What is the difference between the truth of propositions and the validity of arguments?

Truth and validity are two different notions. Truth is predicated of propositions whereas validity is predicated of arguments. Propositions are either true or false.

Deductive arguments are either valid or invalid. We have noted earlier that a deductive argument claims to provide conclusive proof for its conclusion.

A deductive argument is valid if and only if the premises provide conclusive proof for its conclusion. This notion of validity of deductive argument can also be expressed in either of the following two ways.

i) If the premises of a valid argument are all true, then its conclusion must also be true.

(ii) It is impossible for the conclusion of a valid argument to be false while its premises are true.