Thermal Properties of Matter MCQs for NEET — Physics Questions with Answers

The text states: "The molar specific heat capacity of a substance is defined by $C = \frac{1}{\mu} \frac{\Delta Q}{\Delta T}$ where $\mu$ is the number of moles of the substance." This matches option 1.

The definition provided is: "The latent heat of fusion ($L_f$) is the heat per unit mass required to change a substance from solid into liquid at the same temperature and pressure." This aligns with option 2.

As per the text: "The latent heat of vaporisation ($L_v$) is the heat per unit mass required to change a substance from liquid to the vapour state without change in the temperature and pressure." This corresponds to option 2.

The text states: "The unit of C is J mol$^{-1}$ K$^{-1}$." This is consistent with the definition of molar specific heat capacity being per mole, as opposed to specific heat capacity being per unit mass.

According to the NCERT text, 'The latent heat of fusion (Lf) is the heat per unit mass required to change a substance from solid into liquid at the same temperature and pressure. The latent heat of vaporisation (Lv) is the heat per unit mass required to change a substance from liquid to the vapour state without change in the temperature and pressure.' This clearly indicates that latent heat is associated with a change of state at constant temperature.

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.

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.

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.

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 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, $Q = 2 ext{ kg} imes 3.33 imes 10^5 ext{ J kg}^{-1} = 6.66 imes 10^5 ext{ J}$.