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MCAS Test Questions

Open Response

Standard: 6.3 - Using the kinetic molecular theory, describe and contrast the properties of gases, liquids,

and solids. Explain, at the molecular level, the behavior of matter as it undergoes phase transitions.

At sea level, water boils at 100°C. The boiling point of water varies with changes in air pressure, as shown

in the diagram below.

a. Based on the diagram, describe what happens to the boiling point of water as air pressure decreases.

b. Use the kinetic molecular theory to describe the process of boiling.

c. Use the kinetic molecular theory to explain why the boiling point of water is affected by pressure.

a. Based on the diagram, as the air pressure decreases, the boiling point also decreases. This occurs

because as the pressure drops, the point on the phase boundary between water’s liquid and solid

phases get lower (farther down the line).

b. As water is boiling, heat is added to the system. This resuts in an increase of kinetic energy of the w

water molecules. Kinetic energy (the energy of the motion) causes the particles to become more

excited. At this time, the intermolecular forces (forces between the molecules) are being broken.

When the pressure of the water molecules (vapor pressure) is equal to the atmospheric pressure, that

is, when the kinetic enery of the water molecules is enoughly high, the intermolecular forces break

and water boils.

c. In order for water to boil, there must be enough kinetic energy so that the pressure of the water

(vapor pressure) is equal to the atmospheric pressure. If the atmospheric pressure is lower, the

molecules need less kinetic energy to reach this point, and vice versa. As kinetic energy is directl

proportional to the temperature, at lower pressure, water boils at a lower temperature.