Deducing the identity of unknown compounds has been a central aspect of chemistry since its origin. In fact, early chemists had to come up with a system for classifying before they could have any reference for identification. They divided species up as gasses, solids, and liquids; they declared sour compounds acids and bitter compounds bases; the categories go on and on. With increasingly sophisticated taxonomies, and a growing understanding of the atoms that make up molecules, chemists were able to place new unidentified compounds in the appropriate categories. We still do this today: think of forensics. Chemists are needed to identify drugs, determine what chemicals remain on bomb fragments, and find the cause of a fire among arson debris.

Today, you will deduce the identity of four unknowns based on your observations and what you know of chemical properties and reactivity. Like a forensic scientist, you'll do diagnostic "tests" on known samples and compare these "tests" with an unknown sample's behavior. The procedure that you are about to undertake has helped scientists, including forensic specialists, identify the unknown contents of water samples, soil samples, and even specimens of human remains!

You'll be performing a lot of different reactions in this lab period, but don't let the sheer number overwhelm you. While each combination of reagents will behave in a unique way, we can categorize them all fairly easily.

Let's investigate the following categorization of chemical reactions:

Precipitation reactions occur when two or more compounds react to form a new, insoluble compound (the precipitate) that "falls out" of solution. This is perhaps the easiest type of reaction to identify visually. For example, when clear solutions of potassium iodide and lead(II) nitrate are combined, a bright yellow solid appears as if from nowhere. What happened? To understand this, we'll need to turn to chemical equations and solubility rules.

Given our reagents, we have the following:

(Equ. 1)

To form our reactants, we do a double exchange-essentially a "repairing". A simple example makes this clear. Notice that each compound (both reactant and product) is made of a cationic and an anionic component.

(Equ. 2)

In terms of our original reaction, we form:

(Equ. 3)

We're almost there. All we have to do now is determine which product is insoluble, and thus the yellow solid precipitate. In order to do so, we will turn to the solubility rules you've learned about in lecture and read about in your chemistry text.

The following chart is adapted from Zumdahl, Chemistry, 6th Edition, Table 4.1, pg. 152.

Table 1: Solubility Rules

Soluble Insoluble Exceptions

Most nitrates (NO3-) and ammonium (NH4+) compounds


Most Group I compounds


Most chloride, bromide, and iodide (Cl-, Br-, I-) compounds


Compounds that also contain silver, lead, or mercury (Ag+, Pb2+, Hg22+) are insoluble

Most sulfates (SO42-)


Compounds that also contain barium, lead, mercury, or calcium (Ba2+, Pb2+, Hg22+, Ca2+) are insoluble


Most hydroxides (OH-)

Compounds that also contain Group I elements are soluble and compounds that also contain Group II elements are marginally soluble


Most sulfides, carbonates, chromates, and phosphates (S2-, CO32-, CrO42-,PO43-)

Compounds that also contain Group I elements or ammonium (NH4+) are soluble

So we see that while KNO3 is soluble, PbI2is not, and must be the yellow solid.

(Equ. 4)

A step further will allow us to simplify this equation dramatically. The aqueous (aq) subscript indicates that a species is soluble in water, and exists mainly as ions. Therefore, we write:

(Equ. 5)

Then, we cancel out the species that appear on both sides of the equation. Essentially, these do not change in the reaction (that's why we call them spectator ions), and we have:

(Equ. 6)

You'll run into many reactions of this type today, and you can use this procedure on each one.

Acid-Base Reactions, for our purposes, there are two ways of thinking about acids and bases. In the Arrhenius definition, acids produce protons (H+) in water, and bases produce hydroxide ions (OH-). It's a straightforward and useful definition for many cases, but it doesn't include all acids and bases. The Brønsted-Lowry concept describes acid-base reactions as the transfer of a proton from one compound to another. A molecule that donates a proton is called an acid, and a molecule that accepts a proton is called a base.

(Equ. 7)

We see that HBr is the acid because it gives up a proton, and that H2O is the base because it accepts the proton. In equation form, we have:

(Equ. 8)

Often, you will see this reaction written in a simpler form. The idea is the same, however.

(Equ. 9)

This equation reflects the fact that HBr can be described with the Arrhenius definition of an acid, as well as the Brønsted-Lowry definition.

HBr is a fairly simple case, and you will come across reactions that are more difficult to write. The key is to remember that for it to be an acid-base reaction, one compound has to gain a proton, and the other has to lose one. But what if you just can't tell which species will donate and which will accept? What if you have a reaction between ammonia (NH3) and water? Both contain hydrogen. Experimentally, we use indicators. Indicators are substances that respond differently to acidic and basic solutions-generally in terms of color. For example, phenolphthalein is a common indicator that turns pink in basic solutions and is clear in acidic solutions. In this lab, you will use Universal Indicator. Just like phenolphthalein, its color depends on the acidity of the solution. The trick is that you'll have to figure out how!

Complexation Reactions are, well, a little complicated, but that's not the source of their name. A complexation reaction can be described as a reaction that forms a "complex". For instance, in adding a cobalt salt, such as CoCl2 (s), to water, we form [Co(H2O)6]2+ (aq). The part of this substance written in brackets is called a complex ion; it involves more than one species but still carries an overall charge (making it an ion) while in solution. The entire group has a charge, much like a polyatomic ion (ex: [Cu(CN)2]-, [Mn(OH2)6]2-). The list below helps describe complexation reactions in more detail:

  • Complexes generally form around a transition metal cation (ex: Ni2+, Au3+, Cu+).
  • Whether or not a complex forms depends on concentration, among other factors. A reaction between two compounds may proceed normally until it is "flooded" by one of the reagents, at which point a complex ion is more likely to form.
  • The formation of a complex is generally indicated by a color change. In fact, most complex ions are very brightly colored.
  • Don't worry! You won't be expected to figure out the formulas for any complex ions you create.

Redox Reactions (oxidation reduction reactions) involve the transfer of electrons. That's what oxidation and reduction mean.

Loss of electrons = oxidation.
Gain of electrons = reduction.

A useful way to remember this is with the mnemonic: Leo says Ger

Let's look at an example:

(Equ. 10)

It may not look unusual at first, but this isn't like the other reactions we've seen. No atoms are moving from one compound to another, and nothing is dissociating! We need a way to explain how solid aluminum and zinc ions transformed into aluminum ions and solid zinc. To do this, chemists break the reaction into two half reactions, one for each element.

(Equ. 11)

(Equ. 12)

Equation 11 is an oxidation reaction; each mole of solid aluminum loses three moles of electrons to become a mole of aluminum cations. Equation 12 is a reduction reaction; as each mole of zinc cations gains two moles of electrons to become a mole of solid zinc. It's important to note that for a reaction to occur there must be both oxidation and reduction. One species can only lose electrons if another species is in solution to take them.

It isn't always obvious from observation alone that a redox reaction is taking place, but keep the option in mind. If you observe a reaction, and you can't think of a combination of anions and cations that could form new products, consider redox before you give up!