Crystallization is a separation technique that is used to separate a solid (SOLUTE) that has dissolved in a liquid (SOLVENT) and made a solution (homogenous mixture— not necessarily a liquid).

Crystal Making General Background: We know that salt dissolves in water. Salt (NaCl) is an ionic compound composed of a cation (Na + ) and an anion (Cl - ). Only so much salt can dissolve in water. Once you reach the point where no more salt will dissolve the solution is said to be saturated. Heating the solution increases the amount of salt that can be dissolved. When you slowly cool a heated salt solution it ends up having more dissolved content than it otherwise could and this is known as a supersaturated solution. Using cooled, supersaturated solutions is the key to crystal-making. I am going to use preheated, supersaturated sodium acetate (NaC 2 H 3 O 2 ) to demonstrate an exothermic reaction (hot ice) You are going to make borax snowflakes/crystals per the handout. You are also going to dissolve 30g of copper (II) sulfate in 50ml of deionized water via gentle heating to make beautiful blue crystals Complicated Background for Hot Ice: At temperatures above 58C, Na(C 2 H 3 O 2 )3H 2 O (sodium acetate trihydrate) begins to lose its water of hydration and begins to dissolve in that water. The solution, once completely dissolved, can be cooled to give a solution which is supersaturated in the anhydrous and trihydrate form of sodium acetate. In the presence of a seed crystal, this solution will “freeze” but in doing so must reach its freezing point which is 58C and thus the crystallizing solution warms up (exothermic reaction).

Crystallization can be used to “filter” impurities!

Acetate (C 2 H 3 O 2 ) Molecule If this molecule is neutral how many protons does it have? In order for acetate to form it must gain an electron. How many electrons does the molecule have and what is its overall charge? Since acetate has a -1 charge (cation or anion?) a sodium (cation or anion?) will electrostatically attach to it. If this molecule is neutral how many protons does it have? In order for acetate to form it must gain an electron. How many electrons does the molecule have and what is its overall charge? Since acetate has a -1 charge (cation or anion?) a sodium (cation or anion?) will electrostatically attach to it.

Sodium Acetate: Na(C 2 H 3 O 2 ) 1- Charge Na 1+ Charge Electrostatic force: Opposites attract

Lab Write-up (wiki/solutions) What are characteristics of solutions? Can you filter a solution? What are three types of solutions and provide a specific example of each? Define: solvent, solute, solubility, saturation, super saturation and crystallization. Write a paragraph (IN YOUR OWN WORDS) explaining the science of crystal making.