A liquid is left in an open dish. After several days, there is less liquid in the dish. Which of the following statements explains this observation? The

Step 1: Analyze the question A liquid left in an open dish gradually reduces in volume. This process is evaporation, and we need the statement that explains it at the molecular level.

Step 2: Explain the concept The molecules in a liquid move randomly, with different kinetic energies. Some have high energy, and some have low energy.

• Molecules at the surface that have enough kinetic energy can overcome the attractive forces (intermolecular forces) holding them in the liquid.
• These molecules escape into the air.
• Because the most energetic molecules are the ones leaving, the average kinetic energy of the remaining liquid falls. This is why evaporation causes cooling.

Step 3: Evaluate the options

• A. Most energetic molecules leave the surface and escape into the air. Correct. This is exactly how evaporation happens.
• B. Least energetic molecules escape. Wrong. They lack the energy to overcome the intermolecular attraction.
• C. Most energetic molecules leave and return. Wrong. If they returned, the liquid level would not fall. In an open dish, the escaped molecules are carried away by air currents.
• D. Least energetic molecules leave and return. Wrong. It is both physically incorrect and does not explain the loss of liquid.

Step 4: Answer The liquid decreases because its fastest, most energetic surface molecules escape into the air, so the answer is A.

Exam tip: Remember "Evaporation = Energetic molecules Escape." Also, evaporation causes cooling because the high-energy molecules leave, lowering the average energy of those left behind. In an open dish, escaped molecules do not return, so the liquid steadily reduces.