Physics · Thermodynamics · NEET
No. This is the most common mistake. The latent heat Q = mL is the total heat you supply. But by the first law, Q = delta U + W. During boiling the substance expands a lot (liquid to vapour), so it does work W = P times delta V on the surroundings. Only the leftover, delta U = Q minus W, stays inside as internal energy. So delta U is always LESS than mL for boiling. For 1 g of water: Q = 2256 J, W = 169.2 J, delta U = 2086.8 J.
Because vapour takes up far more volume than liquid. 1 g of water is about 1 cubic cm as liquid but about 1671 cubic cm as steam. This huge expansion pushes against the atmosphere (constant pressure), so the gas spends some energy as work W = P times delta V. That work energy leaves the system, so what remains as internal energy is smaller than the total heat you put in.
No. During melting or boiling the temperature stays constant (for example water boils at 100 degree C the whole time). The heat does not raise temperature; it breaks the bonds between molecules. That is exactly why this heat is called latent (hidden) heat - you add heat but the thermometer does not move.
Use W = P times delta V, where delta V is the volume change. For melting ice, the volume change is tiny (ice and water have almost the same volume), so W is nearly zero and delta U is almost equal to Q. For boiling, delta V is huge, so W matters a lot. Always convert cubic cm to cubic metre: 1 cc = 10 to the power minus 6 cubic metre.
Positive. The system (the substance) expands and does work ON the surroundings, so W is positive in the convention W = P times delta V (work done BY the gas). That makes delta U = Q minus W smaller than Q. If instead vapour condenses to liquid, the volume shrinks, work is done on the system, and W is negative.
0.1 g of water at 100 degree C and normal pressure (1.013 x 10^5 N/m^2) requires 54 cal to convert into steam at 100 degree C. If the volume of the steam produced is 167.1 cc, the change in internal energy is
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Start with Q = mL for the heat during a phase change. Then apply the first law Q = delta U + W. So delta U = mL minus P times delta V. Temperature is constant, but internal energy still changes because bonds break and the substance expands.
Because you add heat but the temperature does not rise. The heat is hidden inside as increased potential energy of the molecules (bonds broken) plus a small amount of work done in expanding, instead of showing up as a temperature increase.
Yes. When ice melts to water the volume change is very small, so the work W = P delta V is almost zero. Therefore delta U is nearly equal to the latent heat Q = mL. The big difference appears only during boiling, where the expansion is large.
Latent heat of fusion of ice is about 3.34 x 10^5 J/kg (about 80 cal/g), and latent heat of vaporisation of water is about 2.256 x 10^6 J/kg (about 540 cal/g, or 2256 J/g). Learn both in SI and cal units.
No. Internal energy delta U is only the energy stored inside the substance. The work W = P delta V leaves the system to push back the surroundings, so it is counted separately. That is why delta U = Q minus W.
