05 Chem Theater and dissolving salt Science By the Horns Summer 2006

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 Cl - Na + + Cl

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).

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; Na + Cl

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)

Supersaturation Melting a special salt solution (Na 2 S 2 O 3 5H 2 O) 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

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 it’s original state, before dissolving in water. Salt evaporation pond in France.France

Crystal growing Melting point48.3 ｡ C