[A really-good answer to this question should include quantitative evidence, but here is my hand-waving version...]

Solvated sodium ions ($\ce{Na+}$) aren't all that reactive.  Surrounded by a lot of polar water molecules, they're actually quite stable.  What you're thinking of is solid sodium (Na) which will "react violently with water" until it's all dissolved - dissolved that is, into stable sodium ions.

Same kind of logic applies to chlorine.  Solvated chloride ions ($\ce{Cl-}$) are stable and not nearly as poisonous as chlorine gas. (Note:  everything is poisonous at some level, i.e. "the poison is in the dose".  In saying chlorine gas is more poison, that really just means it is a more 'potent' poison:  it will kill at lower doses compared to chloride ions.)



Another way to think of it: 
Solid sodium and chlorine gas are at high energy states.  Since they can reach much lower energy states by ionzing in the presence  with water, the WILL break down and ionize in the presence of water  

Why is it not dangerous to dissolve NaCl?
With multiple monitors, I have so far been dragging windows manually around. Is there a way to make a window open on a specific screen in a dual-head setup? (Not to be confused with multiple X server displays.)

Something like: SCREEN=2 firefox or open-in-screen 2 firefox.

If it matters, my environment is KDE using the KWin window manager. KWin window rules can only match some properties like title, but I could not find an environment variable as filter.

The secondary display extends the primary screen:

$ xrandr -q
Screen 0: minimum 320 x 200, current 3520 x 1080, maximum 32767 x 32767
LVDS1 connected 1600x900+0+0 (normal left inverted right x axis y axis) 382mm x 214mm
   1600x900       60.1*+
   1024x768       60.0  
   800x600        60.3     56.2  
   640x480        59.9  
VGA1 disconnected (normal left inverted right x axis y axis)
HDMI1 connected 1920x1080+1600+0 (normal left inverted right x axis y axis) 477mm x 268mm
   1920x1080      60.0*    50.0     59.9  
   1920x1080i     60.1     50.0     60.0  
   1680x1050      59.9  
   1280x1024      75.0     60.0  
   1440x900       59.9  
   1280x960       60.0  
   1280x720       60.0     50.0     59.9  
   1024x768       75.1     70.1     60.0  
   832x624        74.6  
   800x600        72.2     75.0     60.3     56.2  
   720x576        50.0  
   720x480        60.0     59.9  
   640x480        75.0     72.8     66.7     60.0     59.9  
   720x400        70.1  
DP1 disconnected (normal left inverted right x axis y axis)

