1. Observation and description of a phenomenon or group of phenomena.
2. Formulation of an hypothesis to explain the phenomena. In physics, the hypothesis often takes the form of a causal mechanism or a mathematical relation.
3. Use of the hypothesis to predict the existence of other phenomena, or to predict quantitatively the results of new observations.
4. Performance of experimental tests of the predictions by several independent experimenters and properly performed experiments.
SINGLE DISPLACEMENT REACTIONS
In this type of reaction, one element displaces (replaces) another in a compound. This happens in two situations:
1. When a metal displaces a metal. e.g.
Li(s) + NaCl(aq) ® LiCl(aq) + Na(s)
2. A non-metal displaces a non-metal.
F2(aq) + 2KI(aq) ® I2(aq) + 2KF(aq)
In most single displacement reactions there is an element and an ionic compound as reactants.

The general pattern is:
A + BC ® AC + B
element A displaces element B
A and B are elements
BC and AC are compounds
e.g. Cu(s) + 2AgNO3(aq) ® 2Ag(s) + Cu(NO3)2(aq)
2Li(s) + 2H2O(l) ® H2(g) + 2LiOH(aq)
Whether or not one element can displace another element in a compound depends on the relative reactivities of the two elements.
e.g. Cu can replace Ag in the reaction above because Cu is more reactive then Ag.
For example:
Cl2(aq) + 2NaBr(aq) ® Br2(aq) + 2NaCl(aq)
Bromine appears below chlorine, therefore chlorine will replace it.
Br2(s) + 2NaCl(aq) ® No Reaction
Chlorine is above bromine, therefore the reaction will not occur.
PREDICT THE PRODUCTS OF THE FOLLOWING REACTIONS:
1. Zn(s) + Pb(NO3)2(aq) ®
2. Pb(s) + Zn(NO3)2(aq) ®
3. Al(s) + NiCl2(aq) ®
4. Mg(s) + H2SO4(aq) ®
5. Br2(aq) + CaCl2(aq) ®


One element replaces another in a compound.
A + BC --> AC + B (positive ion is replaced)
or
D + BC --> BD + C (negative ion is replaced)
• A reactive metal will replace any metal found below it in the activity series (Table 7.2, pg 155).
• Compare the reactivity of Cu, Zn, and Mg in HCl(aq).
Our example:
Fe(s) + CuCl2(aq) --> FeCl2(aq) + Cu(s)