EXPERIMENTAL: AN INVESTIGATION INTO SOME OF THE VARIABLES INVOLVED IN 
THE CONDUCTIVITY AND ELECTROLYSIS OF AQUEOUS SOLUTIONS OF METAL SALTS

Introduction
Studies have shown that, in order to conduct electricity, a substance
must contain either free-moving electrons or free-moving ions; an ion
is either a charged atom or a charged group of atoms (e.g., ammonium
and hydrogencarbonate).  In the solid state, an ionic compound consists
of a lattice of oppositely charged ions which are not free to move
because they are held together by strong electrostatic attractions.  
However, when an ionic compound is either melted or dissolved in water, 
these ions are free to move, and so the electrolyte formed does conduct 
an electric current.

Directly or indirectly, conductivity is important in a wide variety of
scientific contexts; e.g., nerve impulses, electroplating, electrical
cells, and the extraction of metals by electrolytic reduction.

Sea water is an important natural resource because it is rich in water-
soluble metal salts (e.g., LiI, NaCl, and MgBr2).  The extraction of
pure substances from sea water involves a series of physical processes
(e.g., evaporation and fractional crystallization), as well as chemical
processes (in particular, electrolysis; i.e., the decomposition of an
electrolyte by an electric current, as a result of redox reactions
which occur at electrodes).  So, quite reasonably, a knowledge of the
effects of the variables involved in the electrolysis of metal salts is
important (not least because the process uses vast amounts of expensive
electrical energy).

A few semi-quantitative relationships between some variables have been
determined (see the three examples summarized in Note 1). Nevertheless,
purely quantitative relationships for a wide range of variables are not
readily available ...

In this investigation, you are required to examine at least four
variables involved in the conductivity and/or electrolysis of aqueous
solutions of metal salts; at least two of these variables must be
quantitative, and at least one must be qualitative.

Notes   
1.  The three examples are as follows. One, at room temperature (23°C),
using carbon-graphite electrodes at a depth of 46 mm and a distance of
7 mm, the conductance (C) of 100 cm³ of NaCl(aq), when a small voltage
is applied (~6.0 V), is in inverse proportion to concentration (M;
within the range 0.20 - 0.80 mol dm-³); i.e., C = k × M-¹ + c.
Two, the resistance (R) of 0.45 mm diameter nichrome wire, when a small
voltage is applied (~5.0 V), is in linear proportion to length (L;
within the range 0.15 - 0.40 m); i.e., R = k × L + c.
And three, the rate of energy transfer (P) in 0.20 m of nichrome wire,
when a small voltage is applied (~5.0 V), is in inverse proportion to
diameter (D; within the range 0.35 - 0.55 mm); i.e., P = k × D-¹ + c.

2.  Apart from your notes and standard textbooks, sources of scientific
knowledge include encyclopaedias in libraries, on CD-ROMs, and on the
Web; it is good practice to include a bibliography in your write-up.

3.  You are provided with different types of electrodes (e.g., carbon-
graphite, copper, and iron), various aqueous solutions of metal salts
and acids (each with an initial concentration of 1.00 mol dm-³), a low-
voltage d.c. supply, and a range of electrical components. In addition,
you will need to use - within reason - other suitable apparatus.

4.  The proposed plans of your investigation should be presented in
detail; these plans may be modified as the investigation proceeds.

Dr. R. Peters                  Next                      Contents' List