Concentration change redraws all reaction species

[learnthinkteach]

When the concentration slider is used (or when the window is resized), all of the particles in solution in the Molecules View are redrawn. When increasing and decreasing concentration, this performance makes it difficult to tell which species are increasing / decreasing, and whether they are doing so equally. In the java version of the sim, using the Concentration slider adds molecules/ions one particle at a time, so illustrate that more are being reacted forward or backward in the reaction equation.

I understand if this is not possible / not worth the time to enable, but I wanted to have the suggestion on record for future improvements. The performance of the java sim (where additional particles appear/disappear) is more pedagogically useful, I think, because it is clear what is happening to the number of particles.

[pixelzoom]

This is my bad, I forgot about this aspect of the 'Molecules' view. It can (and should) be addressed in ~2 hours. It will be a very isolated change, so will not affect other features in the sim.

[emily-phet]

@pixelzoom - I agree with Julia, this is significant and worth the effort to fix.

[pixelzoom]

Work completed, fix appears in http://www.colorado.edu/physics/phet/dev/html/acid-base-solutions/1.0.0-dev.9/acid-base-solutions_en.html