Page 85 - SCERT Kerala State Syllabus 8th Standard Basic Science Textbooks English Medium Part 1_Neat

Salt that turns black in sunlight

Take some silver bromide in two dry watch glasses. Wrap one of
them with a black paper. Keep both of them in sunlight for some
time.
What do you observe?
In which watch glass does the colour of silver bromide change?

Which form of energy was responsible for this chemical change?
This was due to the deposition of silver produced as a result of the
decomposition of silver bromide by the absorption of light.

What could be the reason for storing certain medicines and chemicals
in brown bottles? Think of it.
Chemical reactions which liberate or absorb light energy are known
as photochemical reactions.
Electrochemical reactions

Let us do an experiment.

Set up the apparatus as shown in Figure 6.5.
What changes do you observe when electricity
is passed through the circuit?
In the colour of copper sulphate solution:

Carbon
rod
On the carbon rods :

Dilute copper
sulphate solution
The change in colour of copper sulphate
Fig. 6.5 solution and the deposition of copper at the
Electrolytes electrode proves that a chemical change has
occurred.
Electrolytes are substances which
undergo chemical change when Does it happen if a carbon rod is simply
electricity is passed through them. kept in copper sulphate solution?
Sodium chloride, copper sulphate, silver Which form of energy was responsible for
nitrate etc., are electrolytes. Acids,
this chemical change? Was the energy
alkalis and salts in their molten form as
liberated or absorbed?
well as in aqueous solutions are
electrolytes. The rods which are In this reaction, copper sulphate decomposed
connected to the poles of a battery and with the absorption of electrical energy. The
which transfer electricity to the process in which a substance undergoes
electrolyte are known as electrodes.

84 Basic Science VIII