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What is the SI unit of latent heat?
The NCERT text states, 'The amount of heat per unit mass transferred during change of state of the substance is called latent heat of the substance for the process. ... Its SI unit is J kg–1.' Therefore, the SI unit is Joules per kilogram.
A burn from steam at 100 °C is more severe than a burn from boiling water at 100 °C because:
The NCERT text explains, 'For water, the latent heat of fusion and vaporisation are $L_f = 3.33 imes 10^5 ext{ J kg}^{-1}$ and $L_v = 22.6 imes 10^5 ext{ J kg}^{-1}$, respectively. That is, $3.33 imes 10^5 ext{ J}$ of heat is needed to melt 1 kg ice at 0 °C, and $22.6 imes 10^5 ext{ J}$ of heat is needed to convert 1 kg water into steam at 100 °C. So, steam at 100 °C carries $22.6 imes 10^5 ext{ J kg}^{-1}$ more heat than water at 100 °C. This is why burns from steam are usually more serious than those from boiling water.' This additional heat is the latent heat of vaporization that is released when steam condenses.
When heat is continuously added to a substance undergoing a phase change, what happens to its temperature?
The NCERT text clearly states, 'Note that when heat is added (or removed) during a change of state, the temperature remains constant.' This is a fundamental characteristic of phase transitions.
What is the term for the change of state from solid to vapor directly, without passing through the liquid state?
The NCERT text describes, 'The change from solid state to vapour state without passing through the liquid state is called sublimation, and the substance is said to sublime. Dry ice (solid CO2) sublimes, so also iodine.'
The latent heat of fusion (Lf) for water is $3.33 imes 10^5 ext{ J kg}^{-1}$. How much heat is required to melt 2 kg of ice at 0 °C into water at 0 °C?
The quantity of heat required for a change of state is given by $Q = mL$. Given $m = 2 ext{ kg}$ and $L_f = 3.33 imes 10^5 ext{ J kg}^{-1}$. So, $Q = 2 ext{ kg} imes 3.33 imes 10^5 ext{ J kg}^{-1} = 6.66 imes 10^5 ext{ J}$.
When 1 kg of water at 100 °C is converted into steam at 100 °C, the heat absorbed is approximately:
The NCERT text states, 'For water, the latent heat of fusion and vaporisation are $L_f = 3.33 imes 10^5 ext{ J kg}^{-1}$ and $L_v = 22.6 imes 10^5 ext{ J kg}^{-1}$, respectively. ... $22.6 imes 10^5 ext{ J}$ of heat is needed to convert 1 kg water into steam at 100 °C.' This refers to the latent heat of vaporization.
In the process of sublimation, which states of matter coexist in thermal equilibrium?
The NCERT text mentions, 'During the sublimation process both the solid and vapour states of a substance coexist in thermal equilibrium.'
The latent heat of an irreversible process like melting or boiling depends on:
The NCERT text states, 'Its SI unit is J kg–1. The value of L also depends on the pressure. Its value is usually quoted at standard atmospheric pressure.' Also, $Q = mL$, so it depends on the mass. Thus, it depends on the mass of the substance and the characteristic of the substance, which implies inherent properties and conditions like pressure.
What is the relationship between the latent heat ($L$) and the amount of heat ($Q$) required for a change of state for a mass ($m$) of a substance?
The NCERT reference clearly defines this relationship as '$Q = m L$ or $L = Q/m$ (10.13)', indicating that $Q = mL$ is the correct formula.
Consider a plot of temperature versus heat added for water at 1 atm pressure. What does a horizontal segment on this graph represent?
The NCERT text states, 'Note that when heat is added (or removed) during a change of state, the temperature remains constant.' A horizontal segment on a temperature-heat plot signifies that heat is being added without a change in temperature, which is characteristic of a phase transition.
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