Brock Middle School 6th Grade Science
Day 1 | Day 2 | Day 3 | Lab | Skills Test

Define a solid, liquid, and gas in terms of volume and shape. Explain why solids, liquids, and gases have these properties. Explain the difference between crystals and amorphous solids

Assumptions of the kinetic theory:

All matter is composed of small particles. The particles of matter are in constant motion. All collisions between the particles of matter are perfectly elastic.

Particle motion: atoms move in a straight line between collisions.

• An O₂ molecule in the air travels about 443 m/s (about 1700 km/hour) at 25 ^oC.
• The diameter of an O₂ is about 0.339 nanometers. Each molecule travels about 314 times its own diameter between collisions. At this rate, each oxygen molecule has over four and a half billion collisions per second.

• The SI unit for pressure is the pascal.
• The pascal is a derived SI unit equal to Newtons / m².
  • 1 N = (1 kg)(1 m) / s²
• The average air pressure at sea level is 101.325 kilopascals (kPa).
• Scientists agree to call this average "standard atmospheric pressure".

• Kinetic energy is the energy an object possesses because of its motion.
• The SI units for energy is the joule.
• The joule is a derived SI unit equal to (kilograms)(meters) / s².
• KE = mv² / 2
• The calorie is sometimes used to describe to describe heat. This unit is defined as the amount of heat needed to raise the temperature of one gram of water one degree Celsius. A "food calorie" is 1000 calories, or one Kilocalorie.

In-class Assignment 161:
This assignment must be turned in by the end of class today to receive credit.
Scoring criteria

1. What is meant by a "perfectly elastic" collision?
2. At 25 ^oC, how many nanometers does an oxygen molecule travel between collisions?
3. A force of 2.5 Newtons is acting on a 300 square centimeter surface. What pressure is exerted on the surface?
4. Which has the greater kinetic energy: an object with a mass of 10g traveling at 2m/s or an object with a mass of 2g traveling at 10m/s?
5. Calculate the average kinetic energy of an oxygen molecule at 25 ^oC.

Day 2

Physical states of matter: solid, liquid, gas, plasma.

The physical states of matter can commonly be defined by their physical characteristics. A solid has a definite volume and shape, a liquid has a definite volume but not a definite shape, and a gas has neither definite volume nor shape. While these are true characteristics, they are not the best description of the physical states.

(chem lab 054)
The kinetic theory can be used to describe the physical states of matter:

• Solid - a substance whose particles have a low kinetic energy. The particles of a solid are held close together by intermolecular forces of attraction. Because the particles are so close together, they appear to vibrate around a fixed point.

  When the temperature of a solid is raised, the velocity of the particles increases. The collisions between the particles occur with greater force, causing the particles to more farther apart. The ordered arrangement of the solid breaks down and a change in physical state occurs.
  

  

• Liquid - a substance whose particles have enough kinetic energy to stretch the intermolecular forces of attraction. Collisions between the particles a strong enough to force the particles apart. The particles appear to have a moving vibration because they are still under the influence of the intermolecular forces of attraction.

  As the temperature of a liquid is raised, the velocity of the particles increases. The collisions eventually become so great that the particles break all intermolecular forces, begin moving independently between collisions, and a change in physical state occurs.
  

  

• Gas - a substance whose particles have enough kinetic energy to break all intermolecular forces of attraction. The particles of a gas move independently of each other. The particles move at random because they have overcome the intermolecular forces of attraction.

  When a gas is raised to extreme temperatures, over 5000 ^oC, they have so much kinetic energy that their collisions will break electrons out of the atoms, and a change in physical state occurs.

• Plasma (a special form of gas) - a plasma can be defined as a charged gas. The particle collisions are violent enough to break electrons out of the atoms, producing particles with charges (electrons and positive ions).
• Physical state at room temperature (25 ^oC) and standard atmospheric pressure:

  Under these conditions, the physical state of a substance depends mostly on the chemical bond characteristics of the substance. Ionic compounds have strong electric charges holding the ions together and exist as solids. Nonpolar molecular compounds of low molecular mass tend to be gases. Greater molecular mass and greater polarity both tend to make substances more dense, producing either liquid or solid.

• Intermolecular forces of attraction: Within an atom, these forces are called weak forces, because they are much weaker than chemical bonds between atoms. Weak forces involve the attraction of the electrons of one atom for the protons of another atom. When these forces interact between molecules, they are known as van der Waals forces. Here are some forms of van der Waals forces:
  • Dipole-dipole forces - two molecules that are both permanent dipoles are attracted to each other.

  A water molecule is a dipole (polar molecule). The hydrogen electron is attracted toward the eight protons of oxygen. The shape of the water molecule causes one side of the molecule to have a slightly positive charge and the other side of the molecule to have a slightly negative charge.
  	Water molecules are attracted to each other because of their polar characteristics. The negative side of one molecule is attracted to the positive side of another molecule. Many of the special properties of water are caused by these dipole to dipole forces.

  
  • Dipole-induced dipole forces - a dipole transforms a nonpolar molecule into a dipole and an attraction occurs.
  • Dispersion forces - the attraction of two nonpolar molecules. Dispersion forces account for over 85% of the van der Waals forces.

Test Your Concept Understanding: A sample of gas is moved from a 100cm3 to a 200cm3 container. Describe any changes in each of these values: The average kinetic energy of the gas molecules. The pressure exerted by the gas. The average speed of the gas molecules. The number of collisions of gas molecules with other gas molecules. What is a dipole? Explain how an induced dipole is formed? In terms of the kinetic theory, what is the significance of absolute zero? Define the term vapor.