In the following equation, which species is the oxidizing agent?

Question

KCLO3 --> KCL + KCLO4

In the same are, How do you know that Cu is the reducing agent in this equation:
AgNO3 + Cu ---> Cu(NO3)2 + Ag

Answer 1

Two definitions cover the whole gamut of questions (actually, only one definition is needed; if you know one the other must be the opposite).
Oxidation is the loss of electrons. An easy way to remember this is LEO the lion goes grrr. LEO stands for loss electrons oxidation. Easy, huh?
(You don't need to remember the definition for reduction but it is "Reduction is the gain of electrons."

Now use oxidation states. For the first one, which is a tricky one.
One half reaction is ClO3^- ==> Cl^-
and the other is ClO3^- ==> ClO4^-
So Cl is +5 going to -1 in the first one (reduction) and +5 going to +7 (oxidation) in the second one.

For the Ag/Cu equation,
Ag goes from +1 on the left to zero on the right which is a gain of electrons.
Cu goes from zero on the left to +2 on the right which is a loss of electrons. Cu is oxidized (which makes it the reducing agent).