Heat — Important Questions

100°C

Step 1: Boiling occurs when vapor pressure equals atmospheric pressure. Step 2: At standard atmospheric pressure (1 atm), water molecules gain enough energy to form bubbles. Step 3: This happens at exactly 100°C or 373 Kelvin for pure water. Step 4: At this temperature, liquid water rapidly converts to steam throughout the liquid. Step 5: 0°C is the melting point, while 273°C and 373°C are incorrect values for boiling point in Celsius.

It remains constant

Step 1: During melting, heat energy is supplied to break intermolecular bonds in ice. Step 2: This energy goes into changing the state from solid to liquid, not increasing temperature. Step 3: The temperature stays at 0°C until all ice melts completely. Step 4: Only after complete melting does temperature start rising again. Step 5: This constant temperature during phase change is characteristic of all pure substances.

80 cal/g

Step 1: Latent heat of fusion is energy needed to convert 1g solid to liquid at melting point. Step 2: For ice, this represents energy to break hydrogen bonds holding water molecules in crystal structure. Step 3: Experimentally determined value for ice is 80 cal/g at 0°C. Step 4: This means 80 calories are needed to melt 1g of ice without temperature change. Step 5: 540 cal/g is the latent heat of vaporization, not fusion.

Mass of the container

Step 1: Evaporation depends on molecular escape from liquid surface. Step 2: Larger surface area provides more escape routes for molecules. Step 3: Higher temperature gives molecules more kinetic energy to escape. Step 4: High humidity means more water vapor in air, reducing evaporation rate. Step 5: Container mass has no effect on molecular behavior at the liquid surface, making it irrelevant to evaporation rate.