Phase Changes Intro

Phase changes Part 1

Phenomenon: Why does water appear blue?

Open Ended

Prediction: Why do you think water appears blue?

Phenomenon: Why does water appear blue?

Water appears blue because of the motion of water molecules.

Semester 1 Review

Phase Changes

Multiple Choice

What is a characteristic of a gas phase?

• Molecules move quickly throughout their container

• Molecules move slowly throughout their container

Multiple Choice

What is a characteristic of a solid?

• They take the shape of their container

• They hold their own shape

Multiple Choice

What is a characteristic of a liquid?

• The molecules move slower than the molecules in solids

• the molecules move slower than the molecules in gases.

Kinetic Theory 

and Motion

• The kinetic theory states that atoms and molecules are constantly in motion.

• Kinetic energy is the highest in the gas phase.

• How do you think this relates to temperature?

Temperature

The temperature of any substance is a measurement of that substance's kinetic energy, or molecular movement.

Multiple Choice

As the temperature increase, what happens to the kinetic energy (energy of motion) of the molecules?

• The molecules move faster

• The molecules move slower

• The molecules stay the same

Vibrations

• Vibrations within molecules can absorb energy.

• In water, the bonds between the oxygen atom and each hydrogen atom can vibrate at just the right frequency to absorb all colors of light except blue.

• Because all the colors except blue have been absorbed by the water molecules, the "leftover" blue light makes water look blue.

Pressure

Pressure comes from gas molecules colliding with the walls of their container.

As the gas molecules move around, they frequently collide with the walls of their container. The force of gas molecules colliding with their container is called pressure.

Pressure is often measured in the

unit atmosphere (atm). Normal air pressure at sea level is 1 atm.

Phase Diagrams

Phase diagrams show the phase of a substance at a variety of temperatures and pressures.

Reading a Phase Diagram

Pressure is shown on the vertical (y-axis) of a phase diagram, and temperature is shown on the horizontal (x-axis).

The different sections on the phase diagram show the temperatures and pressures at which a substance will be in each phase. The lines within a phase diagram indicate where one phase ends and the next begins.

Multiple Choice

When water is at 25°C and has a pressure of 0.5 atm, what phase is it in?

• Solid

• Liquid

• Gas

Melting and Boiling Points on a Phase Diagram

The lines on a phase diagram indicate the melting and boiling points.

Finding Melting and Boiling Points

When you need to find a melting point or boiling point of a substance at a given pressure, first draw a line across the phase diagram at that specific place. For example, say you want to find the temperature at which water boils at 1.0 atm.

Multiple Choice

Would the boiling point of water at 0.5 atm be lower, the same, or higher than the boiling point of water at 1 atm?

• Lower

• The same

• Higher

Comparing Phase Diagrams

The phase diagram for every substance is unique.

For example, here are the phase diagrams for carbon dioxide (CO₂) and water (H₂O).

Water Is the Exception

Water has an unusual phase diagram.

The melting point lines on the carbon dioxide (CO₂) and water (H₂O) graphs slope in different directions. As the pressure is increased, CO₂ goes from gas to liquid to solid. However, as pressure is increased in solid water, it melts.

Almost all substances have melting point lines that slope like CO₂.

Intermolecular Forces

• All molecules have Van der Waals forces, also called dispersion forces. These are intermolecular forces that result from momentary attractions between electrons and protons.

• Polar molecules have dipole moments leading to dipole-dipole intermolecular forces.

• Molecules with H-N, H-F, or H-O bonds can have hydrogen bonds.

Intermolecular Forces and Melting

Intermolecular forces must be overcome in order for melting to occur.

As you add heat to a solid, the kinetic energy of the molecules increases. Eventually, the molecules have enough kinetic energy to overcome the intermolecular forces. It is at this point that the solid melts into a liquid.

Intermolecular Forces and Boiling Point

The stronger the intermolecular forces in a liquid, the higher the boiling point.

Multiple Choice

Molecules with which intermolecular forces will have the highest melting and boiling points?

• Van der Waals

• Dipole-Dipole

• Hydrogen Bonds