BACKGROUND:
Water is a compound that has strong
"bonds" among its constituents. In other words, it is difficult to
break apart the hydrogen and oxygen atoms without some sort of energy input.
"Salts," on the other hand, usually have weak "bonds" and the atoms of
salts can easily be separated into its appropriate ions. When a salt,
like sodium chloride (table salt) is dissolved in water the sodium and
chloride separate temporarily. The sodium atom will become a positively
charged ion and the chloride atom will become a negatively charged ion.
An ion is an atom or group of atoms that has a negative or positive electric
charge. Negative ions are formed by atoms gaining electrons, and
positive ions are formed by atoms losing electrons.
Substances that conduct electric current are
called electrolytes. They are formed as a result of a
dissociation into positively and negatively charged particles called ions,
which migrate toward and ordinarily are discharged at the negative and
positive terminals of an electric circuit, respectively. The most familiar
electrolytes are acids, bases, and salts, which ionize when dissolved in
such solvents as water. Many salts, such as sodium chloride, behave as
electrolytes when dissolved in water. Pure water will not behave
as an electrolyte.
PROCEDURE:
- Review with the students
that fresh water is basically pure "water" whereas salt water has many
other elements dissolved in water.
- Students will discover in this lab that the ions in
the water make salt water an electrolyte. An electrolyte is a
nonmetallic electric conductor in which a current is carried by the
movement of ions.
- Go over the lab set-up with
the students as diagramed on the students' sheet. The battery can
be a six volt or smaller, depending on the type of bulb you have.
(Remember if the voltage is too high it will burn out the light bulb).
An electrode refers to whether a current enters or leaves a conductor
- Ask the students to determine if the
light bulb will glow brighter if more salt is added. Yes, but there
is a maximum point that the bulb will reach and will not glow any brighter.
The salt acts as an electrolyte, making electricity transfer more efficiently.
Fresh water has no ions that can make it act as a conductor. Have
the students record their data on the graph. Then when you discuss
the experiment with the students have the student write a summary of the
class results.
