Q.Distinguish between the latent heat of fusion and the latent heat of vaporisation of a substance. Using the idea of latent heat, explain why a burn caused by steam at $100\,^\circ\text{C}$ is generally far more severe than a burn caused by an equal mass of boiling water at the same temperature.

Showing the 12 most recent of 17 on this concept.

1. CBSE 2025Set ANNUAL1 markMCQ
   Q.Triple point of water is (A) 273 K (B) 100 K (C) 273.16 K (D) None of these

   ›Reveal solutionSolution

   The triple point of water is 273.16 K.

   The triple point of a substance is the unique combination of temperature and pressure at which its solid, liquid, and gaseous phases can all coexist in thermal equilibrium simultaneously. For water, this occurs at exactly:

   $T_{tr} = 273.16\text{ K} \;(\text{at a pressure of } 4.58\text{ mm of Hg})$

   …

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2. CBSE 2025Set ANNUAL1 markMCQ
   Q.The change from solid state to vapour state without passing through the liquid state is called (A) Fusion (B) Regelation (C) Vaporisation (D) Sublimation

   ›Reveal solutionSolution

   The direct solid-to-vapour phase change (without an intermediate liquid state) is called sublimation.

   • Fusion: solid → liquid.
   • Vaporisation: liquid → gas.
   • Regelation: melting under pressure followed by refreezing when pressure is removed (specific to substances like ice). …

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3. CBSE 2025Set ANNUAL1 markMCQ
   Q.The SI unit of latent heat is (A) joule (B) joule kg^-1 °C^-1 (C) J kg^-1 (D) cal g^-1 °C^-1

   ›Reveal solutionSolution

   The SI unit of latent heat is J kg⁻¹.

   Latent heat $L$ is defined by $Q = mL$, where $Q$ is the heat absorbed or released during a phase change (with no temperature change) and $m$ is the mass undergoing the change:

   $L = \dfrac{Q}{m}$

   …

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4. CBSE 2025Set ANNUAL1 markMCQ
   Q.20 gm ice at 0°C is mixed with 40 gm water at 10°C. Then the resultant temperature will be (A) 0°C (B) +5°C (C) +6.6°C (D) -5°C

   ›Reveal solutionSolution

   The heat available from the warm water is not enough to melt all the ice, so the resultant temperature stays at 0°C.

   Heat that could be released by 40 g of water cooling from 10°C to 0°C:

   $Q_{released} = mc\Delta T = 40\times1\times10 = 400\text{ cal}$

   Heat required to melt all 20 g of ice at 0°C (latent heat of fusion of ice $L_f \approx 80$ cal/g):

   $Q_{needed} = mL_f = 20\times80 = 1600\text{ cal}$

   …

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5. CBSE 2025Set ANNUAL1 mark
   Q.What is the S.I. unit of latent heat of vaporization?

   ›Reveal solutionSolution

   Latent heat is defined via $Q = mL$, so its unit is simply energy divided by mass.

   Latent heat $L$ is defined by the relation:

   $Q = mL$

   where $Q$ is the heat absorbed/released (in joules) during a phase change (here, liquid ↔ vapour) of a mass $m$ (in kg) at constant temperature.

   Rearranging: $L = Q/m$

   …

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6. CBSE 2024Set ANNUAL1 markMCQ
   Q.On increasing the pressure, the melting point of ice (A) increases (B) decreases (C) remains constant (D) none of these

   ›Reveal solutionSolution

   Increasing pressure lowers the melting point of ice.

   Most solids expand on melting, and pressure raises their melting point. Ice is an anomaly: it is less dense than the water it melts into (ice contracts as it melts). For such substances, the Clausius-Clapeyron relation predicts the opposite behaviour — increasing external pressure favours the denser …

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7. CBSE 2024Set ANNUAL1 markMCQ
   Q.The specific latent heat of vaporisation of water is (A) 80 kcal/kg (B) 536 kcal/kg (C) 4.2 kcal/kg (D) 1 kcal/kg

   ›Reveal solutionSolution

   Latent heat of vaporisation of water ≈ 536 kcal/kg (about $22.6\times10^5\,\text{J/kg}$).

   Latent heat of vaporisation is the amount of heat needed to change 1 kg of a liquid at its boiling point into vapour at the same temperature, with no temperature change during the process. For water at $100°\text{C}$ under normal atmospheric …

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8. CBSE 2024Set ANNUAL1 markMCQ
   Q.Temperature at which phase transition of matter occurs, depends (A) on volume (B) on pressure (C) on density (D) on mass

   ›Reveal solutionSolution

   The temperature of a phase transition depends on the pressure applied.

   A substance's P-T phase diagram shows curves along which solid-liquid, liquid-vapour, and solid-vapour phases coexist. Changing the pressure shifts the temperature at which a given phase boundary is crossed — e.g. water boils below $100°\text{C}$ at reduced (high-altitude) pressure and above …

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9. CBSE 2024Set SET-AP55001 mark
   Q.The point at which all three states of a substance are in equilibrium is called the ________ point.

   ›Reveal solutionSolution

   The triple point of a substance is the unique combination of temperature and pressure at which its solid, liquid, and gaseous phases all coexist simultaneously in thermodynamic equilibrium.

   On a substance's phase diagram (pressure vs temperature), the three phase-boundary curves — fusion (solid-liquid), vaporization (liquid-gas), and sublimation (solid-gas) — meet at exactly one point. At this specific pressure and temperature, all three phases can exist together in a stable, uncha …

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10. CBSE 2024Set SET-NDP60001 mark
    Q.The boiling point .................... (increases/ decreases) with increase in pressure.

    ›Reveal solutionSolution

    The boiling point of a liquid increases when the external (surrounding) pressure is increased.

    A liquid boils at the temperature where its saturated vapour pressure equals the external pressure acting on its surface. If the external pressure is raised (for example, inside a pressure cooker), the liquid must be heated to a higher temperature before its vapour pressure can rise to match this larger external pressure — so the boiling point increases with pressure. This is exactly why food cooks faster in …

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11. CBSE 2024Set ANNUAL1 mark
    Q.Fill in the blank: The amount of heat required to change the state of any matter is called ______.

    ›Reveal solutionSolution

    Latent heat is the heat absorbed or released during a change of state at constant temperature, without any temperature change.

    When a substance changes its physical state (e.g., ice melting to water, or water boiling to steam), heat is absorbed or released, but the temperature of the substance remains constant throughout the change of state. This heat is used entirely to break or form intermolecular bonds, not to raise the temperature. …

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12. CBSE 2023Set ANN1 mark
    Q.The change of state from liquid to solid is called ________.

    ›Reveal solutionSolution

    When a liquid loses heat and turns into a solid at a fixed temperature (its freezing point), the process is called freezing or solidification.

    Matter can exist in solid, liquid, and gaseous states, and it can change from one state to another when heat is added or removed at constant pressure. When a liquid is cooled, its molecules lose kinetic energy; at a particular temperature called the freezing point, the liquid begins to solidify as the molecules settle into a fixed, ordered arrangement. For a pure substance at a given pressure, this happens at a fixed temperature (equal to the melting point of the solid) and the te …

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