80. A. rate B. extent C. level D. point
Task 2: READING COMPREHENSION
Read the passage and choose the best answer for each question below.
The ability to conduct electricity is one of the key properties of a metal. Other solid material
such as silicon can conduct electricity but only effectively at certain temperatures. Also, some
substances such as salt (sodium chloride) can conduct when molten or when dissolved in water. The
ability of metals to conduct electricity is due to how their atoms bond together. In order to bond
together the metal atoms lose at least one of their outermost electrons. This leaves the metal atoms
with a positive charge and they are now strictly ions. The lost electrons are free to move in what are
known as a sea of electrons. Since the electrons are negatively charged they attract the ions and this
is what keeps the structure together.
An electric current is a flow of charge and since the electrons in the sea of electrons are free
to move they can be made to flow in one direction when a source of electrical energy such as a
battery is connected to the metal. Hence we have an electric current flowing through the wire, and
this is what makes metals such good conductors of electricity. The only other common solid
conducting material that pencil users are likely to encounter is graphite (what the ‘lead’ of a pencil
is made from). Graphite is a form of carbon and again the carbon atoms bond in such a way that
there is a sea of electrons that can be made to flow as an electric current. Likewise, if we have an
ionic substance like salt we can make the electrically charged ions flow to create a current but only
when those ions are free to move, either when the substance is a liquid or dissolved in water. In its
solid state an ionic substance like salt cannot conduct electricity as its charged ions cannot flow.
Electrical insulators are substances that cannot conduct electricity well either, because they
contain no charged particles or any charged particles they might contain do not flow easily. Water
itself is a poor conductor or electricity as it does not contain a significant amount of fully charged
particles (the ends of a water molecule are partly charged but overall the molecule is neutral).
However, most water we encounter does contain dissolved charged particles, so it will be more
conductive than pure water. Many of the problems that occur when touching electrical devices with
wet hands result from the ever-present salt that is left on our skin through perspiration and it
dissolves in the water to make it more conductive.
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