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Kinetic Molecular Theory

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A series of free High School Chemistry Video Lessons and solutions.

In this lesson, we will learn
  • three states of matter
  • phase change
  • phase diagrams
  • boiling points
Three States of Matter
The three states of matter, also called the phases of matter, are solid, liquid and gas. Matter changes phases based upon thermodynamic principles like enthalpy and entropy. At room temperature, different elements are in different states because of their intermolecular forces.

The properties of the three phases of matter
• definite shape and volume
• Are the particles in motion?
• density?
Types of Crystalline Solids and their characteristics
1) atomic
2) molecular
3) covalent network
4) ionic
5) metallic

• no definite shape but has definite volume
• fluidity - ability to flow
• viscosity - how much it resists flow
• surface tension - measurement of inward flow

• no fixed shape or volume

Gases and Kinetic Molecular Theory
How to describe the behavior of gases in terms of motion?
Assumptions or postulates of the kinetic molecular theory of gases • Gases do not attract/repel each other.
• Gas particles have no volume.
• Gas particles are in constant/random motion.
• No kinetic energy is lost when particles collide with each other or the walls of the container.
• Al gas particles have the same kinetic energy at a given temperature. Plasma - the fourth state of matter
For certain substances, if we continue to apply heat to their gaseous form, another change of state could occur. These substances can go from a gas to a state of matter called plasma. For this change of state to occur, very strong heat must be applied. When heat is sufficiently strong, the electrons are stripped from their respective atoms, creating free electrons and positive ions. Although there are both negative and positive particles, overall, plasma is neutral as there are equal amounts of oppositely charged particles. Because there are free electrons, substances in a plasma form can conduct electricity.
This is what separates a gas from plasma – gases cannot conduct electricity, but plasma can.
Naturally occurring plasma include lightning and the Northern lights. Stars also exist in plasma form – in fact stars are just really hot balls of plasma. Plasma can be found in fluorescent light bulbs, neon signs, and plasma TVs.
Introduction to the states or phases of matter and intermolecular forces Phase Change
Phase changes are the transformations from one state of matter to another due to thermodynamics. The processes of phase change between solid and liquid are called melting and freezing. Phase changes between liquid and gas are vaporization and condensation. Phase changes between gas and solid are deposition and sublimation. Phase changes can be spontaneous or non-spontaneous.

How to predict and understand the circumstances under which matter changes phase?

What does a phase change look like at the molecular level?
We'll look at the molecular structure in solid, liquid, and gas phases, and see how the kinetic energy of the particles changes. We'll talking about melting, vaporization, condensation, and freezing. Phase Diagrams
Phase diagrams graphically depict the state of matter in varying temperatures and pressures. The x-axis of a phase diagram is always temperature while the y-axis is always pressure. There is a point on a phase diagram called the triple point at which all three phases of matter exist simultaneously.

How to draw and interpret phase diagrams? How phase change diagrams are made and how to read them? Boiling Point
Boiling point is the particular temperature where vapor pressure equals to of a liquid equals to the surrounding environmental liquid. At this temperature the liquid begins to bubble as liquid below the surface turns to gas and escapes upwards.

The point at which matter boils. How to relate vapor pressure to boiling point?
Why there are different boiling points at different elevations?

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