05 Chem Theater and dissolving salt Science By the Horns

1
05 Chem Theater anddissolving saltScience By
the Horns

• Summer 2006

2
Salt

• Salt is made up of sodium (Na) and Chlorine (Cl).
  
• Na and Cl form a bond where the Cl basically
  takes an electron from the Na.
• This gives Chlorine a negative (-) charge and Na
  a positive () charge. Particles with full
  charges like Na and Cl- are called ions.
• When placed in water, the Na and Cl dissociate.
  The positive side of some water molecules
  attracts the negative chlorine ion and the
  negative side of other water molecules attracts
  the positive sodium ion.

3
Salt Solution

• Water is the solvent, salt the solute and
  together they make a solution.
• Dissolving is a physical process, not a chemical
  reaction. Salt and water are attracted to each
  other but do not form new chemical bonds (i.e. -
  share or exchange electrons).

4
Chem Theater

• Students can be water, Na, or Cl-.
• Start with water molecules and solid salt in
  separate spaces
• Ask 3 or 4 student water molecules to approach
  solid Na and Cl- to dissolve one ion at a
  time.
• Remind students that like charges repel,
  opposite charges attract (like a magnet), then
  let them surround and pull out of the solid one
  ion using the appropriately charged side of the
  water molecule (see picture)
• This is a dynamic process - i.e., water molecules
  can trade ions with other water molecules AND
  can return to solid to pull more ions into
  solution

5
Saturation

• When all the salt that can dissolve is
  dissolved,the solution is saturated.
• HOWEVER, the movement of ions out of the solid
  and into the water continues, just at a pace that
  does not change the ratio of water molecules to
  ions dissolved. In other words, if one Na or
  Cl- ion goes into solution, another one comes out
  (I.e., is deposited back in the solid lattice)
• This is an example of equilibrium!
• (See equilibrium game to experience this dynamic
  equilibrium)

6
Supersaturation

• Melting a special salt solution (Na2S2O35H2O)
  results in a supersaturated solution
• Adding a seed crystal allows the supersaturated
  solute particles to crystallize and solidify
• This is an example of a solution NOT at
  equilibrium!

Melting point48.3?C
http//en.wikipedia.org/wiki/Sodium_thiosulphate
7
evaporation

• Simulate evaporation where the water leaves and
  the Na and Cl- ions rejoin the solid lattice
• This illustrates that dissolving is a physical
  process because the separation (of water and
  salt) through evaporation leaves salt chemically
  identical to its original state, before
  dissolving in water.

Salt evaporation pond in France.
8
Melting point48.3?C
Crystal growing
http//www.waynesthisandthat.com/crystals.htm
http//www.pbase.com/pganzel/through_the_microscop
e