Latent heat — phase transitions: Question bank

Symbols used here are exactly those built in the parent note: $Q = m L$ (heat to change phase of mass $m$ ), $Q = m c Δ T$ (heat to change temperature), latent heat of fusion $L_{f}$ (solid↔liquid) and vaporisation $L_{v}$ (liquid↔gas). Nothing new is introduced.

True or false — justify

Adding heat to a substance always makes it hotter.

False. On a melting or boiling plateau every joule becomes potential energy that breaks bonds, so the temperature stays flat while heat keeps flowing.

During melting, the molecules' average kinetic energy increases.

False. Temperature (∝ average KE) is constant during melting, so KE is unchanged — the energy goes entirely into potential energy pulling the lattice apart.

Latent heat is "hidden" because it produces no temperature change a thermometer can read.

True. Latent means hidden; the energy is present in the substance as broken-bond potential energy but invisible to a thermometer, which only tracks temperature.

Freezing water releases exactly the latent heat that melting it absorbed.

True. Fusion is reversible in energy terms:

Q = m L_{f}

is absorbed on melting and the same magnitude is given out on freezing at the same temperature and pressure.

L_{v}

for water is larger than

L_{f}

because gas molecules are simply hotter.

False. Both plateaus are at constant temperature.

L_{v}

is larger because vaporisation fully separates molecules against attraction and does work pushing back the atmosphere, whereas fusion only loosens a lattice.

A substance at its melting point is necessarily all liquid.

False. At the melting point solid and liquid coexist; the mixture can be any ratio while heat is still being supplied to finish melting.

Steam at

100° C

carries more energy than water at

100° C

of the same mass.

True. To make that steam you had to add

m L_{v}

on top of the water's energy, so the steam holds that extra latent (potential) energy — which is why steam burns are far worse.

The plateau on a heating curve is flat because heat stops flowing there.

False. Heat keeps flowing steadily; the plateau is flat because

Δ T = 0

— the heat is going into phase change (

Q = m L

), not into raising temperature.

Spot the error

"Ice → water → steam needs two latent terms, so

Q = m L_{f} + m L_{v}

and we're done."

Missing the slopes. You must also warm the water from

0° C

100° C

(

m c_{w} Δ T

) between the two plateaus — the

m c Δ T

terms are not optional.

"On the melting plateau

Δ T = 0

, so

Q = m c Δ T = 0

; no heat is needed to melt ice."

Wrong formula for a plateau.

Q = m c Δ T

describes sloped regions; on a plateau you must use

Q = m L

, which is nonzero. Getting

Q = 0

is the tell that you used the wrong equation.

"To find heat released when water cools and freezes, just use

Q = m c Δ T

down to

0° C

You stopped too early. Cooling to

0° C

m c Δ T

, but freezing the water then releases an extra

m L_{f}

at constant

0° C

; both terms count.

L_{f}

and

L_{v}

have units of joules, since they are heats."

Units are wrong.

L

is specific latent heat: J/kg. The actual heat is

Q = m L

, and only

Q

has units of joules.

"Boiling and evaporation are the same process, so both need

Q = m L_{v}

Same latent heat, different process. Both are liquid→gas and use

L_{v}

, but boiling happens throughout the liquid at a fixed temperature, while evaporation happens only at the surface at any temperature — see Evaporation vs boiling.

"A calorimetry balance is: heat lost by water

= m c Δ T

of the ice."

Wrong bookkeeping for the ice. Ice at

0° C

gains heat as latent fusion (

m L_{f}

), not

m c Δ T

— its temperature isn't rising while it melts.

Why questions

Why does temperature refuse to rise while ice melts, even under a strong flame?

Because temperature reflects average kinetic energy, and during melting all incoming heat converts to potential energy breaking the crystal lattice — none speeds molecules up, so temperature holds.

Why is a scald from steam at

100° C

worse than from boiling water at

100° C

The steam first condenses on your skin, dumping its large latent heat

m L_{v}

before it even starts to cool, so it delivers far more energy than same-temperature water.

Why does the First law of thermodynamics tell us

L_{v}

includes more than just pulling molecules apart?

Because

Q = Δ U + W

: vaporisation raises internal energy and does work

W

expanding against the atmosphere, so

L_{v}

pays for both — pushing it above the pure bond-breaking cost.

Why do we split the heating-curve total into five separate terms instead of one formula?

Because sloped regions obey

Q = m c Δ T

(with three different

c

values) and flat plateaus obey

Q = m L

; the physics changes at each kink, so each region is computed independently and then summed.

Why does the Kinetic theory of gases support the idea of a constant-temperature plateau?

Kinetic theory ties temperature directly to mean molecular kinetic energy; if that KE is unchanged during a phase change, temperature must stay constant regardless of how much heat flows.

Why does sweating cool your body?

Evaporating sweat draws its latent heat of vaporisation from your skin; that energy leaves with the escaping water molecules, lowering the skin's temperature (see Evaporation vs boiling).

Edge cases

If you add heat to ice that is below

0° C

, does any of it melt immediately?

No. The ice must first warm up to

0° C

via

m c_{ice} Δ T

; only once it reaches the melting point does further heat go into fusion.

What happens if you drop ice at

0° C

into water and the water cannot supply enough heat to melt it all?

The mixture ends at

0° C

with both ice and water present — a coexistence equilibrium. Only part of the ice melts, using up all the available heat.

At exactly the melting point with heat still flowing, what fraction of the substance is solid?

Indeterminate from temperature alone — it depends on how much heat has been added; the substance passes smoothly from all-solid to all-liquid while the thermometer reads the same value.

Does pressure change where the plateaus sit on the temperature axis?

Yes. Boiling and melting points shift with pressure — that is exactly what a phase diagram maps in

P

–

T

space; the

100° C

boiling plateau assumes

1

atm.

If a substance sublimes (solid straight to gas), how many plateaus does its heating curve show?

One — a single plateau at the sublimation point using the latent heat of sublimation, since there is no separate liquid stage to warm through.

Can latent heat be released while a substance's temperature is simultaneously falling?

Not during the transition itself. The latent release happens at the fixed transition temperature; any temperature fall is a separate sloped

m c Δ T

stage before or after the plateau.

If two objects reach the same final temperature by mixing, why must we still check whether all the ice melted?

Because the energy balance might leave the system stuck at

0° C

with leftover ice — assuming full melting can overstate the heat exchanged and give a wrong final state.

Recall One-line summary of the traps

Slopes use $Q = m c Δ T$ , plateaus use $Q = m L$ ; temperature is frozen during a phase change because heat becomes potential energy; and $L_{v} > L_{f}$ because full separation plus atmospheric work costs more than merely loosening a lattice.

Connections

Latent heat — phase transitions — the parent note these traps interrogate.
Specific heat capacity — the sloped-region partner formula misused in several traps.
Calorimetry — method of mixtures — the "did all the ice melt?" edge cases.
Kinetic theory of gases — why constant KE means constant temperature.
Evaporation vs boiling — surface vs bulk vaporisation, both using $L_{v}$ .
First law of thermodynamics — why $L_{v}$ also pays for atmospheric work.
Phase diagrams — how pressure moves the plateaus.