Passage 9: Thermodynamics

In a series of experiments that investigated the thermodynamics of phase changes,

the journey of a water molecule as it transitions from ice to steam and back to

liquid water was documented. It began with a block of ice at -10°C, which was

gradually heated with a controlled heat source. The ice melted, boiled into steam,

and finally condensed back into water. Temperature changes were meticulously

recorded, and heat input at each stage was determined via sensors on the heat

source.

The previous results applied to an open system (the water was open to the

atmosphere). The team also monitored how energy input values changed when a

closed system was used instead. They found that the amount of energy required

to boil the water was dramatically affected by this adjustment, but not the energy

required to melt the ice.

Click to reveal answer

Correct answer: D. Pressure change is the key factor here. In a closed

system, the pressure increases most dramatically during boiling because the

change in volume between water and steam is much larger than from ice to water.

This increased pressure requires more energy to convert water to steam, as the

equilibrium is shifted toward the liquid phase. The energy required to melt ice is

less affected by pressure changes because the volume change from ice to water is

relatively small. For this reason, it requires more energy to boil water as the partial

pressure of water vapor in the air above the water increases dramatically, shifting

equilibrium toward liquid water.

If you have any questions or see any issues with this page, please get in touch with matthew.russell@inspiraadvantage.com

Day 52 MCAT Practice Question