Melting Point Temperature Behavior

The question asks about the temperature behavior of ice specifically while it is melting. Ice has a well-defined melting point, which is the temperature at which it transitions from a solid state to a liquid state (water). For ice under standard atmospheric pressure, this melting point is $0^\circ C$.

Explanation of Constant Temperature During Melting

When ice is at its melting point ($0^\circ C$) and begins to melt, it absorbs heat energy from the surroundings. This process is called phase transition. The crucial point is how this absorbed energy affects the temperature.

The heat energy absorbed during melting is not used to increase the kinetic energy of the water molecules (which is what temperature measures). Instead, this energy, known as the latent heat of fusion, is used entirely to overcome the forces holding the water molecules in a fixed, rigid structure in the solid ice. It breaks the bonds that define the solid state.

• Heat Absorption: Energy is absorbed by the ice.
• Purpose of Energy: The energy breaks intermolecular bonds in the solid structure.
• Kinetic Energy: The average kinetic energy of the molecules does not increase during this phase change.
• Temperature: Since temperature is a measure of the average kinetic energy, it remains constant.

Therefore, as long as there is still some ice left to melt, the temperature of the mixture of ice and water will stay steady at $0^\circ C$. Only after all the ice has converted into liquid water can the temperature of the water begin to rise if more heat is supplied.

Analyzing Incorrect Options

Let's look at why the other options are incorrect during the melting phase:

• Option 1: first decreases and then remains same - The temperature might decrease to reach $0^\circ C$ *before* melting starts, but it does not decrease further *during* the melting process itself.
• Option 2: increases - This is incorrect because the added heat energy is used for the phase change (breaking bonds), not for increasing the molecular motion (temperature). Temperature only increases after all ice has melted.
• Option 3: decreases - This is incorrect. The temperature remains stable at the melting point during the phase transition.
• Option 4: remains same - This accurately describes the behavior. The temperature stays constant at $0^\circ C$ throughout the entire melting process.

This phenomenon is fundamental to understanding phase changes in thermodynamics.