**31.** An amount of ice of mass \( 10^{-3} \, \text{kg} \) and temperature \( -10^\circ \text{C} \) is transformed to vapour of temperature \( 110^\circ \text{C} \) by applying heat. The total amount of work required for this conversion is: (Take: Specific heat of ice = \( 2100 \, \text{Jkg}^{-1}\text{K}^{-1} \), Specific heat of water = \( 4180 \, \text{Jkg}^{-1}\text{K}^{-1} \), Specific heat of steam = \( 1920 \, \text{Jkg}^{-1}\text{K}^{-1} \), Latent heat of ice = \( 3.35 \times 10^5 \, \text{Jkg}^{-1} \), Latent heat of steam = \( 2.25 \times 10^6 \, \text{Jkg}^{-1} \)) Options: - (1) 3022 J - (2) 3043 J - (3) 3003 J - (4) 3024 J

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Published July 8, 2025
Physics
Thermodynamics
Calorimetry
Phase Change
Specific Heat
Latent Heat

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Detailed Explanation

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Simple Explanation (ELI5)

๐ŸงŠโžก๏ธ๐Ÿ’งโžก๏ธ๐Ÿ’จ Turning ice-cold ice into hot steam

Imagine you have a tiny ice cube that is really cold ( โˆ’10 ยฐC ). You want to turn it into hot steam ( 110 ยฐC ).
To do that you must keep adding heat while the ice passes through several stages:

  1. Warm the ice so it is not so cold.
  2. Melt the ice into water โ€” this needs extra energy called latent heat.
  3. Warm the water up to boiling point.
  4. Boil the water into steam โ€” again a latent heat step.
  5. Warm the steam a little more. Add the energy for every step and you get the total heat you must supply.
    Because the ice piece is very tiny (only 1 g), the final answer is only about 3 kilo-Joules of energy.

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Step-by-Step Solution

Given Data

Mass: m=10โˆ’3โ€‰kgm = 10^{-3}\,\text{kg}

Specific heats
Ice: ci=2100โ€‰Jkgโˆ’1Kโˆ’1c_i = 2100\,Jkg^{-1}K^{-1}
Water: cw=4180โ€‰Jkgโˆ’1Kโˆ’1c_w = 4180\,Jkg^{-1}K^{-1}
Steam: cs=1920โ€‰Jkgโˆ’1Kโˆ’1c_s = 1920\,Jkg^{-1}K^{-1}

Latent heats
Fusion (ice): Lf=3.35ร—105โ€‰Jkgโˆ’1L_f = 3.35\times10^{5}\,Jkg^{-1}
Vaporisation (steam): Lv=2.25ร—106โ€‰Jkgโˆ’1L_v = 2.25\times10^{6}\,Jkg^{-1}

Temperature changes
Ice: โˆ’10โˆ˜Cโ†’0โˆ˜Cโ‡’ฮ”T1=10โ€‰K-10^{\circ}C \to 0^{\circ}C \Rightarrow \Delta T_1 = 10\,K
Water: 0โˆ˜Cโ†’100โˆ˜Cโ‡’ฮ”T3=100โ€‰K0^{\circ}C \to 100^{\circ}C \Rightarrow \Delta T_3 = 100\,K
Steam: 100โˆ˜Cโ†’110โˆ˜Cโ‡’ฮ”T5=10โ€‰K100^{\circ}C \to 110^{\circ}C \Rightarrow \Delta T_5 = 10\,K


Step-wise Heat Calculations

  1. Warm the ice Q1=mciฮ”T1=(10โˆ’3)โ€‰(2100)โ€‰(10)=21โ€‰JQ_1 = m c_i \Delta T_1 = (10^{-3})\,(2100)\,(10) = 21\,J

  2. Melt the ice Q2=mLf=(10โˆ’3)โ€‰(3.35ร—105)=335โ€‰JQ_2 = m L_f = (10^{-3})\,(3.35\times10^{5}) = 335\,J

  3. Warm the water Q3=mcwฮ”T3=(10โˆ’3)โ€‰(4180)โ€‰(100)=418โ€‰JQ_3 = m c_w \Delta T_3 = (10^{-3})\,(4180)\,(100) = 418\,J

  4. Vaporise the water Q4=mLv=(10โˆ’3)โ€‰(2.25ร—106)=2250โ€‰JQ_4 = m L_v = (10^{-3})\,(2.25\times10^{6}) = 2250\,J

  5. Warm the steam Q5=mcsฮ”T5=(10โˆ’3)โ€‰(1920)โ€‰(10)=19.2โ€‰JQ_5 = m c_s \Delta T_5 = (10^{-3})\,(1920)\,(10) = 19.2\,J


Total Heat / Work Required

Q_{\text{total}} &= Q_1 + Q_2 + Q_3 + Q_4 + Q_5 \\[4pt] &= 21 + 335 + 418 + 2250 + 19.2 \\[4pt] &= 3043.2\,J \approx 3043\,J \end{aligned}$$ **Therefore, the required work/heat is closest to option (2) 3043 J.**

Examples

Example 1

Pressure cookers: more energy is needed to convert water to steam at higher boiling points.

Example 2

Meteorology: Large latent heat of water drives storm formation when moist air condenses.

Example 3

Refrigerators: Reverse processโ€”removing latent heat to freeze water into ice.

Example 4

Steam engines: Phase change energy is converted into mechanical work.

Visual Representation

References

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