Introduction
Double displacement reactions are among the most common simple chemical reactions and com-
paratively easy to study. In each part of this experiment two water solutions, each containing posi-
tive and negative ions, will be mixed in a test tube. Consider the hypothetical reaction:
AB (aq) + CD (aq) AD (?) + CB (?) (EQ 8.1)
where AB (aq) exists as A+
(aq) and B-
(aq) ions in solution and CD (aq) exists as C+
(aq) and D–
(aq) ions
in solution. As the ions come in contact with each other, there are six possible combinations that
might conceivably cause a chemical reaction. Two of these combinations are the meetings of ions
of like charge; that is A+
(aq) + C+
(aq) and B-
(aq) + D–
(aq). Since like charges repel, no reaction will
occur. Two other possible combinations are those of the original two compounds; that is, A+
(aq) +
B–
(aq) and C+
(aq) + D–
(aq). Since we originally had a solution containing each of these pairs of ions,
they can mutually exist in the same solution; therefore they do not recombine. Thus the two possi-
bilities for the combination of each of the positive ions with the negative ion of the other compound
is: A+
(aq) + D–
(aq) and C+
(aq) + B–
(aq).
In order for a double displacement reaction to occur one or more of the following must happen:
1. A precipitate is formed.
2. A gas is produced.
3. Water or another slightly ionized substance is formed
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