Salts

Identification of an unknown inorganic substance.

Introduction. In this experiment you will apply your knowledge of properties of inorganic compounds to the problem of identification of a salt of unknown composition. Two samples of different salts will be provided. You should establish their chemical formulas. On of the samples will be colored, another colorless.

Each common salt has only one anion and only one cation. Possible cations you may find in the samples in our laboratory are NH₄⁺, Na⁺, K⁺, Ca²⁺, Ba²⁺, Al³⁺, Cr³⁺, Mn²⁺, Fe²⁺, Fe³⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺. Possible anions are Cl^-, Br^-, I^-, CO₃^2-, SO₄^2-, PO₄^3-, NO₃^-.

Suggested procedure. In this experiment, you are very flexible in the ways you perform your analysis. Therefore, we can only provide some guidance, and you are applying all the technique you have learned in this laboratory yourself.

Color. Look at the color of your compound. Which cations may give you such color, and which cannot?

Solubility in water. Put a small amount (around 0.01-0.02 g) of your sample in a test tube and add some water. Try to dissolve the sample. If it does not happen, bring the water in the test tube to boiling. some salts have much better solubility in hot water.
If your compound is insoluble in water, it cannot be a nitrate. All nitrates are water soluble. Therefore, you may try to dissolve your compound in a diluted nitric acid. That may help in case of some phosphates and carbonates. Watch for bubble formation: this is the evidence of the carbonate anion.
After you have dissolved your compound in water, make tests for anions. Remember, you have only one of them. Thus, try group reagents first: AgNO₃ + HNO₃ for Cl^-, Br^-, I^-, and BaCl₂ for CO₃^2-, SO₄^2-, PO₄^3-. After you have a correct group, perform the individual test reactions for each of the possible anions until you find one.
Before doing tests for cations, cross out impossible variants. If your salt is soluble, and anion is carbonate or phosphate, you may have only NH₄⁺, Na⁺, K⁺ as a possible option: all other carbonates and phosphates are insoluble in water. Cross out all colored cations if your salt is white; cross out all colorless if your salt is colored.
Perform necessary tests to identify the cation within the remaining group.
Write a formula of compound you have identified and proceed to the second salt.

An example: I have a green salt which appeared to be soluble in water. From the color I would suspect Ni or Cr, but I will think also about Fe(II) and Cu as a possible variant. Because the salt is water soluble, it cannot be phosphate or carbonate. I took several drops of solution and have added several drops of AgNO₃ + HNO₃. No precipitate! That means I have no Cl, Br, and I. Next, I took several drops of solution and have added several drops of BaCl₂. This time I have a precipitate. The only option is SO₄^2-. I have my anion!
Now, I am testing for cation. The very first guess, Ni, appears to be correct: I took several drops of solution, added several drops of dimethylglyoxime, and some ammonia: I see lots of pink precipitate in my test tube. Therefore, my compound is nickel sulfate, NiSO₄.