnace, (4) a cup of coffee cooling on a table.

Practice this conceptself-check · 4 quick reps

From the bank · past-year question

Example 4Heat and ThermodynamicsMODERATE

Given below are four cases of heat transfer: 1. Ice is melting in a glass full of water 2. Water is boiling in an open container 3. A metal rod is heated in a furnace 4. A cup of coffee is allowed to cool on a table In which of the above cases, Newton's Law of Cooling is applicable ?

[Q124 · Sep · 2024]

Newton's law does NOT apply to phase changes or furnace heat

The law needs the body's temperature to actually be changing and the temperature difference to be small. Melting ice and boiling water hold a constant temperature; a furnace-hot rod has a huge difference dominated by radiation. Only gentle convective cooling qualifies.

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (3)

Latent heat of fusion and vaporization
Latent heat
$Q = mL$
Boiling point depends on pressure
Boiling condition
$P_{\text{vapour}} = P_{\text{atmospheric}}$
Newton's law of cooling
Newton's law of cooling
$-\frac{d\theta}{dt} \propto (\theta - \theta_0)$

Watch out for (4)

Latent heat is absorbed at CONSTANT temperature
→ Latent heat of fusion and vaporization
Boiling is vapour pressure = atmospheric, not 'less than'
→ Boiling point depends on pressure
Evaporation happens at ALL temperatures; boiling does not
→ Evaporation versus boiling
Newton's law does NOT apply to phase changes or furnace heat
→ Newton's law of cooling

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