Which of the following best represents the temperature versus heat supplied graph for water, in the range of $-20^\circ\text{C}$ to $120^\circ\text{C}$?
Answer: (C) see figure
Heating ice at $-20^\circ$C to steam at $120^\circ$C:
1. Ice warms from $-20^\circ$C to $0^\circ$C (sloping line).
2. Melting at $0^\circ$C: temperature stays constant (flat part), heat $= mL_f$.
3. Water warms from $0^\circ$C to $100^\circ$C (sloping line).
4. Boiling at $100^\circ$C: flat part, heat $= mL_v$. Since $L_v\ (\approx 2260$ J/g$) \gg L_f\ (\approx 334$ J/g$)$, this flat part is much **longer** than the one at $0^\circ$C.
5. Steam warms above $100^\circ$C.
Only graph (C) has exactly two flat parts, at $0^\circ$C and $100^\circ$C, with the one at $100^\circ$C longer.
Solution by Sreeraj P, M.Sc Physics