6.4: Procedure

Solubility Test:

1) Place a "pea-sized" sample (the amount of unknown that fits on the end of a scoopula, approx. 1/8 tsp.) in a clean medium-sized test tube. Add 5.00 mL DI H₂O.

2) Stir with a stirring rod or parafilm the tube and shake gently. Let stand about 1 minute.

3) If the substance READILY DISSOLVES, proceed to PART B of these procedures.

(Right side of flow chart.) If the substance does NOT readily dissolve, proceed to PART A. (Left side of flow chart.)

Part A:

Three water-insoluble white solids are possible: Cornstarch, CaSO₄, and CaCO₃.

1) TO TEST FOR CORNSTARCH: add 2 drops of 0.0500 M I₂ solution to your sample tube. Parafilm tube and shake gently.

a) If a blue-black solution results, then cornstarch is present.

b) If a yellowish to brown solution results, then either CaSO₄ or CaCO₃ is possible.

2) TO TEST FOR CaCO₃: place a fresh sample of unknown into a clean test tube and add 5.00 mL DI H₂O. Parafilm tube and shake gently. Add 1.00 mL 6.0 M HC₂H₃O₂ (about 20 drops with a pasteur pipette). Parafilm and shake gently.

a) If bubbles evolve, then CaCO₃ is present. With concentrated solutions, bubbles of CO₂ gas evolve immediately. With dilute solutions, the effervescence will be much less obvious. (Do NOT confuse "air bubbles" with the evolution of CO₂ gas.) Warming in a water bath, with stirring, will increase the amount of bubble formation. CO₂ gas is odorless and colorless. The molecular equation for this reaction is:

CaCO₃(s) + 2 HC₂H₃O₂ (aq) ® H₂O(l) + CO­₂(g) + Ca(C₂H₃O₂)₂(aq)

b) If bubbles do not evolve, then your substance must be CaSO₄.

Part B:

One of the eleven water soluble white solids is possibly NaOH, Na₂CO₃, K₂CO­₃, MgSO­₄, NaHCO₃, NaCl, LiCl, KCl, H₃BO₃, or sucrose.

1) Add 2-3 drops phenolphthalein indicator to your sample tube.

a) If the solution remains clear, proceed to PART C of these procedures.

b) If the solution turns pink, then NaOH, Na₂CO₃, or K₂CO₃ are possible, since they all form basic solutions when dissolved in water. Proceed to Step 2.

2) Place a fresh sample of your unknown in a clean test tube and add 5.00 mL DI H₂O. Stir with glass stirring rod until dissolved. Add 5-10 drops 6.0 M HC₂H₃O₂.

a) If bubbles do not evolve, then NaOH is the unknown present.

b) If bubbles evolve, then the evolution indicates the presence of either Na₂CO₃ or K₂CO₃. The molecular equation for this reaction is:

M₂CO₃(aq) + 2 HC₂H₃CO₂(aq) -> H₂O(l) + CO₂(g) + 2 MC₂H₃O₂(aq) where M = Na or K

3) A FLAME TEST will distinguish the Na⁺ ion from the K⁺ ion.

a) Prepare a fresh sample of your unknown by placing a double amount in a clean test tube. Add a few mL DI H₂O and stir (does not have to dissolve). This will increase the solution concentration and give a more distinguishable color.

b) Light a bunsen burner and adjust the flame to produce a clear inner "cone".

c) Saturate the tip of a cotton swab with the unknown solution and place it into the hottest part of the flame. Observe the color of the flame:

Lavender-pink indicates the presence of the K⁺ ion and identifies K₂CO₃ as the unknown.

Bright yellow/orange indicates the presence of the Na⁺ ion and identifies Na₂CO₃ as the unknown.

Part C:

A clear solution remaining after the addition of phenolphthalein may contain one of the following compounds: NaCl, LiCl, KCl, NaHCO₃, H₃BO₃, MgSO₄, or sucrose

1.) TO TEST FOR MgSO₄: place a fresh sample of your unknown into a clean test tube and add 5.00 mL DI H₂O. Add 20 drops of 6.0 M NaOH. Parafilm and shake gently.

a) If a white precipitate forms, MgSO₄ is present. The molecular equation for this reaction is:

MgSO₄(aq) + 2 NaOH(aq) -> Mg(OH)₂(s) + Na₂SO₄(aq)

b) If no precipitate forms, then NaCl, LiCl, KCl, NaHCO₃, H₃BO₃, and sucrose are possible.

Part D:

LiCl, KCl, NaCl, NaHCO₃, H₃BO₃, and sucrose are possible.

1.) TO TEST FOR NaHCO₃: Place a fresh sample of unknown into a clean test tube. Add 5.00mL DI H₂O. Parafilm and shake gently. Add 10 drops of 6.0 M HC₂H₃O₂.

a) Evolution of bubbles indicates the presence of NaHCO₃. The molecular equation for this reaction is:

NaHCO₃(aq) + HC₂H₃O₂(aq) -> NaC₂H₃O_2­(aq) + CO₂(g) + H₂O(l)

b) If no bubbles form, proceed to PART E.

Part E:

The remaining possibilities are sucrose, LiCl, KCl, NaCl, and H₃BO₃.

1) Prepare a fresh sample of the unkown by placing a double amount in a clean test tube. Add a few mL DI H₂O and stir (does not have to dissolve).

2) Perform a FLAME TEST. (see PART B - 3)

a) K⁺: lavender/pink unknown is KCl

b) Na⁺: bright yellow/orange unknown is NaCl

c) Li⁺: magenta unknown is LiCl

d) B³⁺: bright green unknown is H₃BO₃

e) no distinguishable color, but if the tip of the swab smells like “toasted marshmallows” (use a “wafting motion” to smell odor), then the unknown is sucrose.