Qualitative Analysis
Supporting original reading. Follow the AP topic pathway for reviewed scope, worked reasoning and targeted practice.
Chemistry II · original Chapter 4 · combined Chapter 12
Does the salt look crystalline or powdery?
Does the salt have a color?
Does the salt appear to be uniform?
All of these questions can help you to verify results you get during the lab. For example, if you somehow come up with lab tests resulting in your salt being Calcium Carbonate, but the salt looks like shiny crystals, then you know something might be wrong.
The first step towards identifying your salt is simply by looking at it. Obviously, even the best eye can’t simply declare a salt based on appearance alone (though some might like to try). In order to truly know what the salt is, we have to prove the presence of the constituents: the cation and the anion.
To start off with, there is a very simple test you can do to try and identify a cation within the salt. This test is the flame test.
If you have an open flame running, usually due to a bunsen burner, then the flame test involves using a sturdy wire to insert a speck of the salt into the flame and looking at what color comes off of the salt.
While it sounds simple, there are a lot of things that can disturb your results. First, the wire has to be clean of any potentially color-producing chemical. An easy way to make sure of this is to wipe it down and stick the tip into the flame. If you hold it too long, sometimes the wire itself starts to flare up a color. That isn’t what you want, so this test will have to be over within seconds.
The other item to make sure of is that your base flame color isn’t the color you’re noting. This means that if your flame is orange, you aren’t thinking that the flame test is resulting in an orange for your salt.
Sodium - Yellow
Barium - Yellow-Green
Lithium - Red
Copper - Green or Blue-Green if a halide is present
Iron - Gold
Potassium - Lilac (Whitish-Purple)
Calcium - Reddish-Yellow
The flame test doesn’t help to distinguish every single cation, disappointingly. If it does help to identify our cation, then great, but we still don’t know the anion. To do that, we move towards the basic solubility test.
As you recall from the previous chapter we covered solubility rules briefly. Those only deal with solubility of compounds in water, but sometimes, that’s a great help. Say your flame test resulted in a Red flame, indicating the presence of Lithium. You then take some of your salt and attempt to dissolve it in water, but it doesn’t. This means that you have one of two salts: Li3PO4 or Li2CO3. In only two tests, you’ve narrowed down all the salts to only two. Solubility rules allow us to identify what category the salt falls under. This gives us the opportunity to make a list of the potential salts we have. From there, it is easier to identify tests to perform to eliminate certain salts or to confirm certain ions.
The solubility rules are as follows:
- Write acetate as CH₃COO⁻. It is the conjugate base of acetic acid, CH₃COOH. A salt of acetate combines that whole anion with cations in a charge-balanced ratio; retaining the acid proton and appending a minus sign gives the wrong species.
- Salts formed with Nitrate (NO3-) will be soluble.
- Salts formed with Silver (Ag+) will be insoluble.
- Salts formed with Acetate (CH3COOH-) will be soluble.
- Salts formed with S2-, OH-, CO32-, PO43- are insoluble.
- Salts formed with Lead (Pb2+) are insoluble.
- Salts with the first few halogens after Fluorine: Cl-, Br-, I- are soluble UNLESS they are bonded with Mercury (Hg2+).
- Solubility rules help us choose hypotheses for ordinary dilute aqueous conditions. Silver nitrate is commonly soluble, but it is not the only silver compound capable of dissolving. An acetate ion is CH₃COO⁻. Use the specified candidate set and supplied observations instead of turning a simplified rule into a unique identification proof.
No visible precipitate on adding a silver-ion reagent does not uniquely prove nitrate. Several anions may remain soluble under the test conditions, and a dilute precipitate can be below the visible detection threshold. Combine the result with blanks, positive controls, other independent tests and an explicit candidate list. Report an inference supported by the evidence rather than a unique identity that the observation cannot establish.
Glossary and deeper reading · 8 refreshers
These editorial refreshers supplement the reading. Definitions and examples are added guidance; original source wording and recorded answers remain in their original context.
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Qualitative analysis
Qualitative analysis seeks the identities or presence of substances rather than their numerical amounts. Individual observations often require supporting evidence because different species can produce similar signals. For example, a supplied flame-color observation can suggest an ion's identity without establishing it uniquely.
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Solubility
Solubility describes how much of a substance can dissolve in a specified solvent under stated conditions. It depends on the solute, solvent, temperature, and other relevant conditions and is distinct from how fast dissolution occurs. For example, two salts can have different equilibrium solubilities in water at the same temperature.
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Cation
A cation is a positively charged ion. Its total positive nuclear charge exceeds its total electron charge. For example, calcium commonly forms a cation with charge +2 by losing two electrons.
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Anion
An anion is a negatively charged ion. It has more electrons than needed to balance its positive nuclear charge. For example, a chlorine atom gaining one electron forms a chloride anion.
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Ion
An ion is an atom or group of atoms with a net electric charge. Ordinary ion formation changes electron count without changing the element's proton count. For example, a sodium atom losing one electron becomes a sodium ion with charge +1.
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Conjugate acid–base pair
A conjugate acid–base pair consists of species differing by one proton. Losing a proton forms an acid's conjugate base, while gaining one forms a base's conjugate acid. For example, ammonium and ammonia form a conjugate acid–base pair.
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Aqueous solution
An aqueous solution is a solution in which water is the solvent. The dissolved substance may originate as a solid, liquid, or gas, so aqueous does not mean that a solid has melted. For example, sodium chloride dissolved in water is aqueous even though the pure salt is solid.
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Precipitate
A precipitate is a solid phase that forms from species in a solution. Whether it forms depends on the ion concentrations and the relevant equilibrium conditions. For example, silver ions and chloride ions can produce a silver chloride precipitate.