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JEE Advanced 2019 · Thermal Properties of Matter

A liquid at 30∘C30^\circ\text{C} is poured very slowly into a Calorimeter that is at temperature of 110∘C110^\circ\text{C}. The boiling temperature of the liquid is 80∘C80^\circ\text{C}. It is found that the first 5 gm5\,\text{gm} of the liquid completely evaporates. After pouring another 80 gm80\,\text{gm} of the liquid to its specific heat will be ___∘C^\circ\text{C}. [Neglect the heat exchange with surrounding]

Answer: 270°C270°C

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Written solution

Step 1: Identify the initial parameters of the system.

The given parameters are:

Tliquid=30∘CT_{\text{liquid}} = 30^\circ\text{C} Tcalorimeter=110∘CT_{\text{calorimeter}} = 110^\circ\text{C} Tboil=80∘CT_{\text{boil}} = 80^\circ\text{C}

Step 2: Analyze the heating and evaporation of the first 5 g5\text{ g} of liquid.

As the liquid is poured slowly, it absorbs heat from the calorimeter and its temperature rises from 30∘C30^\circ\text{C} to 80∘C80^\circ\text{C} according to:

Q=mcΔTQ = mc\Delta T

Once the liquid reaches its boiling point of 80∘C80^\circ\text{C}, it begins to evaporate. The heat required for this phase change is:

Q=mLQ = mL

Since the first 5 g5\text{ g} of the liquid completely evaporates, the liquid has reached its boiling point.

Step 3: Determine the temperature after adding more liquid.

When another 80 g80\text{ g} of liquid is added, it continues to boil. During a phase change (boiling), the temperature of the liquid remains constant at its boiling point.

T=Tboil=80∘CT = T_{\text{boil}} = 80^\circ\text{C}

Answer: 80∘C80^\circ\text{C}

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