Science By the Horns Summer 2004

INTRODUCTION TO STATES OF MATTER Science By the Horns Summer 2004

State Model The model allows: –Students to start to think of different states of matter as being associated with different KINDS OF MOTION by the SAME kinds of molecules. Water in solid, liquid or gaseous form is still water. Students can begin to associate increased temperature with increased motion of the molecules Use of this software should be accompanied by Chemistry Theater activities (have the students move as particles in motion - the different kinds of motion permitted are: –fixed location and vibration (solid); –sliding near each other but still touching and together (liquid); and, –moving freely and independently, filling all available space (gas). Introduce the idea that adding heat provides the energy needed to change state and results in increased disorder (entropy).

Open State Model Follow sequence on left. How like Chem Theater Change of phase requires heat (energy) Observe increase in disorder (entropy) b/c solid is more ordered than liquid (and then gas)

Some Caveats This is just a introductory activity - many aspects of it are not technically correct. These include: –Motions of particles in solid and liquid phases are influenced by attractive and repulsive forces between the particles; –The speeds of individual particles are not uniform but are distributed in a predictable pattern (at equilibrium). We discussed the (Maxwell- Boltzman distribution) in order to explain the process of evaporation.

A More Accurate Account A more accurate model of gases can be found in the models library of NetLogo under Sample Models, Chemistry & Physics, GasLab and then the file GasLab in a box. Even if the molecules all start off at the same speed they will, through the collisions, redistribute themselves in the Maxwell-Boltzman distribution. The distribution is lopsided (more on the left than the right) and has more slow molecules than comparably fast molecules. This is NOT a “bell curve”.

evaporation

Evaporation Particles with “high energies” have enough energy to spontaneously leave the liquid state and enter the gaseous state.

Connection to later Material Students need to develop a better sense of histograms before the graph of distribution of speeds (the M-B distribution) will make much sense. This is something we’ll be working on.

Connection to Fractions in Saturation Game Later we will talk about a fraction of the molecules moving fast enough to shift “up” in terms of phase or slow enough to move down. Think about this distribution as an oddly shaped cake. If the cake is a liquid then some fraction (at the right) is moving fast enough to become a gas. If another cake represents a gas, some fraction is moving slow enough to drop down to become a liquid. Heating/cooling the system causes these fractions to change. Heating causes the bump to move to the right, resulting in a greater fraction of the liquid cake being ready to move “up” to gas. “slice” or fraction going fast enough to move up… Liquid “cake” Gas “cake” slow “slice”